Testing the No-Hair Theorem with Sgr A*

TL;DR

This paper proposes a new theoretical framework for testing the no-hair theorem of black holes using VLBI imaging of Sgr A*, which could confirm or challenge general relativity's predictions about black hole properties.

Contribution

It introduces a Kerr-like spacetime model with a free parameter, enabling observational tests of the no-hair theorem through imaging, astrometry, and timing data.

Findings

A new Kerr-like spacetime framework for observational tests.

Potential for VLBI imaging to verify the no-hair theorem.

Combines imaging with astrometric and timing observations.

Abstract

The no-hair theorem characterizes the fundamental nature of black holes in general relativity. This theorem can be tested observationally by measuring the mass and spin of a black hole as well as its quadrupole moment, which may deviate from the expected Kerr value. Sgr A*, the supermassive black hole at the center of the Milky Way, is a prime candidate for such tests thanks to its large angular size, high brightness, and rich population of nearby stars. In this review I discuss a new theoretical framework for a test of the no-hair theorem that is ideal for imaging observations of Sgr A* with very-long baseline interferometry (VLBI). The approach is formulated in terms of a Kerr-like spacetime that depends on a free parameter and is regular everywhere outside of the event horizon. Together with the results from astrometric and timing observations, VLBI imaging of Sgr A* may lead to a secure test of the no-hair theorem.