Title: Testing General Relativity with LISA observations of extreme mass ratio inspirals

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Abstract:
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Testing the nature of gravity and the validity of Einstein’s theory in the strong field regime is one of the most ambitious goals of gravitational wave (GW) detectors, and a primary target for the space satellite LISA.

Among binary sources observable by LISA, extreme mass ratio inspirals (EMRIs) in which a stellar mass object follows hundreds of thousands of orbits around a supermassive black hole, harbour  the potential for very precise tests of gravity.

This simplest modifications of General Relativity (GR) predict the existence of scalar fields which modify the gravitational interaction. However modelling EMRIs and their GW emission beyond GR is a major challenge, so far slowed down by the mathematical complexity of the problem.

In this talk I will present a new study showing that, for a vast class of theories in which compact objects are endowed with a scalar field witch couples to the gravity sector, the description of EMRIs  greatly simplifies, reducing the problem to the motion of a charged body around a Kerr black hole. In particular I will show how all the information on the GR modifications is universally captured by a single parameter, the scalar charge of the stellar mass object. This promises drastic simplifications in terms of waveform modelling, and offers the opportunity to use EMRIs for agnostic strong field tests of gravity. I’ll present some recent results on the constraints that LISA can potentially be able to infer on the scalar charge, and how they can be mapped to bounds on the fundamental parameters which control GR deviations in specific modified theories of gravity.