Precision Cosmology with Emission-Line Galaxies

Abstract

In the recent decade, low redshift weak lensing surveys have shown a ~2-3σ tension compared to cosmic microwave background (CMB) anisotropy surveys when it comes to measuring the amplitude of matter clustering, σ8. If the same tension is found at a higher precision, then it could potentially point to the existence beyond ΛCDM physics. Next-generation surveys such as the Dark Energy Spectroscopic Instrument aims to answer tensions like these by probing the Universe at higher redshift, especially with emission-line galaxies (ELG) that trace large-scale structure in the critical turnover period when the Universe was being affected by dark energy substantially for the first time. This dissertation aims to address such cosmological tensions by (i) improving the DESI ELG target selection to secure a high yield of reliable spectroscopic redshifts, (ii) studying the impact of biases arising from galaxy window function estimation, and (iii) cross-correlating DESI-like ELGs from the Legacy Surveys and Planck CMB lensing. The findings of this work resulted in (i) validating a DESI ELG target selection algorithm that meets survey specifications, (ii) detecting biases in galaxy window function estimation that can shift the amplitude of clustering inference by ~2.6% and degrade the error ellipse of the galaxy linear bias-A_s plane by ~25%, and (iii) providing a high-precision measurement of σ8 = 0.696 ± 0.028. These findings collectively result in one of the highest tensions in σ8 compared to CMB anisotropy probes. More importantly, these findings show that future cross-correlation analyses of ELGs and CMB lensing will have the power to definitively discern whether the tensions point to new physics.