Cross-correlation of stochastic gravitational wave background and galaxy number counts and Its angular power sectrum

Abstract

In this dissertation, I present my study of the cross-correlation between the anisotropy in stochas- tic gravitational-wave background (SGWB) and the distribution of galaxies across the sky. This study will improve the sensitivity of future searches for anisotropy in the SGWB and expand the use of SGWB anisotropy to probe the formation of structure in the Universe. It has three parts.First, I use the spherical harmonic decomposition of the anisotropic SGWB measured in the second observing run (O2) of LIGO in 50 Hz-wide frequency bands and convert them into pixel-based sky maps in HEALPIX basis. Then I take the galaxies in the Sloan Digital Sky Survey (SDSS) photometric and spectroscopic catalog in 0.1-wide redshift bins and count them in pixels of HEALPIX basis to form number count sky maps at the same angular resolution as the SGWB maps. I compute the pixel-based coherence between the SGWB maps in frequency bands and galaxy count maps in redshift bins and find that the coherence is dominated by the null measurement noise in the SGWB maps and is not statistically significant. Second, I use the anisotropic SGWB measured in the first three observing runs (O1-O3) of LIGO in 10 Hz-wide frequency bins and galaxy count over-density from the SDSS spec- troscopic catalog without binning in redshift to compute the angular power spectra of their auto-correlations and cross-correlation. I compare the observed cross-correlation to the spec- tra predicted by astrophysical models. I apply a Bayesian formalism to explore the parameter space of the theoretical models, and I set constraints on a set of astrophysical parameters of the astrophysical kernel describing the galactic process of gravitational-wave (GW) emission with or without the inclusion of shot noise. Third, I estimate the amplitude and spatial anisotropy in the SGWB energy density due to compact binary coalescence (CBC) events occurring in galaxies in the Flagship Mock Catalogue developed by the Euclid Consortium. For each galaxy in the catalogue, I use the simulated stellar mass and star formation to constrain the galaxy’s star formation history and predict its contribution to the SGWB. I also compare these predictions to the astrophysical models above and explore the parameter space with an ideal covariance dominated by cosmic variance to set constraints on the parameters.