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.

Unless explicitly stated otherwise, the regular (not-lunch) relativity seminars
take place in Room 218
**2nd floor, Währingerstrasse 17.**

The Mathematical Physics Seminars take place on Tuesdays at 14.15.

The Particle Physics Seminars take place on Tuesdays at 16.15.

- Thursday,
**May 4**, Währinger Straße 17, Victor-Franz-Hess-Hörsaal, Kavalierstrakt, 1. Stock, 14:00, joint relativity - mathematical physics seminar, Robert Seiringer (ISTA):*The excitation spectrum in a Bose fluid*

Abstract: Of particular relevance for an understanding of the low-temperature properties of a quantum system is the excitation spectrum. With the exception of exactly solvable models in one dimension, rigorous results on its structure are lacking and one has to resort to adhoc approximations to make predictions. We present recent progress on this question for bosonic many-body quantum systems with weak two-body interactions. Such systems are currently of great interest, due to their experimental realization in ultra-cold atomic gases. We investigate the accuracy of the Bogoliubov approximations, which predicts that the low-energy spectrum is made up of sums of elementary excitations, with linear dispersion law at low momentum. The latter property is crucial for the superfluid behavior of the
system.

- Thursday,
**May 11**, 14:00, Katharina Radermacher (Stockholm):*Strong Cosmic Censorship and the initial singularity in Bianchi spacetimes*

Abstract:
For given initial data to Einstein's field equations, one can find a spacetime solving these equations, and one can do so in a unique way (up to isometries) if one assumes the spacetime to be maximal globally hyperbolic. Both statements were proven by Choquet-Bruhat and Geroch in the 1950s and 60s.
When dropping the additional condition of global hyperbolicity, it is an open question whether one can extend this spacetime, possibly in a non-unique way. Strong Cosmic Censorship conjectures that no such extension exists, at least not for generic initial data.
In my talk, I focus on spacetimes where the initial data is symmetric under the action of a three-dimensional Lie group (a so-called Bianchi spacetime) and the stress-energy tensor is that of vacuum or a perfect fluid. I present results proving the Strong Cosmic Censorship conjecture for orthogonal Bianchi class B spacetimes and explore in more detail the asymptotic behaviour towards the initial
singularity.

- Friday,
**May 12**, 13:00, lunch seminar, Jose P. S. Lemos (Lisbon):*Black hole entropy from matter entropy*

Abstract:
TBA

- Thursday,
**May 18**, 14:00, Stefan Hollands (Leipzig):*TBA*

Abstract:
TBA

- Wednesday,
**May 24**, 13:00, lunch seminar, Ehsan Hatefi (TU Vienna):*Black hole formation and Critical Collapse in the Axion- Dilaton System in Diverse Dimensions*

Abstract: In the first part of the talk we try to come up with some basic notions about the subject as well as Black hole formation, explaining some details of the whole subject on GR and also why we have three different assumptions or ans\"atze for axion-dilaton system in type IIB String Theory. We then would like to study the gravitational collapse of the axion-dilaton system suggested by IIB in dimensions ranging from four to ten. We would also like to extend the previous analysis in the literature concerning the role played by the global
SL(2, R) symmetry, evaluating the Choptuik exponents for different elliptic, parabolic and hyperbolic cases. Eventually we describe some of the open questions for two other assumptions and future directions. This talk is based on arXiv:1108.0078 (published in CQG) and 1307.1378,gr ( published in JCAP) in collaboration with my former supervisor Prof. Luis.Alvarez-Gaume and some works in
progress.

- Thursday,
**June 8**, 14:00, Wolfgang Graf (Vienna):*Circular Geodesics in Time-Dependent Spacetimes*

Abstract:
A notion of "circularity" is proposed in geometric terms based on symmetry considerations. The main geometric object turns out to be closely related to the extrinsic curvature of the family of hypersurfaces defined by constant angular momentum. The solution of the geodesic equation in this setting can then be reduced to a single 2nd-order pde involving the mean curvature of these hypersurfaces. Their stability against linear perturbations can be reduced to a system of two odes having the structure of a damped harmonic oscillator. Even in the very well-known setting of time-independent spacetimes, this angular-momentum based approach has some advantages over the more conventional one based on an effective potential. However its true potential lies in a time-dependent setting, where the conventional approach cannot be applied. Some examples are
given.

- Friday,
**June 9**, lunch seminar, 13:00, Manfried Faber (Vienna):*TBA*

Abstract:
TBA

- Thursday,
**June 22**, 14:00, Carla Cederbaum (Tuebingen):*On extensions of CMC-Bartnik data*

Abstract:
Bartnik data are a Riemannian 2-sphere of positive Gaussian curvature equipped with a non-negative function H to be thought of as its mean curvature in an ambient Riemannian 3-manifold. Mantoulidis and Schoen suggested a construction of asymptotically flat Riemannian 3-manifolds of non-negative scalar curvature which allow to isometrically embed given Bartnik data of vanishing mean curvature, i.e. H=0. They use their construction to explore — and disprove — stability of the Riemannian Penrose inequality. We adapt their construction to constant mean curvature (CMC) Bartnik data, i.e. H=const.>0. I will present the construction as well as the motivation for such a construction which is related to Bartnik’s quasi-local capacity/mass functional and its minimizing
properties.
This is joint work with Armando Cabrera Pacheco, Stephen McCormick, and Pengzi
Miao.

- Thursday,
**TBA**, 14:00, Michael Eichmair (Vienna):*TBA*

Abstract:
TBA

Location (unless indicated otherwise): Währinger Str. 17

- Room 218
on the 2nd floor for **standard seminars**, and

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

Gravitational Physics

Faculty of Physics

University of Vienna

Boltzmanngasse 5

1090 Wien

T: +43-1-4277-51501

F: +43-1-4277-9515

Faculty of Physics

University of Vienna

Boltzmanngasse 5

1090 Wien

T: +43-1-4277-51501

F: +43-1-4277-9515