General Relativity & Quantum Cosmology

It is by now well established that black holes emit a thermal radiation and undergo an evaporation process. The original Hawking evaporation scenario, based on quantum fields on a classical background geometry, has been vastly extended and improved, in order to take into account in particular the backreaction of the radiation on the geometry. This can be done for example in a semiclassical setup, where the Einstein equations are sourced by an effective stress energy tensor. In two-dimensional models, this is a powerful method since the conformal anomaly enables one to reconstruct unambiguously the full effective stress energy tensor, or the corresponding effective matter action. However, attempts to generalize this method in four dimensions have been so far partly unsuccessful. In this talk we will review the difficulties that arise in attempts to constructing the effective action for matter fields coupled to a dilaton, and focus on a potential way out of the problem. This could lead to a new expression for the effective stress energy tensor on a black hole geometry, and to progress in the the study of backreaction in four-dimensional spherically symmetric models.

The supermassive black hole in the centre of the Milky Way, Sgr A*, is an ideal target for testing the properties of black holes. A number of experiments are being prepared or conducted, such as the monitoring of stellar orbits, the search for radio pulsars or the recording of an image of the shadow of a event horizon. The talk puts these efforts in context with other tests of general relativity and its alternatives. I will also compare the approaches in the Galactic centre, while concentrating in particular on the prospects of using pulsars as probes, also in combination with event horizon imaging.