The tune of the universe: the role of plasma in tests of strong-field gravity

TL;DR

This paper investigates how the presence of plasma in the universe affects the ability to test strong-field gravity using electromagnetic and gravitational-wave observations, revealing significant suppression of certain signals and effects.

Contribution

It demonstrates that plasma effects can significantly hinder specific astrophysical tests of strong-field gravity and modified theories, highlighting the need to account for plasma in such analyses.

Findings

Plasma effects quench superradiant instabilities.

Electromagnetic emission contributions are suppressed in charged black hole binaries.

Secondary modes in gravitational-wave signals are negligible due to plasma effects.

Abstract

Gravitational-wave astronomy, together with precise pulsar timing and long baseline interferometry, is changing our ability to perform tests of fundamental physics with astrophysical observations. Some of these tests are based on electromagnetic probes or electrically charged bodies, and assume an empty universe. However, the cosmos is filled with plasma, a dilute medium which prevents the propagation of low-frequency, small-amplitude electromagnetic waves. We show that the plasma hinders our ability to perform some strong-field gravity tests, in particular: (i)~nonlinear plasma effects dramatically quench plasma-driven superradiant instabilities; (ii)~the contribution of electromagnetic emission to the inspiral of charged black hole binaries is strongly suppressed; (iii)~electromagnetic-driven secondary modes, although present in the spectrum of charged black holes, are excited to negligible amplitude in the gravitational-wave ringdown signal. The last two effects are relevant also in the case of massive fields that propagate in vacuum and can jeopardize tests of modified theories of gravity containing massive degrees of freedom.