Gravity theory tests with observations of stars near the black hole at the Galactic Center

Abstract

One could say that 2018 is very special year since 55 years ago quasars were discovered, the Kerr solution has been found and the First Texas Symposium on Relativistic Astrophysics has been organized. In addition, 50 years ago, in 1968 the pulsar discovery paper has been published and the black hole concept has presented by J. A. Wheeler in his groundbreaking paper which was based on his report in December 1967. In last years remarkable achievements have been done in gravitational physics and related observations, in particular, gravitational waves and binary black holes with high stellar masses have been discovered, VLT, Keck and GRAVITY observe trajectories of bright stars and gas clouds near the Galactic Center (GC) with improving accuracy. It is clear that the Galactic Center (Sgr A*) is a specific place. Our Sun is located at a distance around 8 kpc from the Galactic Center (GC). Earlier, astronomers suggested a number of different including exotic ones such as boson stars, fermion balls, neutrino balls, a cluster of neutron stars. Some of these models are ruled out or significantly constrained with consequent observations. A supermassive black hole with mass around 4 × 106 Mʘ is the most realistic model for GC. Precision observations of bright stars near the Galactic Center and observations of bright structures near the black hole at the Galactic Center to reconstruct shadow structure around the black hole with current and future observational VLBI facilities such as the Event Horizon Telescope give opportunities to test alternative theories of gravity. In particular, we got a graviton mass constraint which is comparable and consistent with constraints obtained recently by the LIGO-Virgo collaboration. We consider opportunities to improve current graviton mass mass constraints with future observations of bright stars. Similarly, from an analysis of bright star trajectories one could constrain a tidal charge which was predicted by a gravity theory with an additional dimension.