Analysis of tidal dynamics of binary neutron stars and properties of gravitational collapse beyond general relativity

Gravitational wave emission from compact binary sources has the potential to probe the dynamics of matter and gravity in extremely compact regions of spacetime. In this thesis, we explore the ability of gravitational wave observations to probe the internal nature of neutron stars and understand the nature of gravitational collapse in theories beyond general relativity. In Part I of this dissertation, we will examine the ability of gravitational wave observations to understand the dynamical tidal excitations in a binary neutron star system, probe the internal viscous process inside a neutron star, and measure the Hubble constant. We first build models of time-dependent tidal dynamics of neutron star binary systems using tools from relativistic fluid perturbation theory and post-Newtonian theory. To obtain the dynamical and dissipative tidal response of a non-rotating neutron star, we treat the fluid and gravitational perturbations inside a neutron star exactly and re-sum the external gravitational solution obtained in a small frequency approximation to consistently match to a post-Newtonian metric. The resummation procedure allows one to calculate the dynamical tidal response during the late inspiral of a binary neutron star system. Next, we use post-Newtonian theory techniques to calculate the gravitational waveform due to dissipative tidal interactions, which is accurate to 1 post-Newtonian order. The gravitational waveform obtained from this analysis is used to place the first constraint on the dissipative tidal deformability of a neutron star from GW170817 data. We also use binary-love relations to understand how well we can constrain the Hubble constant using third-generation detectors. Our analysis reveals that third-generation detectors with $\mathcal{O}(1000)$ detections could allow one to constrain the Hubble constant to percent-level precision. In Part II, we understand the theoretical properties of gravitational collapse in theories beyond general relativity. In the decoupling limit, we show analytically and numerically that the scalar radiation resulting from the formation of the horizon leads to the loss of monopole and dipole hair in Einstein scalar Gauss-Bonnet gravity and dynamical Chern-Simons theory. We then understand the breakdown of hyperbolicity in Einstein scalar Gauss-Bonnet theory. We show that the breakdown of hyperbolicity is linked to the growth of strong gradients during gravitational collapse and provide sufficient criteria for the breakdown of hyperbolicity during spherical collapse. Using the insights from the analytical result, we numerically explore the process of scalarization of black holes in Einstein scalar Gauss-Bonnet gravity and show that a fine-tuning of initial data is required to achieve scalarization without breaking hyperbolicity.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Hegade Kumbale Raveesha, Abhishek

Contributors dc:contributor

• Yunes, Nicolás
• Noronha, Jorge
• Gammie, Charles Forbes
• Witek, Helvi

• Gravitational Waves
• General Relativity

Statement dc:rights

• Copyright 2025 Abhishek Hegade Kumbale Raveesha

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129904

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129904