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

TL;DR

This paper reviews methods to test the no-hair theorem of black holes using electromagnetic observations across various spectra, aiming to detect deviations from the Kerr metric that could indicate new physics.

Contribution

It provides a comprehensive overview of current and future observational techniques for testing the no-hair theorem through electromagnetic signals from black holes.

Findings

Current observations are consistent with the Kerr metric

Future observations could detect deviations from the no-hair theorem

Multiple electromagnetic methods complement each other in testing black hole properties

Abstract

According to the general-relativistic no-hair theorem, astrophysical black holes depend only on their masses and spins and are uniquely described by the Kerr metric. Mass and spin are the first two multipole moments of the Kerr spacetime and completely determine all other moments. The no-hair theorem can be tested by measuring potential deviations from the Kerr metric which alter such higher-order moments. In this review, I discuss tests of the no-hair theorem with current and future observations of such black holes across the electromagnetic spectrum, focusing on near-infrared observations of the supermassive black hole at the Galactic center, pulsar-timing and very-long baseline interferometric observations, as well as X-ray observations of fluorescent iron lines, thermal continuum spectra, variability, and polarization.