• Thursday, April 27, 14:00, Dennis Raetzel (Vienna): The gravitational field of a laser pulse
Abstract: As Einstein's equations tell us that all energy is a source of gravity, light must gravitate. However, because changes of the gravitational field propagate with the speed of light, the gravitational effect of light differs significantly from that of massive objects. In particular, the gravitational force induced by a laser pulse is due only to its creation and annihilation and decays with the inverse of the distance to the pulse. We can expect the gravitational field of light to be extremely weak. However, the properties of light are premises in the foundations of modern physics: they were used to derive special and general relativity and are the basis of the concept of time and causality in many alternative models. Studying the back-reaction of light on the gravitational field could give new fundamental insights to our understanding of space and time as well as classical and quantum gravity. In this talk, a brief overview is given of the gravitational field of laser pulses in the framework of linearized Einstein gravity. A glimpse is caught of the gravitational interaction of two single photons, which turns out to depend on the degree of their polarization entanglement.
• Wednesday, April 26, 13:00, lunch seminar, Xinliang An (Toronto): On Gravitational Collapse in General Relativity
Abstract: In the process of gravitational collapse, singularities may form, which are either covered by trapped surfaces (black holes) or visible to faraway observers (naked singularities). In this talk, I will present four results with regard to gravitational collapse for Einstein vacuum equation. The first is a simplified approach to Christodoulou’s monumental result which showed that trapped surfaces can form dynamically by the focusing of gravitational waves from past null infinity. We extend the methods of Klainerman-Rodnianski, who gave a simplified proof of this result in a finite region. The second result extends the theorem of Christodoulou by allowing for weaker initial data but still guaranteeing that a trapped surface forms in the causal domain. In particular, we show that a trapped surface can form dynamically from initial data which is merely large in a scale-invariant way. The second result is obtained jointly with Jonathan Luk. The third result addressed the following questions: Can a black hole’’ emerge from a point? Can we find the boundary (apparent horizon) of a black hole’’ region? The fourth result extends Christodoulou’s famous example on formation of naked singularity for Einstein-scalar field system under spherical symmetry. With numerical and analytic tools, we generalize Christodoulou’s result and construct an example of naked singularity formation for Einstein vacuum equation in higher dimension. The fourth result is obtained jointly with Xuefeng Zhang.
• Thursday, April 6, 14:00, Juan Valiente-Kroon (Queen Mary): Non-peeling spacetimes
Abstract: In this talk I will give an overview of Friedrich’s construction of a regular asymptotic initial value problem at spatial infinity and the open questions related to it. In particular, I will show how this framework can be used to identify initial data sets for the vacuum Einstein field equations which should lead to spacetimes not satisfying the peeling behaviour. This is research in collaboration with Edgar Gasperin.
• Thursday March 30, 14:00, Sari Ghanem (Potsdam): The decay of SU(2) Yang-Mills fields on the Schwarzschild black hole with spherically symmetric small energy initial data
Abstract: First, I will present the Yang-Mills equations on arbitrary fixed curved space-times, valued in the Lie algebra associated to any arbitrary Lie group. Thereafter, I will expose recent results with Dietrich Häfner concerning the Yang-Mills fields valued in the Lie algebra su(2) associated to the Lie group SU(2), propagating on the Schwarzschild black hole. We assume that the initial data are spherically symmetric, satisfying a certain Ansatz and have small energy, which excludes the stationary solutions which do not decay. We then prove uniform decay estimates in the entire exterior region of the black hole, including the event horizon, for gauge invariant norms on the Yang-Mills curvature generated from such initial data, including the $L^\infty$ norm of the so-called middle components. This is done by proving in this setting, a Morawetz type estimate that is stronger than the one assumed in previous work, without passing through the scalar wave equation on the Yang-Mills curvature, using the Yang-Mills equations directly.
• Wednesday, March 29, lunch seminar, 13:00, Elik Olami (Jerusalem): Continuum dynamics on manifolds: Applications to Elasticity of Residually-Stressed Bodies
Abstract: In this talk we discuss the dynamics of continua on differentiable manifolds. We present a covariant derivation of equations of motion, viewing motion as a curve in the infinite-dimensional Banach manifold of embeddings of a body manifold in a space manifold. Our main application is the motion of residually-stressed elastic bodies; residual stress results from a geometric incompatibility between body and space manifolds. We then study a particular example of elastic vibrations of a two- dimensional curved annulus embedded in a sphere. Based on a joint work with Raz Kupferman and Reuven Segev.
• Thursday, March 23, 14:00, Andrzej Rostworowski (Cracow): Higher order perturbations of Anti-de Sitter space and time-periodic solutions of vacuum Einstein equations
Abstract: Motivated by the problem of stability of Anti-de Sitter (AdS) spacetime, I will discuss nonlinear gravitational perturbations of maximally symmetric solutions of vacuum Einstein equations in general and the case of AdS in particular. I will present the evidence that, similarly to the self-gravitating scalar field at spherical symmetry, the negative cosmological constant allows for the existence of globally regular, asymptotically AdS, time-periodic solutions of vacuum Einstein equations that bifurcate from linear eigenfrequencies of AdS. Interestingly, preliminary results indicate that the number of time-periodic solutions bifurcating from a given eigenfrequency equals the multiplicity of this eigenfrequency. The talk will be based on the recent preprint https://arxiv.org/abs/1701.07804
• Thursday, March 9, 14:00, Tim Paetz (Vienna): On the smoothness of the cylinder at spatial infinity in vacuum spacetimes
Abstract: It is well-known that spatial infinity cannot be represented as a regular point due to blow-ups of the Weyl tensor whenever the ADM mass is non-zero. Because of this, the construction of vacuum spacetimes which admit a smooth past and future null infinity turns out to be a rather intricate problem. An approach which avoids these blow-ups is a cylinder representation of spatial infinity. However, for generic initial data the solutions will pick up log-terms at the critical sets where the cylinder "touches" null infinity. The goal of this talk is to set up an asymptotic initial value problem with data at past null infinity and to derive necessary conditions for the smoothness of these critical sets.
• Thursday March 2, 14:00, Guillaume Idelon-Riton (Grenoble): On the scattering theory for the Dirac equation in the Schwarzschild-Anti-de Sitter space-time
Abstract: I will first describe the Schwarzschild-Anti-de Sitter spacetime and the geometrical properties that makes it interesting to look at when studying hyperbolic equations. I will then present the Dirac equation in this spacetime and investigate quickly the Cauchy problem. The solution is then analyzed from the point of view of scattering theory. First, I will look at this solution in the asymptotic region of the spacetime and give a result about the asymptotic completeness and the asymptotic velocity. Then, I will look at local properties of these fields for large time and give a lower bound on the local energy decay using the construction of exponentially accurate quasimodes. I will then present some tools to obtain an upper bound will then be such as the resonances and the WKB solutions that should allow to localize these resonances.
• Wednesday March 1, lunch seminar, 13:00, Wolfgang Wieland (Perimeter): New boundary variables for classical and quantum gravity
Abstract: In my talk, I will present a new representation of loop quantum gravity with spinors as the fundamental configuration variables. I will show, in particular, that the discrete loop quantum gravity spin degrees of freedom (on a spin-network) can be related to classical surface degrees of freedom of the gravitational field on a null surface. The approach is based on the covariant Hamiltonian formulation for a manifold with (inner) null boundaries. The underlying action consists of the the self-dual action in the bulk plus an additional boundary term. The boundary term is required, because otherwise the action is not functionally differentiable. On the null boundary, the most natural such boundary term can be written in terms of spinors. The resulting canonically conjugate variables on the null surface are a spinor and a spinor-valued two-surface density. The quantisation of both the constraints (reality conditions) and the boundary symplectic structure reproduces the loop quantum gravity Hilbert space in the spinorial representation. The talk is based on the papers [arXiv:1611.02784, arXiv:1604.07428, arXiv:1107.5002].
• Thursday, Februar 2, 13:00, Philipp Höhn (Vienna), lunch seminar: Can chaos be observed in quantum gravity?
Abstract: Full general relativity is almost certainly 'chaotic'. I will argue that this entails a notion of nonintegrability: a generic general relativistic model, at least when coupled to cosmologically interesting matter, is likely to possesses neither differentiable Dirac observables nor a reduced phase space. The standard notion of observable then has to be extended to include non-differentiable observables. This has severe repercussions as such observables cannot carry Poisson-algebraic structures and do not admit a standard quantization; one thus faces a quantum representation problem of gravitational observables. Nevertheless, in certain cases, one can explicitly quantize such systems. By means of toy models, I will discuss general challenges and some surprising consequences for the quantum theory of nonintegrable constrained systems which presumably will also appear in canonical quantum gravity. Based on arXiv:1602.03237, 1508.01947.
• Wednesday, February 1, 13:00, Robert Beig (Vienna), lunch seminar: Shielding linearised gravity
Abstract: I will present an elementary argument that one can shield linearised gravitational fields using linearised gravitational fields. This is done by using third-order potentials for the metric, which avoids the need to solve singular equations in shielding or gluing constructions for the linearised metric.
• Thursday, January 26, VERA Seminarraum, 14:00, Iacopo Carusotto (INO-CNR BEC Center, Trento): Analog models of Hawking and Casimir physics in atomic and optical systems
Abstract: In this talk I will review the state of the art in the theoretical and experimental study of analog models of quantum field theories in flat, curved, or time-dependent backgrounds using condensed matter and optical systems. In the first part, I will focus on the theory of the stimulated and spontaneous Hawking emission of phonons in flowing fluids of ultracold atoms and of photons in semiconductor microcavities and I will outline the state of the art of experimental investigations. In the second part, I will introduce analogs of two-level emitters coupled to the quantum field and I will present recent works on the observable consequences of Casimir physics and of Ginzburg radiation processes for moving emitters. I will conclude with an outline of more speculative investigations in the direction of highlighting the back-reaction effect of Hawking emission onto a black hole horizon.
• Monday, January 23, 13:00, Jerzy Knopik (Cracow), lunch seminar: Initial data for the Einstein equations with positive cosmological constant
Abstract: In this talk I report on work in progress on Bowen-York type initial data with positive cosmological constant.
• Thursday, January 19, joint theory seminar, VERA Seminarraum, 14:00, Frank Verstraete (Vienna): Many body physics using tensor networks
Abstract: We will describe how the theory of entanglement provides a novel language for describing quantum many body systems. We will demonstrate how the ensuing quantum tensor networks allow for the classification of topological quantum phases of matter.
• Wednesday, January 18, 13:00, Rita Teixeira da Costa (Cambridge), lunch seminar: Mode stability for Kerr black holes
Abstract: The non-linear stability problem for Kerr black holes is still very much open. In my talk, I explain the conjecture, lay out the strategy to prove it and focus on the base step: mode stability for the Kerr black hole.
• Monday, January 16, 13:00, Mieszko Rutkowski (Cracow), lunch seminar: Lower bounds of collisional energy near Kerr black holes
Abstract: The fact that collisional energy of two particles near Kerr black holes can be arbitrary high, has been broadly discussed in the literature in recent few years. However, it has also been noticed that this phenomenon is not significant for distant observers. During my talk I will firstly discuss these two issues, and then move to the analysis of collisions on the innermost stable circular orbit. I will focus on the lower bounds of energy of such collisons in extreme-Kerr limit.
• Thursday, January 12, 14:00, Jérémie Joudioux (Vienna): Hidden symmetries and decay for the Vlasov equation on the Kerr spacetime
Abstract: I will review in this talk the commutator theory for the transport equation on curved spacetimes, and suggest, as an application, the derivation of an integrated energy decay for massless Vlasov fields on Kerr black holes. This work is a direct application of the work by Andersson and Blue (Ann. Math. 15) for the wave equation, combined with the commutator theory for the transport equation developed by Fajman, Joudioux and Smulevici. This is an ongoing work in collaboration with Pieter Blue.

# 2016

• Thursday, December 15, 14:00, Helmut Rumpf (Vienna): Lesser-known facts about Hawking radiation

Abstract: The first fact I will discuss is that Hawking radiation is primarily not an effect of black-hole physics, or even General Relativity, but of a more general character that points towards a simple solution of the so-called black hole information paradox. The second fact concerns the difference between ordinary thermal and Hawking radiation.

• Thursday, December 1 , 14:00, joint relativity - mathematical physics seminar: Jan Rosseel (Vienna): Non-relativistic supergravity

Abstract: Newton-Cartan geometry is a geometric, covariant description of non-relativistic gravity, akin to General Relativity. Recently, it has seen a renewed interest in the context of condensed matter physics and applications of holography to condensed matter systems. In this talk, I will briefly describe the motivation for this renewed interest. I will then outline how Newton-Cartan gravity can be conveniently described as a gauging of a suitable extension of the Galilei algebra of non-relativistic space-time symmetries. Finally, I will show how this gauging procedure can be applied to yield extensions of Newton-Cartan geometry that implement conformal symmetry and supersymmetry.

• Thursday, November 24, 14:00, Daniel Grumiller (Vienna): Soft Heisenberg hair

Abstract: The notion of "soft hair" refers to zero energy excitations in the near horizon region of black holes or cosmologies, advocated by Hawking, Perry and Strominger. I review recent results on soft hair in three spacetime dimensions. In particular, I focus on the near horizon symmetry algebra, which turns out to be surprisingly simple, namely infinite copies of the Heisenberg algebra. The results are universal (in a sense that I shall make precise) and could generalize to higher dimensions. Talk based on arXiv papers 1603.04824, 1607.00009, 1607.05360.

• Thursday, November 17, 14:00, Stefan Fredenhagen (Vienna): Challenges in higher spins

Abstract: Higher-spin gauge theories provide interesting, highly symmetric extensions of gravity. The only known interacting higher-spin gauge theories are the so-called Vasiliev theories. I will give an introduction to these theories and the unfolding formalism on which they are based. I will also discuss recent results which point out that the extraction of concrete equations of motion not only poses a technical, but also a conceptual challenge.

• Monday, November 14, 13:00, lunch seminar, Luis Cortes Barbado (TU Vienna): Hawking versus Unruh effects: What do you see when you fall into a black hole?

Abstract: Arguably, the most important milestone of Quantum Field Theory in curved spacetime is the discovery by Stephen Hawking that black holes should evaporate by emitting a Planckian spectrum of particles, the so-called Hawking radiation. With a similar derivation, Bill Unruh postulated that accelerated observers in empty space should perceive a thermal bath of particles with temperature proportional to their acceleration, the so-called Unruh effect. It seems clear that, for an observer following an arbitrary trajectory outside a black hole, these two effect must be present together. But, how do they combine to give the observer's net particle perception? In this talk we will address this question, within a restricted but conceptually clear framework, by using the so-called effective-temperature function. Far from just getting a set of concrete quantitative results for different trajectories of the observer, we will obtain general results which are clearly interpretable in terms of well-known physical phenomena. Furthermore, these results will let us address some interesting questions: Which part of the radiation perceived can be assigned to Hawking radiation and which to the Unruh effect? Can these two effects interfere destructively? Does always the Unruh temperature scale with the proper acceleration of the observer? Is it strictly necessary to form a horizon in order to have Hawking radiation emitted? Can Hawking radiation make a test particle to float nearby a black hole due to radiation pressure?

• Thursday, November 10, 14:00, Caslav Brukner (Vienna): Decoherence due to time dilation

Abstract: TBA

• Thursday, November 3, 14:00, Piotr T. Chruściel (Vienna): Anti-gravity a la Carlotto-Schoen

Abstract: After an introduction to gluing constructions for initial data in theories with constraints, I will describe the Carlotto-Schoen gluing construction, which allows to screen away gravitation using the gravitational field.

• Thursday, October 27, 14:00, Christopher Hilweg (Vienna): Testing the quantum and gravity interface with single photons

Abstract: Quantum theory and general relativity are considered the two pillars of modern physics. Their predictions are verified with spectacular precision on scales covering several orders of magnitude. Despite their success in describing nature, a unique framework reconciling these two theories is still missing. In this talk we will present a modified version of a Mach-Zehnder interferometer, capable of realizing the first table-top experiments probing jointly the quantum superposition principle and the mass-energy equivalence principle for single photons. The novel gravitational effects to be tested in this project arise when a single photon is travelling in a superposition along two paths located at different heights above earth and which are then brought to interfere. Due to the Shapiro delay, the travel time of a photon depends on the altitude of its path above earth. For the time dilation comparable with the photon's coherence time, the visibility of the quantum interference is predicted to drop, while for shorter time dilations gravity will induce a relative phase, shifting the interference pattern. As required by quantum complementarity principle, there is a trade-off between the possibility to observe interference and the amount of information about the photon's path, in our proposed experiment available from the arrival time of the photon.

• Thursday, October 20 , 14:00, Bernd Schmidt (AEI Golm): Transition conditions for isolated self-gravitating bodies

Abstract: For freely floating self-gravitating bodies the boundary conditions on physical grounds are: the vanishing of the normal stress at the boundary for all times. We expect that these conditions together with initial data determine a unique solution of the evolution equations. However, if the density of the matter at the surface of the body is positive, further "transition conditions" are needed to imply sufficient differentiability of the solution inside and outside the body. I will discuss the origin of these conditions first for a simple model problem and then for self-gravitating bodies in Newton's and Einstein's theory of gravity.

• Thursday, October 13, 14:00, joint theory seminar), Markus Arndt (Vienna): Quantum Optics with Molecules and Nanoparticles: Opportunities and open debates around gravity physics and cosmology.

Abstract: I will review the state of the art in atom and macromolecule interferometry to stimulate discussions on quantum physics, gravity and cosmology. A large part of the talk will be dedicated to open questions the correct answers to which I do not know at all: Do wave functions collapse ‘objectively’ when objects become massive and delocalized over large periods of time? How would this influence the temperature of the universe? Why does mass do if nobody watches? How could the universe not watch at all? How will the gravitational warp of space-time modify the linearity of Schrödinger’s wave mechanics for very massive and highly delocalized clusters? Is there any chance of observing fluctuations of space time in matter-wave interferometry? Can we use nanoparticle matter-waves for gravitational wave detection? What do we learn about the weak equivalence principle and possible modifications of the standard model when we compare the matter-wave fringe shift of macromolecules and single atoms in free fall? Which quantum particle is best suited for probing Non-Newtonian gravity at short distances? Can matter-wave interferometry serve as a detector for dark matter at low energy? What is needed for serious experimental tests?

• Thursday, October 6, 14:00, Ferdinand Horvath (Vienna): Mathur's inequality and the black hole information paradox

Abstract: Ever since Stephen Hawking discovered that black holes emit radiation, the physics community has been trying to accommodate the effects of this phenomenon. One of its consequences is the so-called information paradox. This paradox arises once a black hole evaporates through the emission of Hawking radiation, when those parts of the radiation that left the black hole can't be described as entangled with the hole anymore. While the theory assumes a pure initial state and hence full information about the particles in the hole and those emitted, information is lost once the hole is gone. This implies a loss of unitarity. Several ways to avoid this prospect are conceivable but few of them seem favourable. One such resort is the supposition that Hawking radiation has been treated too superficially since higher order corrections of its state are usually neglected. Their contribution could destroy the particles' entanglement, thus resolving the entire paradox. This work investigates Samir Mathur's research, who tried to disprove this proposal. Mathur shows that as long as these corrections to the Hawking state are assumed to be small, they cannot affect the first order entropy in a decisive way. Mathur's assumptions are examined in greater detail and his results are revised to conform to Hawking's results. We refine the entropy inequalities he proposed and attempt to directly compute the entanglement entropy of the Hawking radiation.

• Thursday, August 18, lunch seminar, 13:00, Paul Klinger (Vienna): Cosmologies with spikes

Abstract: Numerical studies of inhomogeneous singularities have provided strong evidence for the BKL picture of generically spacelike and oscillatory singularities. However the "local" part of the conjecture (which claims that the dynamics is asymptotically given by a spatially homogeneous model at each point) seems to break down at isolated points, where so-called spikes form, i.e. spatial derivatives become non-negligible, at least intermittently. This behavior can also be seen in explicit symmetric models, which have been proposed as the building blocks for the fully inhomogeneous case. I will introduce the dynamical systems formulation of the BKL conjecture introduced by Uggla et al. and the role played by the explicit spike solutions of Lim. These aim to give a complete description of the dynamics close to spacelike singularities, including the formation and resolution of spikes.

• Wednesday, August 10, 14:00, Edgar Gasperin (Queen Mary, University of London): Zero rest-mass fields and the Newman-Penrose constants on flat space

Abstract: Zero rest-mass fields (the electromagnetic field and the linearised gravitational field) prop- agating on flat space and their corresponding Newman-Penrose constants are studied near spatial infinity. The aim of the analysis made in this article is to clarify the correspondence between data for the field on a spacelike hypersurface and their corresponding Newman- Penrose constants at future and past null infinity. To do so, the framework of the cylinder at spatial infinity is employed to show that, expanding the initial data as in terms spherical har- monics and powers of the geodesic spatial distance ρ to spatial infinity, the Newman-Penrose constants correspond to the data for the highest possible spherical harmonic at fixed order in ρ. As a by product of this analysis, it is shown that the electromagnetic constants at future and past null infinity are related as they correspond to the same portion of initial data. Moreover, it is shown that, this is true for generic data (not necessarily time-symmetric) and the mechanism responsible for this identification, encoded in the evolution and constraint equations, is discussed.

• Thursday, June 23, 14:00, Helmut Friedrich (MPI Golm): Smooth conformal Einstein-lambda-dust flows across time-like infinity

Abstract: We consider the Einstein-dust equations with positive cosmological constant $\lambda$ onmanifolds with time slices diffeomorphic to an orientable, compact 3-manifold $S$. It is shown that the set of standard Cauchy data for the Einstein-$\lambda$-dust equations on $S$ contains an open (in terms of suitable Sobolev norms) subset of data which develop into solutions that admit at future time-like infinity a space-like conformal boundary ${\cal J}^+$ that is $C^{\infty}$ if the data are of class $C^{\infty}$ and of correspondingly lower smoothness otherwise. The class of solutions considered here comprises non-linear perturbations of FLRW solutions as very special cases. It can conveniently be characterized in terms of asymptotic end data induced on ${\cal J}^+$. These data must only satisfy a linear differential equation. If the energy density is everywhere positive they can be constructed without solving differential equations at all.

• Thursday, June 23, 12:30, Joint Theory Seminar, Seminarraum A, Markus Aspelmeyer (Vienna): Quantum Tests of Gravity

Abstract: This is an overview talk on the topic. It starts with the early pioneering experiments by Pound and Rebka and by Colella, Overhauser and Werner that demonstrate the effect of the gravitational potential on the frequency of a photon and on quantum interference fringes in a neutron interferometer, respectively.
The latter represents the first experiment that required the use of both Planck’s constant and Newton’s constant (via earth’s acceleration g) to describe the observed interference fringes. Over the following decades, modern quantum physics added new tools and allowed to significantly expand the available quantum experiments that test the effects of weak gravitational fields, including atomic fountains (pioneered by Kasevich and Chu), lab-¬‐based atomic clock tests of the gravitational red shift or the demonstration of gravitationally bound states of cold neutrons. The last few years have seen a renewed interest and a significant increase of experiments (and experimental proposals) to explore the interface between quantum physics and gravity. On the one hand, quantum optics and cold atom experiments have been pushing the sensitivity of measurements of space and time to unprecedented regimes: squeezed states of light have been shown to increase the sensitivity of interferometric gravitational wave detectors, atomic clocks have reached a precision to detect mm-¬‐scale displacements in earth's gravitational field, and atomic fountain experiments can measure Newton’s constant with a precision comparable to the best known values to date (100ppm). Other proposed applications of cold quantum gases and atomic clocks include the measurement of gravitational waves and demonstrations of quantum field theory in curved space-¬‐time. On the other hand, the fast progress in macroscopic quantum experiments may soon allow
to study large quantum superposition states involving clocks or increasingly massive objects. The latter could open a completely new regime of experiments in which the source mass character of the quantum system starts to play a role. This is reminiscent of Feynman’s proposal at the 1957 Chapel Hill Conference on the generation of entanglement through gravitational interaction.

• Tuesday, June 21, 13:00, lunch seminar , Robert Wald (Chicago): Linear memory effect in flat and Friedman-Lemaitre-Robertson-Walker spacetime

Abstract: The "memory effect" is the permanent change in the relative separation of test particles resulting from the passage of gravitational radiation. I will discuss the memory effect for a general, spatially flat FLRW cosmology by considering the radiation associated with emission events involving particle-like sources. Talk based on joint work with Alexander Tolish https://arxiv.org/abs/1606.04894.

• Tuesday, June 14, 13:00 (Lunch Seminar), Juan Margalef (Madrid): Parametrized theories, making EM even "gaugier"

Abstract: Parametrized field theories provide interesting examples of relatively simple diff-invariant systems, which can be then used as good toy models to understand some subtle features of General Relativity. In this talk, relying on the space of embeddings, I will explain some interesting aspects of the parametrized electromagnetic field, as it is one of the simplest models with gauge symmetries. In particular I will focus on how its primary constraint submanifold can be divided into sectors where different Hamiltonian dynamics take place, and show how the Gauss law comes into play (spoiler alert, it is not a constraint).

• Thursday, June 9, 14:00, Herbert Balasin (TU Wien): Impulsive Yang-Mills scattering

Abstract: We discuss the behavior of (classical) and quantum matter in impulsive pp-Yang-Mills fields employing nonlinear generalized functions.

• Wednesday, June 8, 12:00, lunch seminar, Lorenzo Mazzieri (Trento): Monotonicity formulas for static metrics.

Abstract: In this talk we illustrate a method that can be employed to describe qualitative properties of solutions to relevant geometric PDE's. In particular, we present some applications to the study of static metrics in general relativity. In this context, our method produces monotonicity formulas, from which sharp geometric inequalities can be deduced, whose equality case characterizes the model solutions.The results are obtained in collaboration with V. Agostiniani and S. Borghini.

• Thursday, June 2, 14:00, Arkadij Bojko (Uni Wien): Lovelock Theory

Abstract: The Lagrange densities for metrics give Euler-Lagrange expressions which transform as tensor densities and are symmetric and divergence-free. This, together with requiring the tensor density to depend on only up to the second derivative of the metric, allowed Lovelock to find a general dimension-dependent form of such tensorial quantities. I will go through the derivation of his results in this matter. At the end I will also show that for any such divergence-free, symmetric tensor density there exists an associated L-degenerate Lagrange density.

• Thursday, 19.05, 14:00, Antonin Coutant (U Nottingham): Scattering theory in dispersive wave equations with a background flow

Abstract: I will discuss several aspects of scattering theory in linear dispersive wave equations where the time derivative part is modified by a non-homogeneous background flow. The original motivation of this work is the analogy discovered by Unruh between sound propagating in a moving fluid and radiation around a black hole. In such setups, dispersive effect allow for new wave solutions with negative energy. I will describe how these solutions can be produced by linear conversion, their link with the Hawking effect, and several types of instabilities they give rise to.

• Thursday, 17.05, 14:00, SR A, Jiří Bičák (Institute of Theoretical Physics, Charles University, Prague): Palette of gravitoma(ch)gnetic effects (Special Seminar)

Abstract: I shall start with some remarks on Ernst Mach (+1916), who spent many years at the University in Prague and at the Vienna University. I briefly recall his and Einstein's ideas on the origin of inertia and their influence on the construction of general relativity. I mention the direct experiment verifying relativistic dragging/gravitomagnetic effects - the Gravity Probe B; the results were summarized only recently. I shall then turn to several specific general-relativistic problems illustrating the gravitomagnetic effects: the dragging of particles and fields around a rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, linear dragging in a static situation, and the way how Mach's principle can be formulated in cosmology. A more detailed discussion will be devoted to the dragging effects by rotating gravitational waves.

• Thursday, 12.05, 14:00, Michael Pürrer (MPI Golm): Estimating source parameters of GW150914: The role of waveform models and numerical relativity simulations

Abstract: On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational-wave (GW) transient (GW150914). We characterise the properties of the source and its parameters with Bayesian parameter estimation algorithms using waveform models that describe GWs emitted from binary black holes in general relativity. In addition, we compare these models against a set of numerical relativity (NR) waveforms in the vicinity of GW150914. Simplifications are used in the construction of some waveform models, such as restriction to spins aligned with the orbital angular momentum, no inclusion of higher harmonics in the GW radiation, no modeling of eccentricity and the use of effective parameters to describe spin precession. In contrast, NR waveforms provide us with a high fidelity representation of the "true" waveform modulo small numerical errors. We discuss where in the parameter space the above modeling assumptions lead to noticeable biases in recovered parameters.

• Thursday, 28.04, 14:00, Jacques Smulevici (U Paris-Sud): Vector field methods for relativistic transport equations and applications

Abstract: I will present recent results obtained in collaboration with D. Fajman and J. Joudioux concerning the study of relativistic kinetic equations via techniques inspired by the traditional vector field method of Klainerman. In the second part of my talk, I will give some applications to systems of relativistic transport equations coupled to wave equations, such as the Vlasov-Nordström system.

• Thursday, 21.04, 14:00, Yann Brenier (CNRS, CMLS, Ecole Polytechnique, Palaiseau, France): Large deviations, optimal transport and Newtonian gravitation

Abstract: The Vlasov-Monge-Ampere model, based on optimal transport ideas, is an approximate model for classical (Newtonian) gravitation, closely related to the Zeldovich model in Cosmology. A derivation will be proposed, based on a double application of large deviation principles, from the very elementary stochastic model of a Brownian point cloud without interactions.

• Thursday, 14.04, 14:00, Volker Branding (TU Vienna): On the nodal set of solutions to spinorial equations on closed surfaces

Abstract: It is well known that the nodal set of solutions to semi-elliptic Dirac equations on closed Riemannian surfaces is discrete. We will derive an estimate on the nodal set of eigenspinors of the classical Dirac operator, twistor spinors, solutions to a nonlinear Dirac equation and eigenspinors of twisted Dirac operators that arise in quantum field theory. Moreover, we will point out geometric applications of our results.

• Thursday, 17.03, 14:00, Vince Moncrief (Yale University): Euclidean-Signature Semi-Classical Methods for Quantum Cosmology

Abstract: We show how certain microlocal analysis methods, already well-developed for the study of conventional Schrödinger eigenvalue problems, can be extended to apply to the (mini-superspace) Wheeler-DeWitt equation for the quantized Bianchi IX (or ‘Mixmaster’) cosmological model. We use the methods to construct smooth, globally defined asymptotic expansions, for both ‘ground’ and ‘excited state’ wave functions, on the Mixmaster mini-superspace. A crucial step in this extension involves handling the fact that, for spatially closed universe models, all of the relevant eigenvalues to the Wheeler-DeWitt operator must vanish identically-̶̶̶̶-̶ a sharp contrast to the situation normally arising for Schrödinger operators. We then briefly review an expansive, ongoing program to further extend the scope of such microlocal methods to encompass a class of interacting, bosonic quantum field theories and conclude with a discussion of the feasibility of applying this ‘Euclidean-signature semi-classical’ quantization program to the Einstein equations themselves ̶̶ ̶ in the general, non-symmetric case ̶ ̶ by exploiting certain established geometric results such as the positive action theorem.

• Thursday, 10.03, 14:00, Ellery Ames (Chalmers, Gothenburg): A Numerical Study of Axisymmetric Stationary Solutions to the Einstein-Vlasov System

Abstract: The Einstein-Vlasov system describes a large collection of collissionless particles interacting via the mean gravitational field, where gravity is modeled by general relativity. Here we present numerical solutions of these equations which are far-from spherically symmetric in the sense that the particle distributions take flattened and toroidal shapes, and the solutions have non-zero net angular momentum. In addition, certain families of solutions are found to contain ergoregions. This talk will include a discussion of the properties of the solutions obtained as well as the numerical methods.

• Thursday, 03.03, 14:00, Timon Gutleb (Uni Wien): Gravitating waveguides

Abstract: The commonly introduced description of electromagnetism in curved spacetime is concise and elegant but not particularly useful when describing materials of spatially dependent permittivity $\epsilon$. Thus, discussions of waveguides are typically limited to classical electromagnetism. In this talk, I will work towards a description of electromagnetism in curved spacetime that can be useful to discuss planar waveguides in a weak gravitational field while highlighting the difficulties in notation and convention that arise in the interdisciplinary context.

• Tuesday, January 26, 2016, 13:00, lunch seminar, Helmuth Urbantke (Wien): Hopfions

Abstract: I will discuss a family of topologically non-trivial linearized gravitational field configurations based on the Robinson congruence.

• Monday, January 25, 2016, 12:30, lunch seminar, Bobby Beig (Wien): A generalization of the concept of mass due to Hermann and Humbert.

Abstract: Hermann and Humbert define a concept of mass associated with a class of 2nd order partial differential operators, which can be viewed as a generalized ADM mass and for which they prove a number of interesting properties.

• Friday, January 22, 2016, 13:00, lunch seminar, Birgit Schörkhuber (Wien): Stable blowup for wave equations with focusing nonlinearity

Abstract: In this talk I will present recent results on blowup for wave equations with focusing power nonlinearities in odd space dimensions d \geq 3. It will be shown that in all criticality regimes open sets of radial initial data can be constructed such that the corresponding solution blows up in finite time and converges to the ODE blowup solution locally around the origin.

• Thursday, January 21, 2016, 14:00, joint theory seminar VERA Seminarraum, Jörg Schmiedmayer (TU Wien): Probing non equilibrium quantum fields with cold atoms

Abstract: Ultra cold quantum gases are an ideal system to probe many body physics and quantum fields. In this talk I will give an overview of the different possibilities and what we were able to learn about many body systems and their underlying quantum description.

• Wednesday, January 20, 2016, 13:30, VERA Seminarraum, joint theory seminar, Stefan Fredenhagen (Golm & Berlin): On higher-spin gauge theories

Abstract: I first give an introduction to higher-spin gauge theories. I will discuss the free theory, and the difficulties that arise when one tries to introduce interactions and how they can be overcome. Finally I discuss asymptotic symmetries of higher-spin theories on AdS_3 and their role in the higher-spin AdS/CFT correspondence.

• Thursday, January 14, 2016, 14:00, Sebastiano Bernuzzi (Parma): Modeling gravitational waves from neutron star mergers

Abstract: Neutron stars mergers are among the strongest sources of gravitational waves and among the main targets for ground-based gravitational-wave interferometers Advanced LIGO and Virgo. The observation of these events in the gravitational-wave window can provide us with unique information on neutron stars' masses, radii, and spins, including the possibility to set the strongest constraints on the unknown equation-of-state of matter at supranuclear densities. However, a crucial and necessary step for gravitational-wave observations is the precise knowledge of the dynamics of the sources and of the emitted waveforms. I will talk about recent developments in the modeling of gravitational waves from neutron star mergers using numerical simulations in general relativity.

# 2015

• Thursday, December 17, 2015, 14:00, joint Relativity-Global Analysis Seminar, Martin Taylor (Cambridge, UK): Global Nonlinear Stability of Minkowski Space for the Massless Einstein--Vlasov System

Abstract: Massless collisionless matter is described in general relativity by the massless Einstein–Vlasov system. I will present a proof that for smooth asymptotically flat Cauchy data for this system which is sufficiently close, in a suitable sense, to the trivial solution, Minkowski space, the resulting maximal development exists globally in time and asymptotically decays appropriately. By appealing to the corresponding result for the vacuum Einstein equations, a monumental result first obtained by Christodoulou–Klainerman in the early ’90s, theproof reduces to a semi-global problem. A key step is to estimate certain Jacobi fields on the mass shell, a submanifold of the tangent bundle of the spacetime endowed with the Sasaki metric.

• Thursday, December 10, 2015, 14:00, David Fajman (Vienna): On topology and mass in 2+1 gravity

Abstract: The Einstein flow with vanishing cosmological constant is known to be sensitive to the spatial topology of the spacetime. It is generally believed that initial data with positive curvature has a maximal development which is geodesically incomplete in both time directions, while the development of (certain) initial data with negative spatial curvature has one expanding, complete direction. Except for a few results which concern symmetric solutions or a neighborhood of explicit solutions, this behavior is not rigorously understood. Considering the case of 2+1-dimensional gravity this problem is more accessible, since the classification of closed surfaces without boundary restricts the possible topologies and leaves essentially three cases to study: the sphere, the torus and hyperbolic surfaces. In the talk we present a construction of expanding future complete solutions for all topologies, which - for some cases - require a non-vanishing energy-momentum tensor. This certainly contradicts the initially conjectured behavior. Moreover, the construction requires a certain asymptotic behavior of the energy density, which - as we show - is realized by matter models describing massive particles such as the Einstein-Vlasov system, but fails for massless matter models. Therefore, in 2+1-gravity, future completeness - independent of the spatial topology - is an effect caused by the mass of the individual particles. We discuss a proof for the nonlinear stability of those solutions for the cases of non-negative curvature, which implies that this is a robust phenomenon.

• Friday, December 4, 2015, 13:00, Michal Eckstein and Tomasz Miller (Warsaw): Causality for nonlocal phenomena

Abstract: The postulate of causality is among the most fundamental principles of physics. In relativity theory it is straightforward to implement, as the Lorentzian metric induces a partial order relation between the events. On the other hand, the study of causality for quantum objects --- which are inherently non-local --- is still incomplete. Basing on a recent article (arXiv:1510.06386), we will present a rigorous notion of causality for nonlocal objects, modelled by probability measures on a given spacetime. The work is embedded in the optimal transport theory and explores the borderland between mathematical relativity and measure theory. We will argue that the proposed definition captures an intuitive notion of causality for spread objects and show how various results on causality in quantum theory, aggregated around Hegerfeldt’s theorem, fit into our framework.

• Thursday, December 3, 2015, 14:00, Istvan Racz (Budapest): The many faces of the constraints in general relativity

Abstract: In this talk the constraint equations for smooth spaces satisfying Einstein's equations will be considered. It is shown that, regardless whether the primary space is Riemannian or Lorentzian, the constraints can always be put into the form of an evolutionary system comprised either by a first order symmetric hyperbolic system and a parabolic equation or, alternatively, by a symmetrizable hyperbolic system subsided by an algebraic relation. The (local) existence and uniqueness of solutions to these evolutionary systems is also shown verifying thereby that the proposed evolutionary approach provides a viable alternative to the apparently unique conformal method.

• Thursday, November 19, 2015, 14:00, Marcus Ansorg (Jena): High-accuracy methods for black-hole perturbations: quasi-normal-modes filtering

Abstract: In this talk I will present a spectral decomposition of solutions to relativistic wave equations on a given Schwarzschild-black-hole background. To this end, the wave equation is Laplace-transformed which leads to a spatial differential equation with a complex parameter. This equation is treated in terms of a sophisticated Taylor series analysis. Thereby, all ingredients of the desired spectral decomposition arise explicitly, including quasi normal modes, quasi normal mode amplitudes and the jump along the branch cut. Finally, all contributions are put together to obtain via the inverse Laplace transformation the spectral decomposition in question.

• Thursday, November 12, 2015, 14:00, Juan Valiente-Kroon (London): Conformal properties of the Schwarzschild-de Sitter spacetime

Abstract: In this talk I will show how the asymptotic initial value problem for the conformal Einstein field equations, whereby one prescribes initial data on a spacelike hypersurface representing the conformal boundary, can be used to study various conformal aspects of the Schwarzschild-de Sitter spacetime. The analysis presented covers the subextremal, extremal and hyperextremal cases.

• Thursday, October 29, 2015, 14:00, Norman Gürlebeck (Bremen): A no-hair theorem for non-isolated black holes

In my talk, I prove a no-theorem for static and axially symmetric black holes surrounded by matter. More precisely, I will show that external fields do not induce multipole moments in such black holes that could be read off at infinity. The key ingredients in the proof is the source integral formalism, which will be introduced as well. It allows to define quasi-locally for each region in the spacetime its contribution to the asymptotically defined total multipole moments of that spacetime.

• Thursday, October 22, 2015, 14:00, Volker Schlue (Paris): Non-existence of time-periodic vacuum spacetimes

Abstract: In general relativity, a self-gravitating system such as a binary star is not expected to display time-periodic dynamics, due to the emission of gravitational waves. In my lecture I will present a recent result that rules out the existence of genuinely time-periodic solutions to the Einstein equations, at least in the vacuum region far away from compact sources. I will discuss the relevance of the result to the final state conjecture, and elaborate on the proof which relies on novel uniqueness theorems for a class of ill-posed problems for geometric hyperbolic p.d.e.'s.

• Thursday, October 15, 2015, 14:00, Jérémie Joudioux (Vienna): Vector fi elds method for geometric transport equations, with applications to the Vlasov-Nordström system

Abstract: As for the wave equation, the Vlasov equation admits commutators arising from the geometry. This allows standard PDE techniques, such as the vector fields method, to be applied to this geometric transport equation. In this talk, the relevant geometric structures of the Vlasov equation will be explained, and exploited to apply vector fields methods. The asymptotic behaviour of Vlasov fields, with data in some weighted Sobolev spaces, on flat space-time, can then be described using Klainerman-Sobolev inequalities. Applications to the massless and massive Vlasov-Nordström system are discussed in the last parts of the talk. In particular, a precise asymptotic behaviour for solutions of this system will be derived. This is a collaboration with D. Fajman (Vienna), and J. Smulevici (Orsay-Paris 11).

• Thursday, August 20, 2015, joint ESI - relativity group seminar, ESI seminar room, 14:00, Luc Nguyen (Oxford): Maximal (hyper)surfaces in low dimensions

Abstract: Maximal (hyper)surfaces are sometimes referred to as relativistic strings or membranes. They are objects of considerable interest in relativity and string theory. However little is known about their long-time behavior. We discuss recent progresses in this regards.

• Thursday, August 6, 2015, 14:00, joint ESI-relativity seminar, ESI seminar room, Alexander Strohmaier (Loughborough): The Dirac operator on curved spaces: Index Theorems, Spectral Theory & Gravitation

Abstract: In this introductory talk I will start by explaining some basic properties of the Dirac operator on Riemannian and Lorentzian manifolds. I will revise the Atiyah-Singer index theorem and the Atiyah-Patodi-Singer index theorem for manifolds with boundary and discuss some applications. I will then discuss a recent result about the index of the Dirac operator on a globally hyperbolic spacetime and the relation to physics.

• Monday, June 29, 2015, 13:00, lunch seminar Natascha Riahi (Vienna): Unimodular quantum cosmology

Abstract: I will discuss the steps of quantization of a simple cosmological model. Starting with the unimodular version of General Relativity the result will be an evolving wave function. There is no need for the commonly used frozen time formalism.

• Thursday, June 25, 2015, 14:00, Laszlo Szabados (Budapest): A positive Bondi--type mass in asymptotically de Sitter spacetimes

Abstract: A Bondi-type mass, associated with a cut of the conformal boundary of asymptotically de Sitter spacetimes is suggested. This is based on the integral of the Nester-Witten 2-form and the Witten-type positivity argument on a spacelike hypersurface intersecting the conformal boundary in the cut. It is shown that this integral (1.) can be finite only if the boundary value of the Witten spinor at the cut solves the 2-surface twistor equation, (2.) is positive if the matter fields satisfy the dominant energy condition on the spacelike hypersurface, and (3.) its vanishing is equivalent to the local de Sitter nature of the domain of dependence of the hypersurface. However, this integral gives a well defined notion of mass only in the presence of some extra structure. In particular, when the cut is non-contorted, the integral yields an invariant analogous to the Bondi mass, which is positive and has the rigidity.

• Monday, June 22, 2015, 13:00, lunch seminar Patryk Mach (Cracow): Relativistic Bondi-Michel accretion: global vs. homoclinic solutions

Abstract: A spherically symmetric accretion model introduced by Bondi in 1952 belongs to classical textbook models of theoretical astrophysics. Its general relativistic version is due to Michel, who considered spherically symmetric, purely radial, stationary flow of perfect fluid in the Schwarzschild spacetime. Solutions of the Bondi-Michel flow are usually parametrized by fixing asymptotic values of the density and the speed of sound at infinity; they extend smoothly from infinity up to the horizon of the black hole (and below). In contrast to that, local solutions, that cannot be extended to infinity, were recently discovered in the cosmological context. They correspond to homoclinic orbits on phase diagrams of the radial velocity vs. radius (say). More surprisingly, they also appear in the standard Bondi-Michel model for polytropic fluids with polytropic exponents larger than 5/3. In this talk I will discuss recent results on the existence of those local, homoclinic solutions.

• Thursday, June 18, 2015, 14:00, Ivette Fuentes (Vienna): Quantum systems as spacetime probes

Abstract: Hawking radiation and particle creation by an expanding Universe are paradigmatic predictions of quantum field theory in curved spacetime. Although the theory is a few decades old, it still awaits experimental demonstration. At first sight, the effects predicted by the theory are too small to be measured in the laboratory. Therefore, current experimental efforts have been directed towards siumlating Hawking radiation and studying quantum particle creation in analogue spacetimes.

In this talk, I will present a proposal to test directly effects of quantum field theory in the Earth's spacetime using quantum technologies. Under certain circumstances, real spacetime distortions (such as gravitational waves) can produce observable effects in the state of phonons of a Bose-Einstein condensate. The sensitivity of the phononic field to the underlying spacetime can also be used to measure spacetime parameters such as the Schwarzschild radius of the Earth.

• Monday, June 15, 2015, 13:00, lunch seminar Marcus Khuri (Stony Brook): A Mass-Angular Momentum-Charge Inequality for Multiple Black Holes, Size-Angular Momentum-Charge Inequalities for Bodies, and Existence of Black Holes

Abstract: In the first part of the talk we present a proof of the mass-angular momentum-charge inequality for multiple black holes (joint with Gilbert Weinstein). In the second part, new inequalities relating the size and angular momentum as well as size and charge of bodies is presented. Lastly, black hole existence results due to concentration of angular momentum and charge will be discussed.

• Wednesday, June 3, 2015, 13:00, lunch seminar Lukas Ifsits (Vienna): Are light-cones light? (Lambda<0)

Abstract: I will review the method and results of the elementary proof of positivity of the Trautman-Bondi mass of light-cones with complete generators in asymptotically Minkowskian space-times by P. T. Chruściel T.-T. Paetz and present the changes and our resulting formula for the Trautman-Bondi mass of light-cones with complete generators in asymptotically anti-de Sitter space-times.

• Monday, June 1, 2015, 13:00, lunch seminar Lars Andersson (AEI Golm): Horizons, dominant energy and positive scalar curvature

Abstract: TBA

• Friday, May 29, 2015, 13:00, lunch seminar, Lorenzo Mazzieri (Pisa): Some rigidity results for static metrics

Abstract: We present a new approach to the study of asymptotically flat static metrics in general relativity. Our method works in every dimension and it is based on a conformal splitting technique, which has been previously applied by the authors to the study of the geometric aspects of classical potential theory. The results are obtained in collaboration with V. Agostiniani.

• Thursday, May 28, 2015, 14:00, Emma Jakobsson (Stockholm): Visualizing Limits of Spacetimes

Abstract: The limit obtained when letting a free parameter of a spacetime approach a certain value is in general not unique, but depends on the choice of coordinates. This ambiguity led Geroch to formulate a definition of limits of a one-parameter family of spacetimes in 1969. We have come up with an application of Geroch’s definition, which makes it possible to see the limiting procedure in pictures. The general idea is to let the spacetime under consideration---if possible---be represented by a 1+1-dimensional surface reflecting its essential causal structure, and embed this surface in 2+1-dimensional anti-de Sitter space. With the help of a conformally compactified picture of adS3 the result is reminiscent of a Penrose diagram, with the difference that the picture will change as we vary the parameter. The examples considered here are two different limits obtained when letting the charge parameter e of a Reissner-Nordström black hole approach the mass m. The conformally compactified picture of adS3 and the embeddings of the black hole surfaces will be explained.

• Thursday, May 21, 2015, 14:00, Daniel Grumiller (TU Vienna): How general is holography?

Abstract: The holographic principle was originally motivated by the desire to reconcile black hole evaporation with unitarity and found a concrete implementation in AdS/CFT. However, the way AdS/CFT works makes it logically possible that holography might work for non-unitary theories as well. Moreover, if holography is a true aspect of Nature then it must also work for non-AdS spacetimes. It is therefore of interest to pose the question in the title. I review recent progress on these issues, with particular focus on flat space holography.

• Thursday, May 7, 2015, 14:00, Michael Eichmair (Vienna): Minimal surfaces, isoperimetry, and non-negative scalar curvature in asymptotically flat manifolds

Abstract: It is a classical observation that geodesic balls at points of positive scalar curvature contain more volume than a round ball in Euclidean space with the same surface area. In this talk, I will discuss the global effect of non-negative scalar curvature on isoperimetry in asymptotically flat manifolds.

• Thursday, April 30, 2015, 14:00, Joint Theoretical Physics Seminar. Jakob Yngvason (Vienna): Superfluidity versus Bose Einstein Condensation

Abstract: The two concepts in the title stand for two distinct quantum phenomena whose relation to one another is not obvious although they often occur together. Moreover, there is not a unique concept of superfluidity. In the talk I shall first comment on these general issues and then discuss a simple model involving a tunable random potential where some precise statements can be rigorously proved. The latter is joint work with M.Könenberg, T. Moser and R. Seiringer.

• Wednesday, April 29, 2015, 12:30, lunch seminar, Michael Hoerzinger (Vienna): Kerr-Newman-de Sitter instantons

Abstract: I will review known classes of Einstein-Maxwell instantons, and present a new class of such solutions with lens-space topology.

• Monday, April 27, 2015, 14:00, Joint Theoretical Physics Seminar, Stefan Hollands (Leipzig): Quantum field theory in deSitter spacetime

Abstract: In this talk, I review properties of the so-called "deSitter spacetime", and some properties of quantum field theories that live on this spacetime. The investigation of such theories is highly relevant to cosmology, because deSitter space is thought to describe the earliest epoch of our universe, at least to some approximation. It is also interesting from a Mathematical viewpoint, because deSitter space is a space with maximal symmetry, making possible several explicit constructions and investigations that would be out of reach in quantum field theories on more general Lorentzian manifolds.

• Friday, April 24, 2015, 12:30, lunch seminar, Tim-Torben Paetz (Vienna): On the vanishing of the Mars-Simon tensor in $\Lambda>0$-vacuum space-times

Abstract: In vacuum space-times with an isometry and with $\Lambda=0$, the Mars-Simon tensor (MST) has been introduced to provide a characterization of the Kerr-NUT-metrics. Moreover, it was used by Klainerman et al. to prove uniqueness of the Kerr black hole under certain restrictive hypotheses. Recently, Mars and Senovilla considered this tensor for arbitrary $\Lambda$, and they analyzed the family of metrics characterized by the vanishing of the MST. In this talk, we restrict attention to $\Lambda>0$-vacuum space-times which admit a smooth scri. In this setting we reconsider and extend their analysis from the point of view of an asymptotic Cauchy problem on scri. More specifically, we extract conditions on scri which characterize the vanishing of the MST. Furthermore, we provide a classification of $\Lambda>0$-vacuum space-times with vanishing MST and conformally flat scri which complements the one given by Mars and Senovilla. For this purpose we shall briefly review the asymptotic Cauchy problem in GR and discuss the additional conditions which need to be imposed on the initial data to end up with vacuum space-times with a Killing vector field.

• Thursday, April 23, 2015, 14:00, Roland Donninger (Bonn): Blowup results for nonlinear wave equations

Abstract: In the last 15 years there was spectacular progress in the rigorous analysis of finite-time blowup in nonlinear wave equations. Many of these studies were actually motivated by the desire to obtain a better understanding of singularity formation in Einstein's equations. Mainly based on personal taste, I will discuss some of the most important contributions.

• Wednesday, April 15, 2015, 12:30, lunch seminar, Albert Georg Passegger (Vienna): Algebraic Foundations of the Unruh Effect

Abstract: The Unruh effect is a fundamental phenomenon of quantum field theories in Riemannian spacetimes. In Minkowski spacetime it expresses the fact that a uniformly accelerated observer perceives the Minkowski vacuum state as a thermal equilibrium state at a certain acceleration-dependent temperature. The physical significance of this observation is still a controversial topic. In this talk an algebraic formulation of the Unruh effect (by G.L. Sewell) is discussed. I provide a brief introduction to the necessary tools from quantum statistical mechanics and local quantum physics. This serves as a preparation for a second talk about a new thermal interpretation of the Unruh effect by D. Buchholz and C. Solveen.

• Thursday, April 16, 2015, 14:00, Albert Georg Passegger (Vienna): Introduction to a Local Thermal Interpretation of the Unruh Effect

Abstract: Based on the algebraic setting of the Unruh effect discussed in the previous talk ("Algebraic Foundations of the Unruh Effect"), I present recent results by D. Buchholz and C. Solveen on a new interpretation of the thermal aspects of the Unruh effect for scalar free fields. If the notion of temperature is defined using so-called local thermal observables, the local temperature of the Minkowski vacuum is zero also for the accelerated observer. Finally, I mention some open physical questions in this approach.

• Thursday, March 26, 2015, 14:00, Jacek Jezierski (Warszawa): Hidden symmetries in General Relativity

Abstract: Conformal Yano-Killing (CYK) tensors are natural generalizations of conformal covector fields to the case of higher-rank differential forms. They are often responsible for hidden symmetries. Several spacetimes possess CYK tensors: Minkowski (the components are quadratic polynomials), (anti)de Sitter (a natural construction), Kerr (type-D spacetime), Taub-NUT (they lead to new symmetric conformal Killing tensors). CYK tensors are useful in several situations: Geometric definition of the asymptotic flat spacetime: strong asymptotic flatness which guarantees well-defined total angular momentum; Conserved quantities: asymptotic gravitational charges; Quasi-local mass and "rotational energy" for the Kerr black hole; Symmetries of the Dirac operator; Symmetries of Maxwell equations. These nice geometrical objects are well worth studying in detail.

• Tuesday, March 24, 2015, 16:15, Aneesh Manohar (UC San Diego), joint theoretical physics seminar, E.Schrödinger-HS, Boltzmanngasse 5: The Topology of the Universe from the Cosmic Microwave Background Temperature and Polarization

Abstract: The talk discusses constraints on the global topology of the universe from CMB data, in particular constraints on torus topologies T^3, T^2 x R and S^1 x R^2. The theoretical predictions are compared with experimental CMB data. See also http://particle.univie.ac.at/seminars/particle-physics/

• Thursday, March 19, 2015, 14:00, Romain Gicquaud (Tours): Solutions of the conformal constraint equations with non-constant mean curvature.

Abstract: Constructing broad classes of (physically relevant) initial data for the Cauchy problem is an important issue in general relativity. From the Gauss and Codazzi equations, the 0th order initial data (the metric induced on a Cauchy surface) and the first order initial data (the second fundamental form of the Cauchy surface) cannot be chosen arbitrarily: they have to satisfy some constraint equations. One of the main methods for studying these equations is the conformal method which was highly successful for constructing and classifying constant mean curvature (CMC) initial data. However, constructing non CMC initial data remains a widely open subject. In this talk I will describe recent results on the construction of solutions to the constraint equations with non constant mean curvature by the conformal method.

• Wednesday, March 18, 12:30, Lunch seminar, David Fajman (Vienna): The Einstein flow with a positive cosmological constant

Abstract: We give a concise proof of nonlinear stability for a large class of solutions to the Einstein equations with a positive cosmological constant and compact spatial topology, where the spatial metric is Einstein with either positive or negative Einstein constant. The proof uses the CMC Einstein flow and stability follows by an energy argument. We prove in addition that the development of non-CMC initial data close to the background contains a CMC hypersurface, which in turn implies that stability holds for arbitrary perturbations. This is joint work with Klaus Kroencke (Regensburg).

• Friday, March 13, 12:30, Lunch seminar, Annegret Burtscher (Vienna): On the formation of trapped surfaces in Einstein-Euler spacetimes

Abstract: The concept of a closed trapped surface (a spacelike surface with decreasing area in the direction of the future-directed null normals) was introduced by Penrose for the formulation of his first singularity theorem. It is not a priori clear whether such trapped surfaces are evolutionary, and hence an important question is to understand whether/how trapped surfaces can form starting from initial data that do not contain such surfaces. Christodoulou pioneered this work in the vacuum and scalar field case, results for Einstein-Vlasov spacetimes are also known. In my talk I will present first results for Einstein-Euler spacetimes in spherical symmetry, carried out in joint work with Philippe LeFloch (see also arXiv:1411.3008).

• Thursday, March 5, 2015, 14:00, Radoslav Rashkov (TU Vienna): An invitation to String(Gravity)/gauge theory dualities

Abstract: The understanding of the strong coupling phenomena at qualitative and quantitative level is a challenging task. The best way to attack this problem is at present is the duality between two (or more) theories. The purpose of this lecture is to introduce the basic contemporary concepts of string (gravity)/gauge theory duality and discuss some of their features. The main focus will be on the so-called AdS/CFT correspondence. I'll briefly discuss simple examples of the so-called "brane engineering" of some gauge theories. The "magic" appearance of W-symmetries will be also very briefly discussed.

• Thursday, January 15, 2015, 14:00, Caslav Brukner (Vienna): Quantum clocks and quantum causality

Abstract: Quantum physics differs from classical physics in that no definite values can be attributed to observables independently of the measurement context. However, the notion of time and of causal order preserves such an objective status in the theory: all events are assumed to be ordered such that every event is either in the future, in the past or space-like separated from any other event. The possible interplay between quantum mechanics and general relativity may, however, require superseding such a paradigm. I will approach this problem in two steps. Firstly, I will consider a single "clock" - a time-evolving (internal) degree of freedom of a particle - to be in a superposition of regions of space-time with different ticking rates. While the "time as shown by the clock" is not well-defined, there is still the notion of global time. Secondly, I will consider that space-time itself is in a superposition, and show that this situation gives rise to quantum correlations for which one cannot say that one event is before or after the other. Finally, I will comment on possible implications of this result for quantum computation.

• Tuesday, January 13, 2015, 16:15, Joint Theoretical Physics Seminar, E.Schrödinger-HS, Jose Espinosa (Barcelona): Stability of the Electroweak Vacuum after the first LHC run

Abstract: Details on URL http://particle.univie.ac.at/de/seminare

• Thursday, January 8, 14:00, Gerhard Hensler (Vienna): The Milky Way satellite galaxies - a serious challenge for the Cold Dark Matter cosmology

Abstract: The so-called cosmological concordance model of a Cold Dark Matter (CDM) dominated universe predicts a huge number of low-mass CDM subhalos to exist and to surround massive galaxies with an almost isotropic distribution. For our Milky Way and the neighboring Andromeda galaxy these both requirements are significantly contrasted by observations. Not only that the observed number of satellite galaxies is orders of magnitude smaller - the so-called missing-satellite problem - moreover, their spatial distributions are confined to thin planes with coherent orbits.
Nevertheless, unusually high mass-to-light ratios are derived for the dwarf spheroidal galaxies around the Milky Way, lending strong support of their large CDM content. In order to approach consistency of the observational restrictions with the CDM cosmology, over the recent years various scenarios are constructed which will be critically illuminated in this talk with respect to their verification. Conclusively, an alternative solution for the formation of dwarf galaxies in general will be discussed.

# 2014

• Wednesday, December 17, lunch seminar, 13:00, Nathalie Rieger (Pasadena): Topology of maximally extended non-Hausdorff Misner Space and the non-flat Generalization

Abstract: We turn away from the idea that the Misner spacetime should be Hausdorff as was already discussed by previous authors. In lieu thereof we allow the notion of a non-Hausdorff spacetime and construct an analytic non-Hausdorff extension of Misner space. On this basis we elucidate the global causal structure of the maximally extended Misner spacetime, with the result that there are two fundamentally different maximal extensions and associated covering spaces. From this we can conclude that there exist two versions of Misner space. Furthermore, we wish to shed some new light on the pathologies, e.g. the quasiregular singularities and CTCs. It turns out that the Misner space is related to the pseudo-Schwarzschild spacetime regarding its properties from a chronological and global point of view. According to this result the pseudo-Schwarzschild cylinder can be regarded as a non-flat generalization of the Misner space. This gives rise to a conjecture which says that 4D Misner space and pseudo-Schwarzschild spacetime are isocausal to each other. Furthermore, we create a new chronology violating spacetime that describes a generalization of the two precedent ones: We derive the pseudo-Reissner-Nordstroem spacetime from the well-known Reissner-Nordstreom spacetime and review our main results in this more general setting.

• Thursday, December 11, 14:00, Patryk Mach (Cracow): Global dynamics of a Yang-Mills field on an asymptotically hyperbolic space

Abstract: We consider a spherically symmetric (purely magnetic) SU(2) Yang-Mills field propagating on an ultrastatic spacetime with two asymptotically hyperbolic regions connected by a throat of radius α. Static solutions in this model are shown to exhibit an interesting bifurcation pattern in the parameter α. We relate this pattern to the Morse index of the static solution with maximal energy. Using a hyperboloidal approach to the initial value problem, we describe the relaxation to the ground state solution for generic initial data and unstable static solutions for initial data of codimension one, two, and three.

• Thursday, December 4, 14:00, Matthew Randall (Hannover): Generalised Ricci Solitons in 2 dimensions

Abstract: We introduce a class of overdetermined systems of partial differential equations of on (pseudo)-Riemannian manifolds that we call the generalised Ricci soliton equations. These equations depend on three real parameters. For special values of the parameters they specialise to various important classes of equations in differential geometry.
Among them there are: the Ricci soliton equations, the vacuum near-horizon geometry equations in general relativity, special cases of Einstein-Weyl equations and their projective counterparts, equations for homotheties and Killing's equation. We provide explicit examples of generalised Ricci solitons in 2 dimensions, some of them obtained using techniques developed by J.Jezierski. This is joint work with Pawel Nurowski available at arXiv:1409.4179.

• Thursday, November 27, 14:00, seminar cancelled
• Thursday, November 20, 14:00, Roland Steinbauer (Vienna): News from low regularity GR

Abstract: We report on recent progress in the study of spacetimes where the metric is C^{0,1} (locally Lipschitz continuous) or C^{1,1} (first derivatives locally Lipschitz). In particular, we focuss on existence and regularity of geodesics in the first case and discuss the prospects of proving Hawking's singularity theorem in the second case.

• Thursday, November 13, 14:00, Harold Steinacker (Vienna): Self-intersecting fuzzy extra dimensions in maximally supersymmetric gauge theory(and matrix models)

Abstract: We explain how fuzzy geometries in extra dimensions can emerge in standard Yang-Mills gauge theory, based on a geometric version of the Higgs effect. In particular, we discuss the 4-and 6-dimensional squashed coadjoint orbits which were recently found in maximally supersymmetric N=4 SYM. The resulting low-energy fluctuation modes lead to 3 generations of chiral fermions coupled to scalar and gauge fields. The discussion is focused on geometrical and group-theoretical aspects. Talk based on arXiv:1409.1440

• Thursday, November 6, 14:00, Johanna Knapp (TU Vienna): Unification of forces in String Theory

Abstract: I will introduce the basic concepts of String theory and show how the quantized string unifies gauge theory and gravity. I will further explain why String Theory requires a ten-dimensional space-time and discuss the concept of compactification of extra dimensions.

• Monday, October 27, 12:45, Helmut Rumpf (Vienna), lunch seminar: Analogue black hole laser and Hawking radiation.

Abstract: I discuss the recent claim of experimental verification of an analogue of the Hawking effect.

• Thursday, October 23, 14:00, Josef Stöckl (Innsbruck): Interaction Processes in Clusters of Galaxies

Abstract: Interaction between the components in galaxy clusters - the galaxies and the gas surrounding the galaxies, the so-called intra-cluster medium - have a variety of effects on the cluster. The gas within the galaxies is compressed and sometimes stripped off. Therefore the galaxies change their morphology and their star formation activity. The intra-cluster gas is enriched by the lost gas from the galaxies, hence it changes the metal content and the temperature. All effects are modelled by simulations on galaxy scales as well as clusters scales. Results of the evolution of various properties (metallicity, gas density, star formation rate, temperature, magnetic fields,... ) are presented.

• Thursday, October 16, 14:00, Rod Gover (Auckland): Geometric compactication and conformal geometry

Abstract: Conformal compactification is a well established tool in GR and many related fields. The model for this construction is often taken to be the Poincare ball model of hyperbolic space. There is a refinement of this idea which reveals the Lie groups and Lie group embeddings behind conformal compactification. These structures at once generalise to the curved setting through the conformal Cartan-tractor calculus (i.e. the natural conformally invariant connection and related objects). This provides a conceptual and calculationally effective way to treat many problems linked to conformal compactification.

• Thursday, October 9, 14:00, Harald Skarke (TU Vienna): The effect of inhomogeneity on the evolution of the universe

Abstract: A method for analysing the evolution of the volume of an inhomogeneous irrotational dust universe is presented. In this framework it is possible to go beyond perturbation theory in a numerical analysis. The results of such computations show that the evolution is strongly affected by inhomogeneities, but nevertheless suggest that a cosmological constant is required to account for the observed acceleration of the expansion. Possible loopholes to this conclusion will be discussed.

• Thursday, October 2, 14:00, Paul Klinger (Vienna): Non-chaotic vacuum singularities without symmetries?

Abstract: We construct a class of vacuum space-times without Killing vectors and with "asymptotically velocity dominated" singularities.

• Thursday, September 25, 13:00 (lunch seminar!), Drazen Vrzan (Vienna): Mass inequalities for axially symmetric, asymptotically flat initial data

Abstract: In 2008 Sergio Dain proved that the ADM mass of axially symmetric, AF initial data is greater or equal than the root of the angular momentum, and equality holds for extreme Kerr (only). We describe recent, stronger inequalities which also contain higher "momenta", focusing on the special case where the data are close to extreme Kerr in a suitable sense.

• Tuesday, September 23, 13:00 (lunch seminar!), Piotr T. Chruściel (Vienna): Bifurcating solutions of the constraint equations

Abstract: I will describe bifurcation phenomena in thevacuum Lichnerowicz equation with positive cosmological constant on $S^1\times S^2$ with $U(1)\times SO(3)$-invariant seed data.

• Monday, September 22, 13:00 (lunch seminar!), Bobby Beig (Vienna): Of rigid rods and strings
• Thursday, September 18, 13:00 (lunch seminar!), Stefan Pletka (Vienna): Initial data for rotating cosmologies

Abstract: I describe the construction of certain classes of axially symmetric initial data with positive cosmological constant via the conformal method.

• Monday and Tuesday September 1 and 2, 13:00 (lunch seminar!), Jérémie Joudioux (Vienna): The vector field method for the Vlasov fields

Abstract: I will explain how the well-known vector field method, which was one of the most important tool to understand the asymptotic behavior of the wave equation, can also be applied to the Vlasov fields.

• Wednesday, August 20, 13:00 (lunch seminar!), Olivier Sarbach (Morelia, Mexico and Vienna): The collisionless Boltzmann equation on a Kerr background

Abstract: I will describe ongoing work on the construction of solutions to the collisionless Boltzmann equation on a Kerr black hole background.

• Tuesday, June 17, 13:00 (! Lunch seminar !), Eliana Chaverra (Mexico and Vienna): Acoustic perturbations of radial accretion flows.

Abstract: I will briefly describe my research project on acoustic perturbations of radial accretion flows.

• Thursday, June 12, 14:00, Gregory Galloway (Miami and Vienna): On the geometry and topology of initial data sets in General Relativity II

Abstract: We continue our discussion of the geometry and topology of asymptotically flat initial data sets, including discussion of a different approach based on solutions of Jang’s equation.

• Thursday, June 5, 14:00-16:00, Erwin Schrödinger Institute (! note change of location and duration !), Grischa Karssen (Köln): The galactic center

Abstract: With a distance of about 8 kpc, the center of the Milky Way is the closest galactic nucleus to us. Hence, it provides us with a unique opportunity to study a galactic nucleus up close. Longterm observations of stellar kinematics of the Nuclear Star Cluster point to the existence of a super-massive black hole (SMBH) at the position of Sagittarius A* (SgrA*), with a mass of 4 million suns. SgrA* shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future.

A relativistic model that could explain the flaring nature of SgrA* are hotspots, overdense compact emitting regions, moving inside an accretion flow. To model compact sources orbiting in the immediate vicinity of SgrA*, it is necessary to use the metric for a rotating black hole, the Kerr-metric. There are a couple of relativistic effects on the emission that need to be taken into account, most importantly the gravitational Doppler-shift and gravitational lensing.

• Tuesday, June 3, 13:00 (! Lunch seminar !), Maximilian Thaller (Vienna): Static solutions of the Einstein-Vlasov system with positive cosmological constant

Abstract: We consider the static Einstein-Vlasov system in spherical symmetry. Existence of different types of solutions to this system for zero cosmological constant has been shown for the isotropic and anisotropic case by Rein-Rendall, Rein and Wolansky. In this talk I review the results on static solutions for the Einstein-Vlasov system and eventually describe a method to prove existence of static solutions to the Einstein-Vlasov system with positive cosmological constant. The energy density and the pressure of these solutions have compact support and outside a finite ball these solutions are identical to a Schwarzschild deSitter spacetime. The results presented in the talk are joint work with H. Andréasson and D. Fajman.

• Monday, June 2, 13:00 (! Lunch seminar !), Anna Nakonieczny (Lublin and Vienna): Dynamical gravitational collapse of scalar fields

Abstract: The dynamical gravitational collapse of a complex scalar field coupled with Maxwell field in dilaton gravity, allowing a phantom coupling to gravity, will be described.

• Thursday, May 22, 14:00, Bernd Schmidt (AEI Potsdam): Isolated self--gravitating bodies

Abstract: Known theorems and work in progress establishing the existence of solutions describing isolated bodies will be discussed. There are results for fluids as well as for elastic matter, with and without gravity in Newtonian and Einstein's theory.

• Tuesday, May 20, 13:00 (! Lunch seminar !), Tim-Torben Paetz (Vienna): Are light-cones light?

Abstract:We give an elementary proof of positivity of the Trautman-Bondi mass of light-cones with complete generators in asymptotically flat space-times.

• Monday, May 19, 13:00 (! Lunch Seminar !), Carla Cederbaum (Tübingen):The geometry of static spacetimes

Abstract: We present a geometric approach to the study of static isolated general relativistic systems for which we suggest the name geometrostatics. After describing the setup, we introduce localized formulas for the ADM-mass and ADM/CMC-center of mass of geometrostatic systems (Huisken-Yau, Metzger, Huang). We then explain the pseudo-Newtonian character of these formulas and show that they converge to Newtonian mass and center of mass in the Newtonian limit, respectively, using Ehlers' frame theory. Moreover, we present a novel physical interpretation of the level sets of the canonical lapse function and apply it to prove uniqueness results.

• Friday, May 16, 13:00 (! Lunch seminar !), Olivier Sarbach (Mexico and Vienna): Initial-boundary value problems for Einstein's field equations

Abstract: We discuss the initial-boundary value problem which arises when formulating the Cauchy problem in general relativity on a finite domain with an artificial outer boundary, like is usually the case in numerical relativity simulations. First, the restrictions on the boundary data that result from the requirement of constraint propagation and the attenuation of spurious reflections will be analyzed. Then, we will introduce the important concept of strong well-posedness and explain it first in the simple example of the wave equation on the half-plane. For systems of wave equations, strong well-posedness allows to treat a certain class of boundary conditions which is general enough to cover many evolution systems in physics, including Einstein’s equations in harmonic coordinates. Finally, open issues related to a geometric formulation of the initial-boundary value problem will be mentioned.

• Thursday, May 15, 14:00, Piotr Jaranowski (Bialystok): 4th post-Newtonian Hamiltonian dynamics of compact binary systems

Abstract: Recent results concerning derivation of the conservative equations of motion of compact binary systems up to the 4th post-Newtonian approximation of general relativity will be presented. The derivation is made within the ADM canonical formalism. It employs Dirac delta distributions to model the compact bodies what leads to divergencies which are regularized by a combination of Riesz-implemented Hadamard's partie finie approach and dimensional regularization. It also requires taking into account tail-transported nonlocal-in-time interaction between the bodies.

• Tuesday, May 13, 13:00 (! Lunch seminar !), Håkan Andréasson (Chalmers): On the rotation curves for flat steady states of the Vlasov-Poisson system

Abstract: I will discuss an ongoing project on flat steady states for the Vlasov-Poisson system, which in astrophysics are used as models of disk-like galaxies. We construct solutions numerically and study in particular the shape of the rotation curves. It is often claimed that a system obeying Newton's law of gravity should have a rotation curve which declines in a Keplerian manner far out in the galaxy. However, observations indicate that the rotation curves are approximately flat and this discrepancy is one of the reasons for introducing dark matter. In our numerical study we find a large class of solutions for which the rotation curves are flat all the way out to the boundary of the steady state. This is a joint work with Gerhard Rein.

• Monday, May 12, 13:00 (! Lunch seminar !), Gregory Galloway (Miami and Vienna): On the geometry and topology of initial data sets in General Relativity

Abstract: We discuss some results concerning the geometry and topology of asymptotically flat initial data sets in three and higher dimensions, with and without horizons. More specifically, we explore the relationship between the topology of such initial data sets and the occurrence of marginally outer trapped surfaces in the initial data. We shall discuss the rationale for this and present relevant background material. This involves work with several collaborators, L. Andersson, K. Baker, M. Dahl, M. Eichmair and D. Pollack.

• Friday, May 9, 13:00 (! Lunch Seminar !), Michael Hörzinger (Vienna):Criterions for the linearization stability of the Einstein equations

Abstract: I will give an introduction to the linearization stability problem for the Einstein equations. Furthermore I will introduce two criterions for linearization stability (established by Vincent Moncrief [1][2]) and sketch the corresponding proofs from those references.
[1] V. Moncrief, Spacetime symmetries and linearization stability of the Einstein equations. I ,
J. Math. Phys. 16, 493 (1975); dx.doi.org/10.1063/1.522572
[2] V. Moncrief, Spacetime symmetries and linearization stability of the Einstein equations. II ,
J. Math. Phys. 17, 1893 (1976); dx.doi.org/10.1063/1.522814

• Thursday, May 8, 14:00, Pawel Nurowski (Warsaw) : Rolling without slipping or twisting. A few surprises.

Abstract: I will describe a configuration space of two surfaces rolling on each other without sleeping or twisting. A relation between this space and totally null planes in 4-dimensional conformal geometry of signature (2,2) will be established and used to construct new surfaces that roll on each other without sleeping or twisting and exhibit the symmetry of the exceptional simple Lie group G2.

• Monday April 28, 13:00 (! Lunch seminar !), Lukasz Nakonieczny (Lublin and Vienna): Fermion fields in spacetimes with topological defects and black holes

Abstract: Some results on the mechanism of interactions among fermion fields and cosmic strings in curved spacetime, as well as on the influence of spinor fields on Yang-Mills black holes, will be presented.

• Thursday, April 10, 14:15, Peter Michor (Vienna): Overview on geometries of shape spaces, diffeomorphism groups, and spaces of Riemannian metrics

Abstract:
1. A short introduction to convenient calculus in infinite dimensions.
2. Manifolds of mappings (with compact source) and diffeomorphism groups as convenient manifolds
3. A diagram of actions of diffeomorphism groups
4. Riemannian geometries of spaces of immersions, diffeomorphism groups, and shape spaces, their geodesic equations with well posedness results and vanishing geodesic distance.
5. Riemannian geometries on spaces of Riemannian metrics and pulling them back to diffeomorphism groups.
6. Robust Infinite Dimensional Riemannian manifolds, and Riemannian homogeneous spaces of diffeomorphism groups.
We will discuss geodesic equations of many different metrics on these spaces and make contact to many well known equations (Cammassa-Holm, KdV, Hunter-Saxton, Euler for ideal fluids), if time permits.

• Wednesday, April 9, 11:00 (! pre-Lunch Seminar !), Oliver Rinne (AEI Golm): Hyperboloidal evolution of the Einstein equations

Abstract: I will review the status of a conformal constrained ADM-like formulation of the Einstein (+matter) equations on hypersurfaces of constant mean curvature, developed with V. Moncrief. This has been adapted and implemented numerically for several applications: a gravitationally perturbed Schwarzschild black hole in axisymmetry, late-time tails of massless scalar and Yang-Mills fields in spherical symmetry, critical phenomena in the Einstein-Yang-Mills system, and massive scalar fields / evolution of (mini) boson stars.

• Monday and Tuesday, April 7 & 8 , 13:00 (! lunch seminar !), Olivier Sarbach (San Nicolas de Hidalgo and Vienna)  : The geometry of the tangent bundle
• Thursday, April 3rd, 14:15, Hartmut Abele (Vienna): Newton's Law and Gravity Resonance Spectroscopy

Abstract: Newton’s Law of Gravity is considered valid from sub-millimetre distances up to inter-galactic space, but fails to describe important features of cosmology like the accelerating expansion component of our universe. While the most straightforward candidate for such a component is Einstein’s cosmological constant, a plausible alternative is dynamical vacuum energy, or ”quintessence”, changing over time. Although it is traditional to neglect (or set to zero) the couplings of this light scalar to the standard model, it is natural for a scalar quintessence field to evolve on cosmological time scales today while having couplings to matter, as expected from string theory. Hence the presence of such a field would provide energy changes to Newton’s gravity potential of the earth at short distances invisible to electromagnetic interactions.

We present a novel direct search strategy with neutrons based on Rabi spectroscopy of quantum transitions in the gravity potential of the earth. The sensitivity for deviations on Newton’s gravity law is right now E = 10-15 eV, providing a severe restriction on quintessence fields and on any possible new interactions on that level of accuracy. If some undiscovered dark matter or dark energy particles interact with a neutron, this should result in a measurable energy shift of the observed quantum states. In the case of some dark energy scenarios with a coupling to matter, the experiment has the potential to find or exclude these hypothetical particles in full parameter space.

• Friday, March 28, 12:00 (! Lunch Seminar !), Helmut Rumpf (Univ. Vienna): Inflation, gravitational waves, and BICEP2

Abstract: I discuss the recently announced discovery of a B-mode signal in the cosmic microwave background and its significance for cosmology.

• Thursday, March 27, 14:15, Juan Valiente Kroon (London): Propagation of massless fields on the Einstein Cosmos

Abstract: In this talk I will make use of a representation of the Einstein Cosmos based on the properties of conformal geodesics to discuss the global evolution in time of massless spin-2 fields. In view of the conformal properties of the massless spin-2 equation, the constructed solutions can be reinterpreted as global solutions in the anti de Sitter space-time. I will discuss how this analysis can be generalized to the case of the conformal field equations.

• Friday, March 21, 12:30 (! Lunch Seminar !), Jacques Smulevici (Orsay): Future Dynamics of T2 symmetric polarized space times

Abstract. Joint Work with Philippe G. LeFloch. We consider vacuum spacetimes with two spatial Killing vectors and with initial data prescribed on T^3. The main results that we will present concern the future asymptotic behaviour of the so-called polarized solutions. Under a smallness assumption, we derive a full set of asymptotics for these solutions. Within this symetry class, the Einstein equations reduce to a system of wave equations coupled to a system of ordinary differential equations. The main difficulty, not present in previous study of similar systems, is that, even in the limit of large times, the two systems do not directly decouple. We overcome this problem by the introduction of a new system of ordinary differential equations, whose unknown are renormalized variables with renormalization depending on the solution of the non-linear wave equations.

• Thursday, March 13, 14:15, Michael Reisenberger (Universidad de Montevideo): Null canonical gravity
• Thursday, January 30, 14:15, R. Beig (Vienna): On local wellposedness of ideal relativistic, and more general fluids

Abstract: We exhibit a class of theories, with the relativistic fluid a special case, which naturally take the form of a symmetric hyperbolic system. The 'reason' for this is that they possess a convex extension, with the role of convex entropy being played by the particle number density. This is joint work with Philippe LeFloch.

• Thursday, January 23, 14:15, Antonin Coutant (AEI Potsdam): Birth of laser instability in black hole geometries

Abstract: In this talk, I discuss a peculiar black hole instability that arises in the presence of short distance dispersion. Its origin is to be found in the spectral properties of the wave equation on a background geometry containing two horizons. I will start by qualitatively describing this effect. In a second part, I will show that the presence of complex eigen-frequencies in the spectrum encodes this instability. Such eigen-frequencies are allowed only because the conserved scalar product is non positive definite. I will then compute the spectrum through a WKB approximation. In a last part, I will present an abstract toy model to discuss general feature of the appearance complex eigen-frequencies. This model is directly inspired from the Friedrich model'' of resonances. This will allow to make contact with quasi-normal modes of black holes and other known black hole instabilities.

• Thursday, January 9, 14:15, Joanna Jalmuzna (Univ. Cracow): Globally regular instability of AdS_3

Abstract: I will describe results of my joint work with Piotr Bizoń on instability of three-dimensional asymptotically AdS spacetime coupled to a massless scalar field. As in higher dimensions, for a large class of perturbations we observe a turbulent cascade of energy to high frequencies, However, in contrast to higher dimensions, small perturbations cannot evolve into a black hole, because their energy is below the threshold for a black hole formation. To determine the long-time evolution we use the analyticity strip method, well known in fluid dynamics, which provides a powerful numerical tool.

# 2013

• Thursday, December 12, 14:15, Clemens Sämann (Uni Wien): On geodesics in impulsive gravitational waves

Abstract: We review recent and older work on impulsive gravitational waves. These space-times have become textbook examples modelling short but intense gravitational wave impulses. Mathematically they have been described by a distributional - the so-called Brinkmann - metric as well as by a continuous metric - referred to as Rosen form. Our main focus will be on geodesics in these geometries. First we will discuss the behaviour and regularity of geodesics in the distributional form and the notion of geodesic completeness in an even wider class of impulsive wave-type spacetimes. Then we will turn to the Rosen form, and examine the regularity of geodesics in the various subclasses of impulsive wave spacetimes.

• Thursday, December 5, 14:15, Abraham Harte (AEI Potsdam): Relativistic motion and self-interaction

Abstract: This talk will explore issues related to the motion of extended bodies in curved spacetimes. Non-perturbative notions of linear and angular momentum will be introduced and some of their properties discussed. Most important among these properties is that forces and torques are “almost” preserved by a certain class of deformations which may be applied to the relevant field (electromagnetic, gravitational, or otherwise). Here, the “almost” refers to terms which can be interpreted purely as finite shifts in an object’s apparent multipole moments. The freedom to choose different fields can be used to dramatically simplify problems where self-interaction affects the motion. Usual results on the self-force emerge as a simple special case of this formalism. In another special case, full multipole expansions for the forces and torques acting on extended test bodies are recovered as well.

• Thursday, November 28, 14:15, Herbert Balasin (TU Wien): Classical and Quantum Impulsive Scattering

Abstract: The behaviour of particles, both from a classical as well as a quantum mechanical perspective, with respect to impulsive background fields is investigated. Due to the singular nature of the problem, which requires the definition of products of distributional objects, a generalized framework like Colombeau's new generalized functions has to be used.

• Thursday, November 14, 14:15, Jeremie Joudioux (Uni Wien): Hertz potentials and the asymptotic behavior of higher spin fields

Abstract: The study of the asymptotic behavior of the Maxwell and gravitational fields is a key point in the understanding of the stability properties of solutions of the Einstein equations. Penrose introduced in the beginning of the 60s a method based on the construction of Hertz potentials satisfying a wave equation to determine the asymptotic behavior of massless free fields of arbitrary spin from a decay Ansatz on solutions of the scalar wave equation. The purpose of this talk is to adapt this idea in the context of a Cauchy problem: consider a Cauchy problem for the Maxwell and gravitational fields on the Minkowski space-time with initial data in weighted Sobolev spaces;  in the framework of this Cauchy problem, the existence of a Hertz potential is proved; finally, from a standard decay result for the scalar wave equation, the asymptotic behavior of these higher spin fields is derived. The classical decay results for Maxwell and gravitational fields are recovered.

• Thursday, November 7, 14:15, Nishanth Gudapati (AEI Potsdam): Critical Self-Gravitating Wave Maps

Abstract: Wave maps are maps from a Lorentzian manifold to a Riemannian manifold which are critical points of a Lagrangian which is a natural geometrical generalization of the free wave Lagrangian. Self-gravitating wave maps are those from an asymptotically flat Lorentzian manifold which evolves according to Einstein's equations of general relativity with the wave map itself as the source. The energy of wave maps is scale invariant if the domain manifold is 2+1 dimensional, hence it is referred to as the critical dimension.

Apart from a purely mathematical interest, the motivation to study critical self-gravitating wave maps is that they occur naturally in 3+1 Einstein's equations of general relativity. Therefore, studying critical self-gravitating wave maps could be a fruitful way of understanding the ever elusive global behavior of Einstein's equations. A few central questions concerning the study of critical self-gravitating wave maps are local and global existence, blow up profile, compactness and bubbling.

In this talk, after a brief discussion on the background and formulation of the Cauchy problem of critical self-gravitating wave maps, we shall present a recent proof of the non-concentration of energy of critical equivariant self-gravitating wave maps before pointing out potential generalizations and applicable methods therein.

• Thursday, October 31, 14:15, Jochen Zahn (Uni Wien): Locally covariant field theory

Abstract: I give an introduction to quantum field theory on curved spacetimes in the framework of locally covariant field theories,   introduced by Brunetti, Fredenhagen, Verch and Hollands, Wald. The main motivation and example will be the covariant definition of the stress-energy tensor of a scalar quantum field.

• Thursday, October 24, 14:15, Albert Huber (TU Wien): Null Foliations and ultrarelativistic Geometry

Abstract: The present work concerns the construction of a lightlike foliation of spacetime which suites the Kerr-Schild framework describing the gravitational field of a massless particle located on the horizon. Despite of being defined only on local grounds, the gained results do not only prove to be consistent to former works of Hayward and Brady, Israel, Droz and Morsink, but fit also former results of Moncrief and Isenberg and, in addition, that of Friedrich, Racz and Wald concerning Gaussian null coordinates. Two simple examples for the construction, describing the situation for a Schwarzschild black hole in Kruskal-Szekeres as well as in Kerr-Schild coordinates, are given. Finally it is explained how the obtained foliation might be used in order to extend the gravitational field of a massless particle off the horizon.

• Thursday, October 10, 14:15, Walter Simon (Uni Wien): Marginally outer trapped surfaces in 5 dimensions

Abstract: I review the basic setup of Kaluza-Klein theory, namely a 5-dim. vacuum with a cyclic isometry (a U(1) fibre bundle over 4-dim. spacetime) which corresponds to Einstein-Maxwell-dilaton theory. I show that the property of compact surfaces of being (stably) marginally trapped is preserved under lift and projection provided the appropriate ("Pauli-") conformal scaling is used for the spacetime metric. I also discuss recently proven area inequalities for stable axially symmetric 2-dimensional and 3-dimensional marginally outer trapped surfaces. This talk is based on joint work with Tim-Torben Paetz, arxiv.org/abs/1302.3052

• Thursday, June 27, 14:00, Werner Zeilinger (Vienna): The Dark Universe

Abstract: More than 95% of the matter in the Universe is invisible. An overview of our current understanding of abundance and properties of dark energy and dark matter is presented. The first part focusses on issues pertaining to dark matter including observational evidence for its existence and current constraints. MOND is briefly mentioned. The second part focusses on dark energy. Observational strategies to detect and quantify dark energy are reviewed. In particular, recent results from the Planck mission are presented and an overview of the new ESA dark energy mission Euclid is given.

• Wednesday, June 26, 15:00, tea seminar (common room, Währinger Straße 17, 1st floor), Walter Simon (Vienna): Area inequalities for stable marginally outer trapped surfaces in Einstein-Maxwell-dilaton theory

Abstract: I describe ongoing joint work with D. Fajman on this topic. Our inspiration comes from the work arxiv.org/abs/1109.5602 on the pure Einstein-Maxwell case, and from the known strange exact solutions in Einstein-Maxwell-dilaton theory.

• June xx, 14:00, Hartmut Abele (TU Wien): Gravity Resonance Spectroscopy: Neutrons test Newton

Abstract: Newton's Inverse Square Law has been examined in detail from the sub-millimetre scale up to inter-galactic distances. His gravity prediction for these systems is considered valid, but fails to describe important features of cosmology like the accelerating expansion of our universe. While the most straightforward candidate is Einstein's cosmological constant , a plausible alternative is dynamical vacuum energy, or "quintessence", changing over time. Although it is traditional to neglect the couplings of this light scalar to the standard model, some scenarios allow scalar quintessence field to evolve on cosmological time scales today while having couplings to matter, as expected from string theory . Hence the presence of such a field would provide energy changes to Newton's gravity potential of the earth at short distances invisible to electromagnetic interactions. We present a novel direct search strategy with neutrons based on Rabi-spectroscopy of quantum transitions |1> ↔|2>, |1> ↔|3>, |2> ↔ |4>, |2> ↔|3>, and |2> ↔ |4>$in the gravity potential of the earth. The sensitivity for deviations on Newton's gravity law is right now E = 10-14 eV, providing a severe restriction on quintessence fields and on any possible new interactions on that level of accuracy. • Friday, June 21, 13:00, lunch seminar (common room, Währinger Straße 17, 1st floor), Tim Paetz (Vienna): KIDs like cones Abstract: I will give an exhaustive description of Killing Initial Data on light-cones, and on transversally intersecting characteristic hypersurfaces, in vacuum space-times. • Thursday, June 20, 14:00, Olaf Müller (Regensburg): Time functions, conformal factors and Cauchy surfaces of bounded geometry Abstract: The talk first resumes some recent progress towards the goal to find time functions for a given globally hyperbolic metric for which basic geometric quantities are bounded. Then we conversely fix a time function and ask whether there is a conformal factor such that the corresponding Cauchy surfaces are of bounded geometry which provides us with Sobolev embeddings and denseness results for spaces of initial values. This is done by using a recently developed method called flatzooming which has proven to be powerful in different contexts of Riemannian and Lorentzian geometry. • Thursday, June 13, 14:00, Andy Cap (Vienna): Projective compactifications Abstract: Via the geodesics of the Levi-Civita connection, a pseudo-Riemannian metric on a smooth manifold M determins a projective structure on M. Similarly to the role of the conformal geometry, this projective structure can be used to identify particularly robust properties of pseudo-Riemannian manifolds. Reporting on joint work with A.R. Gover (Auckland) my talk will be devoted to the projective analog of the notion of a conformally compact Riemannian metric. This exhibits a notion of compactification for Ricci flat metrics and non-Ricci-flat Einstein metrics which are similar to - but different from - the ususal notion of conformal compactifications. • Wednesday, May 29, 17:00, tea seminar (common room, Währinger Straße 17, 1st floor), Marcelo Rubio (Córdoba, Argentina): Charge - size inequality for real objects Abstract: Geometric inequalities have been of interest in General Relativity in recent years. From them, it is possible to relate physical quantities that have a precise geometric meaning--like mass, area, charge and angular momentum--, and thus be able to predict significant consequences on the evolution and stability of some physical systems. In this talk, I present a conjecture relating the electrical charge to the size of a real object, inspired on the hoop conjecture valid for black holes. First I discuss briefly some relevant aspects of the hoop conjecture and then I state the analogous conjecture for real objects in general. Physical motivation of the inequality is discussed, as well as define with precision what we understand about the "size” of a three dimensional object. As a first approach, I study the spherical problem with ECD wherein this conjecture is precisely formulated and I show that it is true outside and in the bound of the sphere. • Thursday, May 23, 14:00, Jose-Luis Jaramillo (AEI Golm): Applications of apparent horizon stability in dynamical black hole spacetimes Abstract: Light bending, characteristic of geometric descriptions of gravity as spacetime curvature, manisfests dramatically in the existence of black hole spacetimes. Global notions associated with the causal disconnection between spacetime regions, on the one hand, and (quasi-)local concepts related to the convergence of light rays, on the other hand, provide complementary tools for the study of black holes. Here we focus on the latter aspects, namely relying on the notion of trapped surface. More specifically, we discuss the role of the limiting case provided by marginally (outer) trapped surfaces (MOTS) as probes into the geometry of dynamical black holes, placing a special emphasis in their notion of stability. We illustrate the discussion with two examples, the first one dealing with a family of geometric inequalities providing a lower bound for the horizon area, and the second one motivating the role of MOTS as inner "test screens" in a heuristic proposal for a "scattering-like approach" to the a posteriori analysis of dynamical black hole spacetimes. • Wednesday, May 22, 15:00, Jerzy Kowalski-Glikman (Wrocław): Quantum gravity and the fate of Lorentz symmetry Abstract: In my talk I will discuss two possible nontrivial scenarios concerning the fate of Lorentz symmetry in the low energy limit of quantum gravity: Lorentz Invariance Violation (LIV) and Lorentz/Poincare symmetry deformation. I will also briefly present some of the experimental bounds on the parameters of the models pertaining to these scenarios • Thursday, May 16, 14:00, Maciej Maliborski (Cracow): Time-periodic solutions in Einstein AdS - massless scalar field system Abstract: There is a strong evidence that anti-de Sitter space is unstable due to small generic perturbations. It is also believed that there might exist solutions that do not lead to the formation of a black hole. I will discuss recent analytical and numerical results concerning time-periodic solutions for Einstein-massless-scalar field system with negative cosmological constant, in particular how to construct such stable configurations. If time permits I will outline the pure vacuum case. The talk will be an extension of joint work with Andrzej Rostworowski presented in the paper arxiv:1303.3186. • Monday, May 13, 13:00, lunch seminar (common room, Währinger Straße 17, 1st floor), Natascha Riahi (Vienna): Quantum cosmology with time Abstract: I will present an alternative to the Dirac quantization of minisuperspaces that admits a time evolution. • Monday, May 6, 13:00, lunch seminar (common room, Währinger Straße 17, 1st floor), Hedda Gressel (Vienna): On a Cosmological Model with Torsion Abstract: Summarising my diploma thesis I will start with introducing the work of Hubert Bray being the paper my thesis is built up upon. The paper deals with a possible explanation of the existence of dark matter by introducing a torsion of space-time. Its basic idea is to derive an extension of General Relativity involving a more general connection from particular axioms for the metric and the connection. According to these axioms the gravitational action functional can only take a specific form. The variation of this action functional leads to Einstein-Klein- Gordon equations. The mass term in the Klein-Gordon equation corresponds to the coupling constant for the torsion. The terms involving the scalar field and its gradient appearing in the Einstein field equations can be interpreted as the effective energy-stress tensor and can be attributed to dark matter. The solution of the Klein-Gordon equation in a spherically symmetric space-time is an oscillating function both in time and space. From the effective energy-stress tensor appearing in the Einstein field equations we derive a Newtonian potential displaying a slowly rotating maximum, which resembles a spoke. In the paper the author performs simulations using this Newtonian potential and obtains results resembling a spiral galaxy. The aim of my diploma thesis is to investigate the measurable effects of the torsion field by analysing the behaviour of a particle with spin-1/2 in the torsion field. The polarization vector of a particle in a torsion field is subject to a torque and hence precesses. To compute the precession two different approaches were chosen: the first one is the supersymmetric approach that enables one to consistently couple a classical spinning particle to the torsion field. The second approach is a quantum mechanical one solving the Dirac equation minimally coupled to the torsion field. The conclusion of my thesis is that the precession of the polarization vector induced by the torsion field results in an oscillatory motion with the deflection of order of magnitude 10^-6 rad. The sense of rotation of the precession changes every half period of the time oscillation of the torsion field. • Friday, May 3, 13:00, lunch seminar (common room, Währinger Straße 17, 1st floor), Herbert Balasin (TU Wien): pp-waves and the Mathisson-Papapetrou equation • Thursday, May 2, 14:00, Jerzy Kijowski (Polish Academy of Sciences, Warsaw): Theories of gravitation based on Lagrangians which are non-linear in curvature. Abstract: A broad class of theories based on non-linear Lagrangians will be discusssed and their equivalence/nonequivalence with Einstein theory (possibly with additional matter fields) will be analyzed. To simplify technical aspects of such theories, a nonstandard theory of curvature will be used. • Thursday, April 25, 14:00, Gernot Heissel (Vienna): Spatially homogenous cosmology and dynamical systems Abstract: Following the parts of my thesis, I will first give a brief introduction to the field of spatially homogenous (SH) cosmology with an emphasis on the use of dynamical systems methods to analyse the evolution of these cosmologies qualitatively. After this, I will summarise the results of the central part of my thesis, which deals with the dynamical system analysis of a special class of SH cosmologies (locally rotationally symmetric Bianchi type VIII). The matter content is thereby chosen out of a very general family which allows for anisotropic pressures, and contains physically relevant models like perfect fluids, elastic matter or collisionless matter. The goal was to investigate how the grade of anisotropy of the matter influences the qualitative dynamics, which was achieved via a comparison with the well known results with perfect fluids. It is shown that there are indeed cases where the qualitative dynamics can differ significantly in both, the past and future asymptotics. If time is left I would like to close my talk with a little eye candy, by presenting a Maple document, which allows to plot the solutions to each matter configuration as a flow diagram by a single click on the matter-parameter space. • Thursday, April& 18, 14:00, Udo Hertrich-Jeromin (TU Wien): Conformally flat hypersurfaces Abstract: We shall discuss conformally flat hypersurfaces in the realm of Moebius geometry. Particular attention will be paid to the transformation theory and integrable nature of this class of hypersurfaces. • Thursday, April 11, 13:00. lunch seminar (common room, Währinger Straße 17, 1st floor), Piotr T. Chruściel: The Hamiltonian mass of asymptotically Schwarzschild-de Sitter space-times Abstract: I will present a Hamiltonian approach to the definition of mass for a class of asymptotically cylindrical initial data sets. This is based on joint work in progress with Jezierski and Kijowski. • [cancelled, new time and date will be communicated later] Pawel Nurowski (Warszawa): Twistors and rolling bodies Abstract: We will consider a configuration space of two solids rolling on each other without slipping or twisting, and will identify it with an open subset U of R5. It turns out that U is naturally equipped with a generic distribution D of 2-planes. We will discuss symmetry properties of the pair (U,D) and will mention that, in the case of the two solids being balls, when changing the ratio of their radii the dimension of the group of local symmetries unexpectedly jumps from 6 to 14 . This occurs for only one such ratio, and in such case the local group of symmetries of the pair (U,D) is maximal. It is maximal not only among the balls with various radii, but more generally among all (U,D)s corresponding to configuration spaces of two solids rolling on each other without slipping or twisting. This maximal group is isomorphic to the split real form of the exceptional Lie group G2. In the remaining part of the talk we will argue how to identify the space U defined above with the bundle T of totally null real 2-planes over a 4-manifold equipped with a split signature metric. We call T the twistor bundle for rolling bodies. We show that the rolling distribution D, can be naturally identified with an apropriately defined twistor distribution on T. We use this formulation of the rolling system to find more surfaces which, when rigidly rolling on each other without slipping or twisting, have the local group of symmetries isomorphic to the exceptional group G2 • Wednesday, March 20, 15:00, Markus Aspelmeyer (Vienna): Quantum Optomechanics: a route for table-top experiments at the interface between quantum physics and gravity? Abstract: Quantum optics provides a high-precision toolbox to enter and to control the quantum regime of the motion of massive mechanical objects. This opens the door to a hitherto untested parameter regime of macroscopic quantum physics. Due to the large available mass range - from picograms in nanomechanical waveguides to kilograms in mirrors for gravitational wave detection - it becomes possible to explore the fascinating interface between quantum physics and (quantum) gravity in table-top quantum optics experiments. I will discuss a few examples. • Monday, March 18, 13:00, Jozef Skakala (UFABC, Santo André); lunch seminar, Common Room, Währinger Straße 17, 1st floor: Asymptotic quasi-normal modes and some conjectured quantum properties of spacetime Abstract: The perturbations of black hole spacetimes, when decaying, show characteristic (damped) oscillations called quasi-normal modes. The asymptotically highly damped modes are widely suspected to carry information about certain black hole quantum properties in the semi-classical limit. We analyse the behavior of asymptotic quasi-normal frequencies of static black hole spacetimes and interpret the meaning of the results, linking them to possible quantum properties of spacetime. We analyse our suggestions in the broader context of spacetime thermodynamics and discuss some open questions. • Friday, March 15, 13:00, Juan Valiente-Kroon (Queen Mary College, London); lunch seminar, Common Room, Währinger Straße 17, 1st floor: Conformal properties of the extremal Reissner-Nodrström spacetime Abstract: I will discuss some conformal properties of the extremal Reissner-Nordström spacetime ---in particular in what concerns the behaviour of the spacetime close timelike infinity. I will show how Friedrich's construction of the "cylinder at spatial infinity" can be used, together with a conformal discrete symmetry of the spacetime, to show that there exists a conformal representation of timelike infinity in this spacetime for which the various conformal field quantities and equations regular. I will also discuss some numerical evidence of this conformal representation. • Friday, March 8, 13:00, Klaus Kröncke (Potsdam); lunch seminar, Common Room, Währinger Straße 17, 1st floor: Linear Stability of Einstein metrics and Ricci flow Abstract: A compact Einstein metric is called Linearly stable if the second variation of the Einstein-Hilbert functional is nonpositive on TT-tensors. We will discuss curvature conditions which ensure stability. Then we will show that under certain conditions on the spectrum of the Laplacian, linear stability implies that the given Einstein manifold is an attractor of the Ricci flow. • Wednesday, March 6, 15:00, Harold Steinacker (Vienna): On the geometry of Yang-Mills Matrix models, and aspects of emergent gravity Abstract: An introduction is given to some recent developments in Yang-Mills matrix models, focusing on the effective geometry of brane solutions and their possible relevance to gravity in a brane-world picture. • Wednesday, January 23, 14:15, Volker Branding (TU Wien):Dirac-harmonic maps and Evolution Equations Abstract: Dirac-harmonic maps are critical points of an energy functional that is motivated from supersymmetric field theories. The critical points couple the equation for harmonic maps with spinor fields. At present, a general existence result for Dirac-harmonic maps is not available. In the first part of the talk we will introduce the notion of Dirac-harmonic maps and explain their basic properties. We will also summarize what is currently known about the existence of Dirac-harmonic maps. In the second part of the talk we present an approach to the existence question by the so-called heat flow method and explain how far this idea can be pushed. • Wednesday, January 16, 14:15, Daniel Grumiller (TU Wien): Introduction to 3-dimensional higher spin gravity Abstract: I provide an introduction to 3-dimensional higher spin gravity, review some of the recent developments with particular emphasis on holography and point out some of the puzzling open questions, especially those concerning a geometric interpretation of the field configurations. • Wednesday, January 9, 14:15, Michał Kahl (Kraków): Yang-Mills field on extreme Reissner-Nordstrom background Abstract: I consider a spherically symmetric SU(2) Yang-Mills field on the exterior of extreme Reissner-Nordstrom black hole. The problem is equivalent to a Yang-Mills field propagating on a regular asymptotically flat spacetime. Infinitely many non-trivial static solutions are shown to exist. I analyze linear perturbations of the solutions and find their spectrum (unstable modes and quasinormal modes). Then I show the dynamics of the field and the approach to a static solution. # 2012 • Wednesday, December 19, 14:00, Vincent Moncrief (Yale), joint seminar with the ESI workshop on dynamics on general relativity, ESI seminar room (note the unusual time and place!): Constrainted propagation of spherically symmetric Einstein-Yang-Mills fields out to Scri Abstract: Oliver Rinne (AEI) and I developed, a few years ago, a fully constrained method for integrating the vacuum Einstein field equations out to Scri. Oliver subsequently implemented this proposal numerically for the case of axially symmetric metrics and showed that it gave stable evolutions, reproducing in particular (in a fully nonlinear code) the well-known quasi-normal ringing modes characteristic of black holes. In this talk I will describe some very recent work with Oliver in which we have extended the theoretical developments to include conformally invariant matter sources, including Yang-Mills fields and implemented these numerically in the case of spherical symmetry. The extra resolution available in this case permits us not only to recover the ringing but also the (Price law) tails in the various radiation fields. • Wednesday, December 12, 14:00, Sergio Dain (Cordoba), joint seminar with the ESI workshop on dynamics on general relativity, ESI seminar room (note the unusual time and place!): The wave equation on the extreme Reissner-Nordstroem black hole Abstract: We study the scalar wave equation on the open exterior region of an extreme Reissner-Nordstr\"om black hole and prove that, given compactly supported data on a Cauchy surface orthogonal to the timelike Killing vector field, the solution, together with its$(t,s,\theta,\phi)$derivatives of arbitrary order,$s\$ a tortoise radial coordinate, is bounded by a constant that depends only on the initial data. Our technique does not allow to study transverse derivatives at the horizon, which is outside the coordinate patch that we use.  However, using previous results that show that second and higher transverse derivatives at the horizon of a generic solution grow unbounded along horizon generators, we show that any such a divergence, if  present, would be milder for solutions with compact initial data.

This talk is based on http://lanl.arxiv.org/abs/1209.0213, and it is joint work with G. Dotti.

• Monday, December 10, 13:00, lunch seminar, Ettore Minguzzi (Firenze): An introduction to topological preordered spaces

Abstract: In many cases the mathematical structures which we use in applications (computer science, dynamical systems, general relativity) present both a topology and an order. There is a beautiful but little known topological theory which unifies these concepts into that of 'quasi-uniformity'. In practice one has simply to drop an axiom in topology to find that an order naturally arises. Most of topology can be still developed, leading to concepts such as normally preordered spaces or completely regularly preordered spaces. I wish to introduce and comment on this generalization of topology which allows us to prove, among the other results, the existence of time functions in stably causal spacetimes.

• Wednesday, December 5, 14:15, Willie Wong (EPFL Lausanne): Towards nonexistence of electrovacuum stationary black holes with multiple components

Abstract: In recent years there were renewed interest in extending the black hole uniqueness theorems to space-times which are neither real-analytic nor axially-symmetric. Thus far the results obtained have either been conditional on an additional rigidity assumption of the black hole event horizon, or on an additional smallness assumption of the space-time being suitably "close" to being Kerr(-Newman). I will describe a result of the latter class: that a weighted point-wise control of local space-time geometry yields topological constraints on the domain of outer communications. This provides a rigorous formulation for the intuitively obvious fact that "if on every patch the space-time looks similar to a Kerr-Newman solution, it cannot contain more than one black hole".

• Wednesday, November 28, 16:00, David Fajman (AEI Golm), joint seminar with the Pauli Institute GPRS Workshop, Seminarraum C715, Wolfgang Pauli Institut, Nordberstrasse 15/7/C 1090 Wien (note the unusual time and  location!)Non-linear stability of the Einstein-Vlasov system in 2+1 dimensions

Abstract: Solutions to the Einstein-Vlasov system describe spacetimes with collisionless matter. The nonlinear stability problem for the Einstein-Vlasov system with symmetries has been considered in a series of works starting with Rein and Rendall in 1992. Recently, the first result for the Einstein-Vlasov system without symmetry assumptions has been established by Ringström, considering a positive cosmological constant. In the talk, we present the proof of future nonlinear stability of the Einstein-Vlasov system in 2+1 dimensions without symmetry assumptions and no cosmological constant. Due to the slow expansion and low spatial dimension in that situation, it is essential to prove strong decay properties of the energy momentum tensor. We obtain these decay rates, by introducing geometric Vlasov energies using a specific metric on the tangent bundle of spacelike hypersurfaces - the Sasaki metric. We present energy estimates for those energies and their application in the proof of nonlinear stability. Finally, we give an outlook to applications and related work in progress on the corresponding higher dimensional problem.

• Wednesday, November 21, 14:00, Volker Perlick (Bremen), joint seminar with the Pauli Institute GPRS Workshop, Seminarraum C715, Wolfgang Pauli Institut, Nordberstrasse 15/7/C 1090 Wien (note the unusual time and  location!) Photon accumulation near a black hole

Abstract: In the first part of the talk a Schwarzschild black hole is considered. We assume that light sources are distributed on a (big) sphere of radius R that emit, at an instant of time, photons isotropically. We calculate the resulting photon distribution and find that in the long-time limit the density becomes infinitely large near the photon sphere at r=3m. This suggests that every Schwarzschild black hole in  nature should be surrounded by a shell of very high photon density which could be detrimental to the health of any observer who comes close to this region. In the second part we discuss how the situation changes if a Kerr black hole is considered. -- The first part is based on the Bachelor Thesis of Dennis Philipp and the second part is ongoing work with Arne Grenzebach.

• Wednesday, November 14, 14:15, Patryk Mach (Kraków): Geometry of Keplerian disk systems and bounds on masses of their components

Abstract: We investigate accreting disk systems with polytropic gas in Keplerian motion. Numerical data and partial analytic results show that the self-gravitation of the disk speeds up its rotation -- its rotational frequency is larger than that given by the well known strictly Keplerian formula that takes into account the central mass only. Thus determination of central mass in systems with massive disks requires great care -- the strictly Keplerian formula yields only an upper bound. The effect of self-gravity depends on geometric aspects of disk configurations. Disk systems with a small (circa 10^{-4}) ratio of the innermost radius to the outermost disk radius have the central mass close to the upper limit, but if this ratio is of the order of unity then the central mass can be smaller by many orders of magnitude from this bound.

• Wednesday, November 7, 14:15, Helmut Rumpf (Vienna): Atoms in free fall and at rest

Abstract: I discuss the classical motion of electromagnetically bound systems in an external gravitational field and associated quantum effects.

• Wednesday, October 24, 14:15, Ettore Minguzzi (Pisa): Lightlike lines and time functions in general relativity

Abstract: "The causal ladder of spacetimes is introduced and the role of stable causality is commented. Some details are given of the recent solution to the problem of the equivalence between stable causality and K-causality. In particular this result is used to show that under reasonable conditions the absence of a cosmological time implies the null geodesic singularity of spacetime."

• Wednesday, October 17, 14:15, Christa Ölz (Vienna): On Kerr-type metrics with cosmological constant.

Abstract: "I will present a class of diagrams, that we call projection diagrams, as a tool to visualise the global structure of space-times, and show how they can be used for the Kerr-Carter family of metrics with cosmological constant. A seemingly new class of overspinning such solutions with negative cosmological constant and unusual global properties will be presented."

• Friday, October 05, 13:00, Tim Paetz (Vienna), common room, Währinger Straße 17, 1st floor, lunch seminar: Conformal versions of the Einstein equations.

Abstract: "I will discuss old and new well posed sets of conformally covariant versions of the vacuum Einstein equations."

• Wednesday, October 03, 14:15, Michael Reiterer (ETH Zurich): Choptuik's critical spacetime exists.

Abstract: "About twenty years ago, Choptuik studied numerically the gravitational collapse (Einstein field equations) of a massless scalar field in spherical symmetry, and found strong evidence for a universal, self-similar solution at the threshold of black hole formation. We give a rigorous, computer assisted proof of the existence of Choptuik's spacetime, and show that it is real analytic. This is joint work with E. Trubowitz."

• Thursday, July 05, 14:15, Moritz Reintjes (Regensburg). Points of General Relativistic Shock Wave Interaction are "Regularity Singularities" where Spacetime is Not Locally Flat. Abstract.
• Thursday, June 28, 14:15, Luc Nguyen (Princeton). Smoothness of closed relativistic strings in 2+1 vacuum spacetimes.
• Thursday, June 21, 14:15, Magdalena Zych (Vienna). General relativistic effects in quantum interference. Abstract.
• Thursday, June 14, 14:15, Gregory Galloway (Miami). Topological censorship from the initial data point of view.
• Tuesday, June 05, 17:15, Helmut Rumpf (Vienna). Gravitational analogs of the Aharonov-Bohm effects. Part II. Place: Common room, 1st floor.
• Tuesday, June 05, 12:00, Claes Uggla (Karlstad). Two research programs: Generic Singularities and Perturbations in Cosmology. Lunch seminar. Place: Common room, 1st floor.
• Thursday, May 31, 14:15, Riccardo Sturani (Urbino). The gravitational two body problem in the post-Newtonian approximation via effective field theory methods. Abstract.
• Friday, May 25, 13:00, Roland Steinbauer (Vienna). The wave equation on space-time and low regularity. Lunch seminar. Abstract.
• Thursday, May 24, 14:15, Helmut Rumpf (Vienna). Gravitational analogs of the Aharonov-Bohm effects.
• Friday May 04, 13:00, Piotr T. Chrusciel (Vienna). The many ways of the characteristic Cauchy problem. Lunch seminar, common room, Währinger Straße 17, 1st floor.
• Thursday, May 03, 14:15, Gerhard Ecker (Vienna). Effective quantum field theories.
• Thursday, April 26, 14:15, Jan-Hendrik Treude (Regensburg). Volume comparison in Lorentzian manifolds and singularity theorems. Abstract.
• Thursday, April 19, 12:45, Juan Valiente-Kroon (Queen Mary College, London). A class of conformal curves on the Reissner-Nordstroem spacetime. Abstract. Lunch seminar, common room, 1st floor.
• Thursday, March 29, 14:15, Albert Fathi (ENS Lyon, et Institut Universitaire de France). Existence of time functions.
• Thursday, March 22, 14:15, Daniel Grumiller (TU Vienna). Recent developments in 3-dimensional classical and quantum gravity. Abstract.
• Friday, March 16, 13:00, Lars Andersson (AEI Potsdam). Conserved charges for linearized gravity. Abstract. Place: Common room 1st floor. Lunch seminar.
• Thursday, March 15, 14:15, Bodo Ziegler (Vienna). 12 Billion Years of Galaxy Evolution. Abstract.
• Thursday, March 08, 14:30, Robert Beig (Vienna). (Weak) solutions of (generalized) fluids. Abstract. Note the exceptional location: Erwin Schroedinger Institute and unusual time.
• Wednesday, March 07, 13:15, Martin Reiris (AEI Golm). Axisymmetric black holes. Abstract. Lunch seminar: Note the exceptional location: Erwin Schroedinger Institute and unusual time.
• FridayFebruary 10, 13:00, Juliette Hell (Berlin). Anisotropic black hole initial data with prescribed scalar curvature. Abstract. Lunch seminar: Library, Währinger Straße 17, 1st floor.
• Thursday, January 26, 14:15, Mark Heinzle (Vienna). On the asymptotic dynamics of G2 cosmologies.
• Thursday, January 19, 14:15, Michael Pürrer (Vienna). Wave Maps in 2+1 Dimensions / Towards hybrid methods for puncture evolution.
• Thursday, January 12, 14:15, Andreas Cap (Vienna). Conformally invariant overdetermined PDEs and Einstein metrics. Abstract.

# 2011

• Wednesday, December 21, 13:00, Nishanth Abu Gudapati (Golm). The Einstein-wave maps system. Place: Währinger Straße 17, 1st floor, common room.
• Friday, December 16, 13:00, Mikolaj Korzynski (Vienna). Numerical finder of isometric embeddings of S2 surfaces in R3 with applications to relativity. Place: Währinger Straße 17, 1st floor, common room.
• Thursday, December 15, 14:15, Vincent Moncrief (Yale). A Modified Semi-Classical Approach to Nonlinear Quantum Oscillations Problems. Abstract.
• Friday, December 09, 13:00, Igor Pikovsky (Vienna). Probing Planck-scale physics with quantum optics. Abstract. Place: Währinger Straße 17, 1st floor, common room.
• Thursday, December 1, 14:15, Mattias Dahl (Stockholm). Estimates of the asymptotically hyperbolic mass. Abstract.
• Thursday, November 24, 14:15, Helmut Rumpf (Vienna). On a new solution of the cosmological constant problem.
• Thursday, November 17, 14:15, Gilbert Weinstein (UAB & Monash). Rigidity in the positive mass theorem with charge.  Abstract.
• Wednesday, November 16, 13:00, Wolfgang Graf (Hamburg). Volumetric Gravity - Cosmic Acceleration without Darkness? Place: Währinger Straße 17, 1st floor, common room. Abstract.
• Thursday, November 10, 14:15, Walter Simon (Vienna). Inequalities between area, angular momentum, charge and the cosmological constant for marginally trapped surfaces. Abstract.
• Thursday, November 3, 14:15, Alan Rendall (Golm). Late-time behaviour of spatially homogeneous solutions of the Einstein equations.
• Thursday, October 27, 14:15, Mikolaj Korzynski (Vienna). Cosmological spacetimes with regular lattices of black holes. Abstract.
• Thursday, October 20, 14:15, Piotr Chrusciel (Vienna). Lorentzian geometry with continuous metrics. Abstract.
• Thursday, October 13, 14:15, Christina Sormani (New York). The Stability of the Riemannian Positive Mass Theorem and the Intrinsic Flat Distance. Abstract.
• Thursday, October 06, 14:15, Tim-Torben Paetz (Vienna). Asymptotically flat vacuum spacetimes with complete smooth light cones. Abstract.
• Tuesday July 26, 13:00, Joanna Jalmuzna (Krakow). Gravitational collapse in 2+1 dimensional AdS spacetime. Place: Währinger Straße 17, 1st floor, common room. Abstract.
• Thursday, June 30, 14:15, Mark Heinzle (Vienna). The initial singularity of ultrastiff perfect fluid spacetimes without symmetries. Abstract.
• Wednesday, June 22, 16:00, Caslav Brukner (Vienna). Quantum interferometric visibility as a witness of general relativistic proper time.
• Thursday, June 16, 14:15, James Grant (Vienna). Energy inequalities on space-times with one-sided geometrical bounds. Abstract.
• Thursday, June 09, 14:15, Erwann Delay (Avignon). Gluing of TT tensors. Abstract.
• Thursday, June 09, 10:00, Donald G. Saari (UCIrvine). Dark matter; is this really a problem? Abstract.
• Thursday, May 26, 14:15, Stoytcho Yazadjiev (Sofia). Black holes in higher dimensions: exact solutions and classification theorems. Abstract.
• Thursday, May 12, 14:15, Walter Simon (Vienna). The Bartnik quasilocal mass of a trapped region, Part II. Abstract.
• Thursday, May 05, 14:15, Stephan Broda (Vienna). Elastic spheres on circular orbits. Abstract.
• Thursday, April 14, 14:15, Walter Simon (Vienna). The Bartnik quasilocal mass of a trapped region. Abstract.
• Thursday, April 07, 14:15, Juliette Hell (Berlin). Towards the BKL conjecture -- a step by Reiterer and Trubowitz. Abstract.
• Thursday, March 31, 14:15, Michael Pürrer (Vienna). Reducing orbital eccentricity for moving puncture simulations of binary black holes.
• Thursday, March 24, 14:15, Philippe LeFloch (Paris). Weakly regular spacetimes with symmetries. Definition, existence, and global geometry. Abstract.
• Thursday, March 17, 15:00, Mark Heinzle (Vienna). Cosmological models and spacelike singularities. Abstract. Place: ESI lecture hall.
• Thursday, March 10, 14:15, Helmuth Urbantke (Vienna). On the principle of symmetric criticality II.
• Thursday, March 3, 13:30, Marc Nardmann (Hamburg). The positive energy theorem in higher dimensions.
• Tuesday, February 1, 14:00, Patrik Sandin (Karlstad). Space-like singularities in cosmologies with multiple perfect fluids.
• Thursday, January 13, 13:30, Helmuth Urbantke (Vienna). On the principle of symmetric criticality I.

# 2010

• Thursday, December 16, 13:30, Christian Spreitzer (Vienna). Symmetric hyperbolic systems with distributional coefficients.
• Tuesday, December 12, 13:30, Michael Eichmair (Stanford). Large isoperimetric surfaces in initial data sets.
• Thursday, December 9, 13:30, Harold Steinacker (Vienna). Emergent gravity from Yang-Mills matrix models. Abstract.
• Thursday, December 2, 13:30, Birgit Schörkhuber (TU Vienna). On stable self-similar blow up for equivariant wave maps - The linearized problemAbstract.
• Friday, November 26, 13:30, Daniel Grumiller (TU Vienna). Place: Common room, Waehringerstrasse 17, 1st floor. TBA
• Thursday, November 25, 13:30, Mikolaj Korzynski (Vienna). A new covariant approach to the backreaction problem in cosmology.
• Thursday, November 18, 13:30, Günther Waxenegger (Vienna). Trumpet initial data sets for the Einstein vacuum equations. Abstract.
• Thursday, November 11, 13:30, Helmut Rumpf (Vienna). Remarks on gravity, thermodynamics and holography, II.
• Thursday, November 4, 13:30, Helmut Rumpf (Vienna). Remarks on gravity, thermodynamics and holography, I.
• Thursday, October 28, 13:30. Walter Strauss (Brown University, Providence). Stability in collisionless plasmas (Vlasov-Maxwell).
• Thursday, October 7, 13:30, James Grant (Vienna). Volumes and areas for null cones. Abstract.
• Wednesday, August 25, 13:30, Milton Ruiz (Jena). Constraint preserving boundary conditions for the Z4c formulation of general relativity. Abstract.
• Monday, August 23, 14:00, Jan Steinhoff (Jena). Canonical formulation of spin in general relativity. Abstract.
• Tuesday, June 15, 13:30. Robert Beig (Vienna). Mountains for neutron stars. Abstract. Note: Room 118, Währinger Straße 17, 1st floor.
• Monday, June 14, 13:00. James Grant (Vienna). Null injectivity radius estimates. Abstract.
• Friday, June 11, 13:30. George Ellis (Cape Town University).
Alternative explanations of "dark energy" in cosmology. Abstract.
• Tuesday, June 08, 11:15. Michael Reiterer (ETH Zürich). Strongly Focused Gravitational Waves.
• Monday, June 07, 13:00. Michael Reiterer (ETH Zürich). The BKL Conjectures for Spatially Homogeneous Spacetimes.
• Friday, June 04, 13:30. Piotr T. Chrusciel (Vienna). The Cauchy problem on a light cone. Abstract.
• Friday, May 28, 13:30. Stefan Hollands (Cardiff & ESI). Black holes and their topology in higher dimensions. Abstract.
• Friday, May 21, 13:30. Luc Nguyen (Oxford). Uniqueness for degenerate Kerr-Newman solutions. Abstract.
• Thursday, May 20, 13:30. Omar Ortiz (Cordoba). Well-posedness, linear perturbations, and mass conservation for the axisymmetric Einstein equations. Abstract. Note: Room 118, Währinger Straße 17, 1st floor.
• Friday, May 14, 13:30. Frank Ohme (Albert Einstein Institute). Combination of analytical and numerical relativity in gravitational waves of binary black holes. Abstract.
• Thursday, May 06, 13:30. Niall O'Murchadha (Cork). The boundary of the space of gravitational degrees of freedom.
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