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

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