Probing new fundamental fields with Extreme Mass Ratio Inspirals

TL;DR

This paper explores how extreme mass ratio inspirals (EMRIs) can be used to detect and differentiate scalar and vector fields around black holes in modified gravity theories through gravitational wave observations.

Contribution

It provides the first detailed analysis of scalar and vector field effects on EMRIs, including post-Newtonian expansions and numerical flux calculations, to identify potential observational signatures.

Findings

Degeneracy in scalar and vector charges in weak-field regimes.

Distinct behaviors of scalar and vector fields in strong-field, fast-motion orbits.

Extra polarizations may help distinguish scalar from vector fields in gravitational wave data.

Abstract

We examine extreme mass ratio inspirals (EMRIs), where a charged compact object spirals into a supermassive black hole, in modified gravity theories with additional scalar or vector fields. Using the Teukolsky and generalized Sasaki-Nakamura formalisms, we provide the post-Newtonian expansion of the energy flux of the vector waves up to $O(v^5)$ beyond the quadrupole formula in the weak field and numerically calculate the energy flux in the strong field for a charged particle moving in circular orbits. Our findings reveal a degeneracy in the scalar and vector charge parameters for weak-field, slow-motion orbits. However, for strong-field, fast-motion orbits close to the innermost stable circular orbit, we observe distinct behaviors between scalar and vector fields. We investigate the potential of using EMRIs detected by space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna to identify whether a black hole carries a scalar or vector charge. The influence of scalar and vector flux on the orbital evolution and tensor GW phase can not help us distinguish scalar and vector fields. However, extra polarizations emitted by the scalar or vector field can break the correlations between the scalar field and vector field and then help us distinguish the scalar and vector field.