General Relativity & Quantum Cosmology

The path from the current EHT to the full array capabilities has been mapped out over the past several years. Through the efforts of the EHT Technical Working Group and the full complement of experts involved in the project, we have a detailed plan and work breakdown to implement these improvements. I will describe the schedule for EHT expansion and enhancement from 2015 onward.

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.

In general relativity, astrophysical black holes are characterized uniquely in terms of their masses and spins and are described by the Kerr metric. The high sensitivity and resolution of the EHT will allow for unprecedented tests of the Kerr nature of black holes and, hence, of general relativity. I will present current and future limits on deviations from the Kerr metric from Sgr A* in the context of radiatively-inefficient accretion flow models. I will also show how largely model-independent constraints on such deviations can be obtained from the angular diameter of the photon ring of Sgr A* in combination with existing mass and distance measurements. Finally, I will describe a new general framework for the parametrization of deviations from the Kerr metric.

I will discuss the prospects for finding additional radio pulsars in the Galactic center region and their utility for probing general relativity and other constituents in the region. This will include discussions of neutron star populations; radio wave scattering in and toward the Galactic center; issues and progress in discovering pulsars; and the precision to which discoverable pulsars can be timed

GRAVITY is a new instrument combining the four 8m ESO Very Large Telescopes in Chile. Other than the BlackHoleCam / EHT with its focus on imaging the shadow of the black hole against the surrounding accretion flow, the goal of GRAVITY is to measure dynamical processes in the immediate vicinity of the black hole, for example the motion of matter close to the last stable orbit and relativistic effects in stellar orbits. Our presentation covers the experimental and astrophysical aspects of this project and highlights the complementarity with the submm interferometry to overcome the degeneracies in modelling the observations.

M87 is one of the most luminous nearby galaxies and hosts one of the most massive black holes known, making it a very important target for extragalactic studies. The supermassive black hole has been the subject of several stellar and gas dynamical mass measurements; however, the best current stellar dynamical black hole mass is larger than the gas dynamical determination by a factor of two, corresponding to a 2-sigma discrepancy. In this talk, I will review the gas dynamical black hole mass measurements that have been made over the years for M87, focusing in particular on the most recent measurement from multi-slit Space Telescope Imaging Spectrograph observations from the Hubble Space Telescope. I will also discuss the strengths and weaknesses generally associated with stellar and gas dynamical black hole mass measurement methods, and the current state of cross-checks between the two methods that have been carried out within the same galaxy.