# 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
- Room 218 on the 2nd floor for standard seminars, and
- Common room, first floor, for lunch seminars. 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, November 23, 14:00, Peter Michor (Vienna): General Sobolev metrics on the manifold of all Riemannian metrics

Abstract: For a compact manifold $M^m$ equipped with a smooth fixed background Riemannian metric $\hat g$ we consider the space $\operatorname{Met}_{H^s(\hat g)}(M)$ of all Riemannian metrics of Sobolev class $H^s$ for real $s>\frac m2$ with respect to $\hat g$. The $L^2$-metric on $\operatorname{Met}_{C^\infty}(M)$ was considered by DeWitt, Ebin, Freed and Groisser, Gil-Medrano and Michor, Clarke. Sobolev metrics of integer order on $\operatorname{Met}_{C^\infty}(M)$ were considered in [M.Bauer, P.Harms, and P.W. Michor: Sobolev metrics on the manifold of all Riemannian metrics. J. Differential Geom., 94(2):187-208, 2013.]
In this talk we consider variants of these Sobolev metrics which include Sobolev metrics of any positive real (not integer) order $s>\frac m2$.
We derive the geodesic equations and show that they are well-posed under some conditions and induce a locally diffeomorphic geodesic exponential mapping.
Based on collaborations with M.Bauer, M.Bruveris, P.Harms.

• Friday, November 24, 11:00, Room 218, Claus Kiefer (University of Cologne): Quantum Geometrodynamics of Einstein and Conformal Gravity

Abstract: I discuss the canonical quantization of general relativity and Weyl-squared gravity. I present the classical and quantum constraints and discuss their similarities and differences.
I perform a semiclassical expansion and discuss the emergence of time for the two theories.
While in the first case semiclassical time has a scale and a shape part, in the second case it only has a shape part. I also address the relevance of these results for the general problem of understanding quantum gravity.
Ref.: C. Kiefer and B. Nikolic, J.Phys.Conf.Ser. 880 (2017) no.1, 012002 (open access) and references therein

• Thursday, November 30, joint relativity-geometric analysis seminar, 14:00, Lorenzo Mazzieri (Trento): TBA

Abstract: TBA

• Thursday, December 7, 14:00, David Fajman (Vienna): Nonvacuum stability of the Milne universe

Abstract: The Milne model is the only cosmological vacuum solution to Einstein’s equations (with vanishing cosmological constant) that is known to be nonlinearly (future-) stable due to the work of Andersson-Moncrief. We present a first generalisation of this result to the nonvacuum case, namely to the Einstein-Vlasov system. In particular, we introduce a new idea to combine earlier approaches to control massive collisionless matter in cosmological spacetimes with a physically motivated estimate that is necessary to establish sufficient decay properties of the matter field. This is joint work with Lars Andersson (Golm).

• Thursday, December 14, 14:00, Dennis Raetzel (Vienna): TBA

Abstract: TBA

• Thursday, January 18, 14:00, Stefan Palenta (Jena): Nonlinear Interactions of Gravitational Waves

Abstract: After an introduction on gravitational waves and nonlinear effects in general, the talk will present the foundations of a new solution technique for the characteristic initial value problem of colliding plane gravitational waves. Assuming plane symmetry, the Einstein equations essentially reduce to the Ernst equation. In the course of the inverse scattering method this nonlinear PDE is tranlated first into an overdetermined linear system of differential equations and secondly into a Riemann-Hilbert problem. Ambiguities in this Riemann-Hilbert problem's solution lead to the construction of families of exact spacetimes generalising the proper solution to the initial value problem. Therefore the presented technique also serves as a solution generating technique. The method is exemplified by generalising the Szekeres class of colliding plane wave spacetimes. A new type of circularly polarised impulsive gravitational waves is identified within this generlisation.