**Beatrice Bonga**, Perimeter Institute

*Angular momentum flux in Einstein-Maxwell theory*

There are three natural currents for Maxwell theory on a non-dynamical background: the stress, Noether and canonical current. Their associated fluxes across null infinity differ by boundary terms for asymptotically flat spacetimes. These boundary terms do not only quantitatively change the behavior of the flux associated with an asymptotic Lorentz symmetry, but also qualitatively: the stress flux contains both radiative and Coulombic information, whereas Noether and canonical ones are purely radiative.

While all methods are equally valid and have their own range of usefulness, it is reasonable to ask if one definition is more natural than the other. In order to answer this question, we turn to general relativity. With Maxwell theory coupled to gravity, we use the Wald-Zoupas formalism to obtain an expression for the flux of angular momentum and find that it is purely radiative. When the gravitational field is ``frozen'', the Wald-Zoupas flux reduces to the Noether flux.

**Catherine Meusburger, **Friedrich-Alexander-Universität Erlangen-Nürnberg

*Ideal tetrahedra and their duals*

Ideal tetrahedra play an important role in 3d hyperbolic geometry, in the construction of hyperbolic 3-manifolds and in the computation of their volumes.

Recently they have been generalized to other 3d homogeneous spaces, namely 3d anti de Sitter space and half-pipe space, a 3d homogeneous space with a degenerate metric.

We show that generalized ideal tetrahedra correspond to dual tetrahedra in 3d Minkowski, de Sitter and anti de Sitter space. They are those geodesic tetrahedra whose faces are all lightlike.

We investigate the geometrical properties of these dual tetrahedra in a unifi ed framework. We then apply these results to obtain a volume formula for generalised ideal tetrahedra and their duals, in terms of their dihedral angles and their edge lengths.

This is joint work with Dr Carlos Scarinci, KIAS.

**Nicola Pinamonti,** University of Genova

*Semiclassical Einstein equations in cosmological spacetimes*

During this talk we shall discuss the backreaction of quantum matter fields on classical backgrounds by means of the semiclassical Einstein equation.

We shall see that self consistent solutions of this coupled system exist in the case of cosmological spacetimes.

Furthermore, Einstein equations governing the backreaction will transfer quantum matter fluctuations to the metric.