Institute for Advanced Study Astrophysics Seminar

Our understanding of astrophysical black holes is predicated on the Kerr solution to Einstein’s equations. A promising avenue for testing the Kerr hypothesis is to detect the shadow a black hole casts on the surrounding emission and compare its size and shape to the predictions of the Kerr metric. New horizon-scale images of the Galactic Center black hole Sagittarius A* (Sgr A*) recently published by the Event Horizon Telescope (EHT) allow for new strong-field tests of the Kerr metric in a previously unexplored regime. In paper VI of the recent EHT publications we conduct a careful calibration between the geometrically defined black hole shadow and the observational ring feature, and arrive at a ~10% error bar for the measured size of the black hole shadow. We use this bound to constrain metrics that parametrically deviate from Kerr as well as the charges of several known metrics. We also explore possible alternatives to an event horizon and use the size of the emission region measured by the EHT combined with multi-wavelength observations to rule out a thermal surface. Additionally, the brightness depression in our images shows that a reflective surface is unlikely. Combined with the results for stellar mass black holes from gravitational wave observations and the EHT constraints for M87, our results show strong agreement with the Kerr prediction over ten orders of magnitude in mass.