Date of Award

May 2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Patrick R. Brady

Committee Members

Paul F. Lyman, Philip Chang, David Kaplan, Jolien D.E. Creighton

Keywords

black hole, cosmology, general relativity, gravitational wave, Hubble constant, neutron star

Abstract

Modern astronomical data sets provide the opportunity to test our physical theories of the Universe at unprecedented levels of accuracy. This dissertation examines approaches to testing gravitational theories using a) observations of stars orbiting the center of the Milky Way; b) observations of the pulsations of Cepheid variable stars in dwarf galaxies; and c) gravitational-wave observations of compact binary mergers.

Observations of stars orbiting the center of the Milky Way have been used to infer the mass of the putative black hole that exists there. I discuss how well present and future measurements of stellar orbits can constrain the black hole properties: both its mass and its spin. Specifically, I used a Markov-Chain Monte Carlo (MCMC) code to compare real and synthetic astrometric and radial velocity data with models for the stellar orbits and black hole mass, accounting for differences in reference frame between different observational campaigns. Unlike previous investigations, our model includes leading order post-Newtonian corrections to the orbit from the black hole's mass, spin, and quadrupole moment, as well as the impact of unknown non-quadrupole internal and exterior potentials. I present strategies for future observations to measure the Galactic Center black hole spin and even the black hole No-hair Theorem.

Chameleon field theory is one of the attempts to explain the observed acceleration of our universe. I demonstrate the testing of chameleon field theory on stellar structure scales with the distance indicator Cepheid variable stars. Using the numerical results obtained for the evolutions of stars from MESA, I calculate the pulsation rates of Cepheid variable stars with both the theory of general relativity (GR) and the chameleon field theory. I find that the period-temperature relation is not that simple as we previously thought, which assumed that the equivalent effect of a chameleon field is an enhancement of gravity and should result in faster pulsations of the Cepheids. I discuss strategies to use observations of Cepheids to test chameleon fields.

The first direct detection of gravitational waves was made on Sep 14, 2015 by the two advanced detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO). The waves came from the coalescence of a binary black hole (BBH) system. Since then, the LIGO-Virgo Collaboration's (LVC) has reported multiple detections of binary black hole mergers. On August 17, 2017 a binary neutron star (BNS) merger was detected by LIGO and Virgo.

Subsequently, an electromagnetic (EM) counterpart was observed in the host galaxy NGC4993. The unprecedented event provides brand new insights into astrophysics and cosmology. I present an approach to using gravitational waves to measure the expansion of the Universe. The methods have been tested in end-to-end simulations of the gravitational-wave analysis chain. After we detected the BNS and its EM counterpart, I applied my tools on the event and constrained $H_0$. I also present studies of the statistical method of measuring $H_0$ carried out in collaboration with the LVC Cosmology group.

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