2025 Theses Doctoral

Gravitational wave topics: black hole ringdown and instrumental noise

Siegel, Harrison

This thesis contributes new understanding of gravitational wave (GW) science in the formof more refined data analysis methods, possible observation of new black hole ringdown phenomena, phenomenological modeling of precessing binary black hole (BBH) coalescences, and theoretical characterization of thermal charge carrier noise in semiconductor optics for future GW detectors.

In chapter 1 (published in Ref. [1]), we lay some of the foundations for the analysis of black hole ringdowns in timeseries data. We demonstrate that the posteriors of our analysis can be corrupted if one does not carefully apply data conditioning operations such as downsampling, filtering, and data segment truncation, and we show that sharp lines in the noise power spectral density can necessitate the analysis of unusually long data segments in order to capture the entire signal.

In chapter 2 (published in Ref. [2]), we perform an analysis of an exceptional ringdown, from the GW190521 signal. We raise the possibility that previous analyses of this ringdown may not have fully characterized the signal. We propose a model of this ringdown which may make it the first known signal to demonstrate a previously underappreciated phenomenon, namely the strong excitation of certain quasinormal modes (QNMs) due to binary spin-orbit misalignment.

In chapter 3 (published in Ref. [3]), we theoretically validate our phenomenological model of QNM excitation and binary spin-orbit misalignment which we proposed in our analysis of GW190521. We demonstrate that our model seems to accurately describe numerical relativity (NR) simulations of precessing BBHs, i.e. BBHs with spin-orbit misalignment. We also show how current GW models of precessing binaries may be systematically biased.

In chapter 4 (from Ref. [4]), we expand upon previous theoretical observations of a trend in the amplitudes of overtone QNMs in NR simulations of BBH coalescences. Overtone amplitude ratios in these simulations seem to respect a strict relationship that depends solely on their excitation factors, parameters which are intrinsic to the geometry of the black hole spacetime. We comment on both the theoretical and observational implications of this finding.

In chapter 5 (published in Ref. [5]), we compute the noise power spectrum from refractive index variations induced by thermal fluctuations of charge carrier density in semiconductor optics for future GW interferometer designs. We compare our computation with a similar previous work, and show that our more general approach produces significantly different results. We conclude with a brief discussion of what next steps could be taken to build on our work.