Testing the Sagittarius A* Spacetime Metric with the 2017 EHT Observations

Astrophysical black holes (BHs) are expected to be described by the Kerr solution of the Einstein’s equations and to respect the no-hair theorems. We will present the first results on how these theorems can be tested using the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*). We also discuss the viability status of alternative models: non-Kerr BHs from general relativity, BHs from alternative theories, wormholes, and other exotic objects. This is facilitated by empirically calibrating the size of the bright ring-like feature that the EHT observes to the size of the “critical curve,” a purely spacetime-dependent feature and the actual observable of interest here. Together with the bounds found for stellar-mass BHs and the BH in M87, our observations provide further support that the external spacetimes of astrophysical BHs are well described by the Kerr metric, independently of their mass. Time permitting, I will briefly comment on the exciting future of BH imaging vis-a-vis studies of strong-field gravity.