Testing the No-Hair Theorem with Observations of Black Holes in the Electromagnetic Spectrum

TL;DR

This paper discusses a new method to test the no-hair theorem of black holes using electromagnetic observations, focusing on deviations in the quadrupole moment from the Kerr metric.

Contribution

It introduces a framework for testing the no-hair theorem by analyzing deviations in the quadrupole moment through electromagnetic observations.

Findings

Potential to test the no-hair theorem with upcoming VLBI imaging of Sgr A*

Spectroscopic observations of iron lines can provide additional tests

Deviations from Kerr quadrupole could indicate new physics

Abstract

According to the no-hair theorem, astrophysical black holes are uniquely described by their mass and spin. In this paper, we review a new framework for testing the no-hair hypothesis with observations in the electromagnetic spectrum. The approach is formulated in terms of a Kerr-like spacetime containing a quadrupole moment that is independent of both mass and spin. If the no-hair theorem is correct, then any deviation from the Kerr metric quadrupole has to be zero. We show how upcoming VLBI imaging observations of Sgr A* as well as spectroscopic observations of iron lines from accreting black holes with IXO may lead to the first astrophysical test of the no-hair theorem.