Research Projects

Despite being successful in the weak field regime, the validity of general relativity remains elusive in the strong field regime. Issues with unification of general relativity with quantum theory in a self-consistent manner have further complicated the situation. Recent GW observations have opened the door to test general relativity in the strong field regime of black holes, mainly through the merger of binary black holes. Even though it turns out that the observed ringdown waveform of the merger remnant is consistent with the general relativistic prediction, the detected ringdown waveform can be constructed by the perturbations of the photon sphere alone. This opens up the possibility that the final remnant may correspond to a horizonless compact object with a photon sphere [see, Cardoso et al., Phys. Rev. Lett. 116, 171101 (2016)]. Even though these compact objects can cure pathologies associated with black holes, consistency with shadow observation requires that they either must be supported by exotic matter fields which violate energy conditions or should be motivated by modified theories, as well as arise from quantum effects at horizon. These horizonless compact objects are known as exotic compact objects (ECOs) [see, Cardoso et al., Living Rev Relativ (2019) 22: 4]. In this project, we wish to analyse the signatures of these exotic compact objects on the gravitational waveforms. Because of the absence of the horizon, the boundary condition for the wave propagation changes drastically compared to that of a black hole. In the case of a black hole, modes will be purely ingoing near the horizon, on the other hand, for ECOs there will be both ingoing and outgoing modes. Imprints of this modified boundary condition on the stability of these objects under linear perturbations and on the gravitational waveform will be a main focus of the project. Beside these we will to study the environmental effects such as how the presence of dark matter in the galaxy clusters affect the gravitational waveforms during ringdown phase or in the inspiral phase. Using extreme mass ratio inspiral consisting of exotic compact objects we will also try to detect the presence of extra non-metric fields that may be present in the alternative theories of gravity.