# 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 13.45.

The Particle Physics Seminars take place on Tuesdays at 16.15.

- Thursday,
**November 22th, 14:00, Seminarraum A**

Igor Khavkine, (Prague University):*Linear local gauge-invariant observables on spacetimes of sub-maximal symmetry*

Abstract: The Killing operator $K_{ab}[v]=\nabla_a v_b + \nabla_b v_a$ is the generator of gauge symmetries (linearized diffeomorphisms) $h_{ab}\mapsto h_{ab} + K_{ab}[v]$ in linearized gravity. A linear local gauge-invariant observable is a differential operator $I[h]$ such that $I[K[v]] = 0$ for any gauge parameter field $v_a$. A set $\{I_i[h]\}$ of such observables is complete if the simultaneous conditions $I_i[h] = 0$ are sufficient to conclude that the argument is a pure gauge mode, $h_{ab} = K_{ab}[v]$. The explicit knowledge of a complete set of local gauge invariant observables has multiple applications from the points of view of both physics and geometry, whenever a precise separation of physical and gauge degrees of freedom is required. Surprisingly, until very recently, such complete sets have been known explicitly only on spacetimes of maximal symmetry (Minkowski or (anti-)de Sitter). I will discuss recent progress that has allowed an explicit construction of complete sets of local gauge invariant observables on backgrounds of sub-maximal symmetry, most notably on cosmological (FLRW) and black hole (Schwarzschild and Kerr) spacetimes.

- Thursday,
**November 29th, 14:00, Seminarraum A**

Stefano Borghini, (Trento University):*Static vacuum spacetimes with positive cosmological constant*

Abstract: Static vacuum metrics are solutions to the Einstein Field Equations with vanishing stress-energy tensor and featuring a very special metric structure (warped product). Such a structure induces a natural foliation of the spacetime into space-like slices which are all isometric to each other, so that the corresponding physical universe is static.

We discuss the problem of the classification of such solutions in the case of positive cosmological constant. To this end, we introduce an appropriate notion of mass, showing that it satisfies a Positive Mass Statement and a Riemannian Penrose-like inequality. Building on this, we prove a uniqueness result for the Schwarzschild-de Sitter solution, which is somehow reminiscent of the well known Black Hole Uniqueness Theorem for the Schwarzschild solution.

- Thursday,
**December 13**, 14:00, Seminarraum A

Philipp Höhn (ESI):*From quantum reference systems to quantum general covariance*

Abstract: Despite its importance in general relativity, a quantum notion of general covariance has not yet been established in quantum gravity and cosmology, where, given the a priori absence of coordinates, it is necessary to replace classical frames with dynamical quantum reference systems. As such, quantum general covariance bears on the ability to consistently switch between the descriptions of the same physics relative to arbitrary choices of quantum reference systems. In this talk, I will summarize a recent systematic method for such switches, which works in analogy to coordinate changes on a manifold, except that these `quantum coordinate changes' proceed between different Hilbert spaces.

I will illustrate this method by means of spatial quantum reference frames and a simple quantum cosmological model. Time permitting, I might also disucss conceptual implications for quantum gravity. (Based on arXiv:1809.00556, 1809.05093, 1810.04153 and 1811.00611.)