Chennai Mathematical Institute

Seminars




PUBLIC VIVA-VOCE NOTIFICATION
Subject: Physics
Date: Wednesday, 09 August 2023
Time: 11:00 AM
Venue: Seminar Hall
Multiparameter tests of general relativity using compact binary coalescences

Sayantani Datta
Chennai Mathematical Institute.
09-08-23


Abstract

Gravitational wave (GW) observations of compact binary coalescences (CBCs) provide a unique opportunity to test general relativity (GR) in the radiative strong-field regime of gravity. The post-Newtonian (PN) approximation to GR accurately describes the inspiral phase of the GW signal emitted by a CBC. The PN coefficients at different PN orders contain information about various nonlinear interactions and physical effects in GR. Hence, in a modified theory of gravity, more than one PN coefficient may deviate from its corresponding GR value. Therefore, simultaneously measuring multiple PN coefficients is crucial to obtain meaningful bounds, which, in turn, may be used to constrain a modified theory of gravity. Nevertheless, performing such a multiparameter test of GR is very challenging due to the high degree of correlations among the PN coefficients. In this thesis, we propose two independent methods to carry out the multiparameter tests of GR using GW observations.
The first method uses principal component analysis (PCA) to optimize multiparameter tests of GR by constructing best-measured, orthogonal linear combinations of generic parametrized deviations from GR at multiple PN orders. For the events detected during the first and second observing runs of Advanced LIGO and Virgo, the two most dominant PCA linear combinations optimally capture the information on most of the PN deviation parameters and were found to be consistent with GR. We also study the usefulness of this method to identify GR violations. Further, we use the Fisher information matrix to forecast the bounds on these PCA linear combinations with next-generation ground-based detectors like Cosmic Explorer (CE) and Einstein Telescope (ET) and a space-based detector such as Laser Interferometer Space Antenna (LISA). This PCA-assisted test of GR significantly reduces the computational cost compared to the standard method of one-parameter tests, where eight independent tests are performed varying only one PN deviation parameter at a time. This method is currently being implemented as a parameter estimation framework to test GR using compact binaries detected in the ongoing fourth observing run of LIGO/Virgo.
We propose a second approach to perform the multiparameter tests of GR by using multiband observations of GWs from heavy stellar-mass (SBBHs) and lighter intermediate mass binary black holes (IMBBHs) by CE/ET and LISA. Multiband observations of heavy SBBHs would measure most of the known PN deviation parameters with 1σ uncertainty below a few percent—two orders of magnitude better than the best bounds achievable from ‘golden’ binaries in the CE/ET or LISA bands. However, this test will only be possible around the mid-2030s when the ground- and space-based detectors are expected to be functional.
All are invited to attend the viva-voce examination.