# Current seminars

In addition to the Vienna relativity seminars, the calendar above sometimes contains other events of interest to members of the relativity group. The seminars of the Vienna relativity group are listed below.

Location (unless indicated otherwise): Währinger Str. 17

- Seminarraum A on the 2nd floor for **standard seminars**, and

- Common room, first floor, for **lunch seminars**.

The Mathematical Physics Seminars take place on Tuesdays at 13.45.

The Particle Physics Seminars take place on Tuesdays at 16.15.

- Thursday, May 16th (as part of the joint relativity-mathematical physics seminar), 14:00, Seminarraum A

Stefan Fredenhagen (Vienna):*Obstructions to interacting higher-spin gauge theories in three dimensions*

Abstract: Free higher-spin Fronsdal fields generalise Maxwell fields and linearised gravity to higher tensor fields. Whereas for spin-1 and spin-2 there are non-linear completions (e.g. Yang-Mills, gravity), no non-linear, gauge-invariant theory of Fronsdal fields is known. A systematic way to construct them is the Noether procedure in which a gauge invariant action is constructed in a perturbative expansion in powers of the fields. In three space-time dimensions, there are strong obstructions to construct such an action leading to the conclusion that the interactions of the higher-spin gauge fields are completely fixed by the cubic vertices in the action.

- Friday,
**May 17th**, 13:00, lunch seminar

Roland Steinbauer (Vienna):*cut-and-paste for impulsive gravitational waves with Λ.*

Abstract: Impulsive gravitational waves in Minkowski space were introduced by Roger Penrose at the end of the 1960s, and have been widely studied over the decades. We focus on non-expanding waves which later have been generalised to impulses travelling in all constant-curvature backgrounds. While Penrose's original construction was based on his vivid geometric 'scissors-and-paste' approach in a flat background, until now a comparably powerful visualisation and understanding have been missing in the ${Λ not=0}$ case. In this work we provide such a picture: The (anti-)de Sitter hyperboloid is cut along the null wave surface, and the 'halves' are then re-attached with a specific shift of their null generators across the wave surface.

- Thursday, May 23th, 14:00, Seminarraum A

Daniel Grumiller (TU Wien):*Soft excitations on horizons in any dimensions*

Abstract: We derive generic properties of non-extremal horizons, assumed to be in equilibrium with a thermal bath, in any spacetime dimension greater than two. The physical properties of the thermal bath are modelled by the way we impose boundary conditions, and we shall describe various different well-motivated choices leading to infinite-dimensional near horizon symmetries, including BMS-like symmetries for arbitrary spin and Heisenberg-like symmetries. We prove that they generically span soft hair excitations in the sense of Hawking, Perry and Strominger.

- Thursday, June 6th, 14:00, Seminarraum A

Anne Franzen (Lisbon):*The wave equation near flat Friedmann-Lemaître-Robertson-Walker and Kasner Big Bang singularities*

Abstract: We consider the wave equation, $\square_g\psi=0$, in fixed flat Friedmann-Lemaître-Robertson-Walker and Kasner spacetimes with topology $\mathbb{R}_+\times\mathbb{T}^3$. We obtain generic blow up results for solutions to the wave equation towards the Big Bang singularity in both backgrounds. In particular, we characterize open sets of initial data prescribed at a spacelike hypersurface close to the singularity, which give rise to solutions that blow up in an open set of the Big Bang hypersurface $\{t=0\}$. The initial data sets are characterized by the condition that the Neumann data should dominate, in an appropriate $L^2$-sense, up to two spatial derivatives of the Dirichlet data. For these initial configurations, the $L^2(\mathbb{T}^3)$ norms of the solutions blow up towards the Big Bang hypersurfaces of FLRW and Kasner with inverse polynomial and logarithmic rates respectively. Our method is based on deriving suitably weighted energy estimates in physical space. No symmetries of solutions are assumed.

- Friday,
**June 7th**, 13:30, lunch seminar

Ighor Khavkine (Prag):*Conformal Killing Initial Data*

Abstract: We find necessary and sufficient conditions ensuring that the vacuum development of an initial data set of the Einstein's field equations admits a conformal Killing vector. We refer to these conditions as conformal Killing initial data (CKID) and they extend the well-known Killing initial data that have been known for a long time. The strategy used to find the CKID is a classical argument involving wave-like "propagation equations". Time permitting, I will how similar strategies might succeed or fail for other geometric equations, like e.g. Killing-Yano.

- Thursday, June 13th (as part of the joint theoretical physics seminar), 14:00, Seminarraum A

Paolo Salucci, Sissa: The mystery of the Dark Matter Phenomenon

Abstract: The distribution of the non-luminous matter in galaxies of different luminosity and Hubble type is much more than a proof of the existence of dark particles governing the structures of the Universe. The deeper we go into the knowledge of the dark component that embeds the stellar component of galaxies, the more we realize the profound interconnection present between the two of them.

They are too complex to be arisen by two inert components that just share the same Gravitational field.

The 30 years old paradigm which rests on a-priori knowledge of the nature of dark matter that has led to the scenario of collisionless dark matter in galaxy halos reveals itself to be insufficient to explain the observations. Here, we will review the complex but well-ordered scenario of the properties of the dark halos in relation with those of the baryonic components they host.

We will present a number of tight and unexpected correlations between selected properties of the dark and the luminous matter that indicate that they interacted in a direct way over the Hubble Time.

- Thursday, June 27th, 14:00, Seminarraum A

Artur Alho (Lisbon): Spherically symmetric steady states of Newtonian self-gravitating elastic matter

Abstract: In this talk I will introduce a new definition of spherically symmetric elastic body in Newtonian gravity. Using this new definition it is possible to introduce Milne-type homology invariant variables which transform the field equations into an autonomous system of nonlinear differential equations. By employing dynamical systems methods I will finally discuss the existence of static balls for a wide variety of constitutive equations, including Seth, Signorini, Saint Venant-Kirchhoff, Hadamard, and John’s harmonic materials.