Gravitational wave ringdown as a probe to test general relativity

The detection of gravitational waves by the LIGO-Virgo-Kagra collaboration has opened up new avenues for testing the principles of relativistic gravity in the domain of strong gravitational interactions. This realm is characterized by simultaneous strong, non-linear, and highly dynamic gravitational effects. The achievement of detecting gravitational waves has provided a means to explore Einstein's general relativity through experimental tests, complementing existing tests involving binary pulsars that have confirmed the predictions of general relativity. In this dissertation, we focus on two specific modifications of general relativity that lack stringent constraints. Our attention is directed towards a subset of theories that introduce modifications through a non-minimal coupling between a pseudo-scalar (scalar) field and a quadratic term in curvature, namely the Pontryagin density (the Gauss-Bonnet term). These theories fall under the categories of dynamical Chern-Simons gravity and Einstein dilaton Gauss-Bonnet gravity, respectively. Throughout this dissertation, we investigate various tests in the strong gravity regime, utilizing binary pulsars and gravitational waves. Our initial test examines the influence of dynamical Chern-Simons gravity on the orbital period of a binary pulsar system. Subsequently, we delve into the study of polarization modes of gravitational waves in both dynamical Chern-Simons gravity and Einstein dilaton Gauss-Bonnet gravity. However, our findings indicate that these observations are independent of modifications to general relativity, thus rendering these tests incapable of imposing stringent constraints on the theories. Subsequently, we explore the potential of utilizing ringdown studies to test general relativity and its modifications. Initially, we conduct a study using metric perturbation analyses to calculate the quasinormal mode spectra of slowly rotating black holes in dynamical Chern-Simons gravity. Encouragingly, our calculations demonstrate promising prospects for the future, as signatures of deviations from general relativity can potentially be observed in the quasinormal mode spectra of black holes in dynamical Chern-Simons gravity and studied using current and future gravitational wave detectors given a strong signal-to-noise ratio and extraction of at least two modes from the signal. Building upon this, we develop a novel formalism, drawing inspiration from the Teukolsky formalism which is applicable in general relativity, to derive a master equation for rotating black holes in modified theories of gravity that admit a Petrov type I black hole under the Petrov classification. This formalism yields completely decoupled evolution equations for the \Psi₀ and \Psi₄ Weyl scalars, which describe the gravitational wave perturbations. Furthermore, we demonstrate the applicability of our new formalism through a toy model of slowly rotating black holes in dynamical Chern-Simons gravity, obtaining a master equation that is separable in the radial and angular coordinates. Integration of this master equation allows for the determination of quasinormal mode spectra for black holes in modified theories of gravity. Overall, this dissertation contributes to the exploration of modifications to general relativity in the context of strong gravitational interactions. The findings shed light on the behavior of gravitational waves and binary pulsars in the presence of non-minimal couplings and provide insights into the potential deviations from general relativity in dynamical Chern-Simons gravity and Einstein dilaton Gauss-Bonnet gravity.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Wagle, Pratik Kanteshwar

Contributors dc:contributor

• Yunes, Nicolas
• Witek, Helvi
• Shelton, Jessie
• Filippini, Jeffrey

• Gravity
• Gravitational Waves
• Tests Of General Relativity

Statement dc:rights

• Copyright 2023 Pratik Wagle

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/121926

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F121926