Testing Multipole Moments of compact objects beyond Kerr paradigm

Abstract: Multipole moments are related to the physical properties of compact gravitating objects; therefore, understanding their structure is useful in accessing the nature of compact objects. We look into gravitational wave observables for black holes with charge, black holes on the brane, black holes with torsion, and regular black holes to see if and how they are correlated to the black hole hairs, which are related to the multipole moments. We find that the gravitational wave observables are indeed related to the hairs of non-vacuum spacetimes (for instance, charge $Q$ in the case of Kerr-Newman black holes). We also constrain the black hole hairs for change in gravitational wave phasing to see if the dependencies are significant and can be observed. The results from the analysis imply that the charge $Q$ in Kerr-Newman black holes should be detectable; thus, we provide a constraint to $Q^2/M^2$ given the spin and mass ratio of an ideal EMRI system for which future detectors like LISA can detect the change in gravitational wave observables. We also look into an analytical approach to find multipole moments of non-vacuum black hole spacetimes, mainly using the Improved Twist Vector approach for the Geroch-Hansen multipole moments and the Thorne formalism. The necessary analytics are computed, and the multipole moments are obtained for various non-vacuum spacetimes. However, the multipole moments don't contain any information about the black hole hairs, and we have commented on this observation in our paper.