Asymmetric binaries such as Extreme Mass Ratio Inspirals (EMRIs) are powerful probes of fundamental physics. In the final year before the plunge, these systems are expected to complete thousands of orbits in the strong-field region surrounding the massive black hole. Tracking such a large number of cycles enables precise measurements of the source parameters, allowing for accurate tests of gravity in the strong-field regime, and potentially revealing deviations of the central object from the Kerr spacetime. The gravitational waves emitted from these systems could also reveal information about the black hole’s environment.
An additional avenue to test gravity arises from probing the structure of the smaller companion. If the secondary carries a charge associated with a new fundamental field, its accelerated motion around the primary will lead to the emission of extra radiation — leaving an imprint on the emitted gravitational waves that could potentially be detected with LISA.