Testing the Kerr Paradigm with the Black Hole Shadow

TL;DR

This paper discusses how upcoming submillimeter VLBI observations of SgrA*'s shadow could test the Kerr black hole paradigm by analyzing the shadow's shape and comparing it to predictions of the Kerr metric.

Contribution

It proposes a method to use black hole shadow imaging to detect deviations from the Kerr solution, aiding in testing general relativity in strong gravity regimes.

Findings

Shadow shape depends only on the background metric.

Imaging can constrain deviations from Kerr.

Potential to test the Kerr paradigm with future observations.

Abstract

Within 5-10 years, submillimeter VLBI facilities will be hopefully able to image the "shadow" of SgrA$^*$. When a black hole is surrounded by an optically thin emitting medium, the boundary of the shadow corresponds to the apparent photon capture sphere and only depends on the background metric. An accurate determination of the shape of the shadow of SgrA$^*$ could constrain possible deviations from the Kerr solution. In combination with other measurements, these observations could test the Kerr black hole paradigm.