Resonance correction to time-dependent finite-size effects in Effective Field Theory of binary black hole inspiral

Abstract

General Relativity predicts that black holes lack an internal structure and that their tidal Love numbers vanish. However, if astrophysical black holes were instead ultra-compact objects (UCOs) with internal structure, their tidal Love numbers would no longer vanish. Then, during a binary inspiral, their tidal deformation could transfer orbital energy to the UCO, increasing the orbital period and leaving an observable imprint on the phase of the emitted gravitational waves. As we discussed previously, the effect is enhanced if one binary companion is resonantly excited. Here we use the point-particle effective field theory framework to calculate this enhancement. We consider circular orbits and focus on excitations of the internal electric quadrupole, modelling the UCO spectrum as a harmonic oscillator spectrum. However, our methods could be straightforwardly extended to more complicated situations. Integrating out the internal quadrupole induces a series of time-dependent finite-size operators with specific numerical coefficients, which we calculate. We find that the static electric quadrupole Love number $k_2$ is negative, and that all finite-size terms are proportional to $k_2$. We further show that resonant excitation induces a leading 2.5 Post-Newtonian term, compared with the 5 Post-Newtonian term in the time-independent case. Our results confirm the expectation that resonant excitation during a binary inspiral offers one of the best ways to detect whether astrophysical black holes can undergo tidal deformation and, if they do, learn about their internal excitation spectrum.