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.