Astrophysical black holes as natural laboratories for fundamental physics and strong-field gravity

TL;DR

Astrophysical black holes serve as natural laboratories for testing general relativity and its extensions, with gravitational-wave observations providing crucial insights into fundamental physics and strong-field gravity.

Contribution

The paper reviews how black holes can be used for both internal and external tests of general relativity, emphasizing recent research and the importance of space-based gravitational-wave detectors.

Findings

Black hole ringdown tests support the Kerr solution.

Superradiant instabilities can reveal new physics.

Gravitational-wave observations are vital for fundamental physics.

Abstract

Astrophysical tests of general relativity belong to two categories: 1) "internal", i.e. consistency tests within the theory (for example, tests that astrophysical black holes are indeed described by the Kerr solution and its perturbations), or 2) "external", i.e. tests of the many proposed extensions of the theory. I review some ways in which astrophysical black holes can be used as natural laboratories for both "internal" and "external" tests of general relativity. The examples provided here (ringdown tests of the black hole "no-hair" theorem, bosonic superradiant instabilities in rotating black holes and gravitational-wave tests of massive scalar-tensor theories) are shamelessly biased towards recent research by myself and my collaborators. Hopefully this colloquial introduction aimed mainly at astrophysicists will convince skeptics (if there are any) that space-based detectors will be crucial to study fundamental physics through gravitational-wave observations.