Hong Qi, PhD Candidate, UWM Department of Physics.

Studies in Gravitational-wave Astronomy & Tests of General Relativity

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.

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 GR and the chameleon field theory.

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 using the data from a Mock Data Challenge.