Cosmological searches for the neutrino mass scale and mass ordering

TL;DR

This thesis advances neutrino cosmology by refining mass limits, exploring mass ordering preferences, addressing galaxy bias issues, and analyzing parameter correlations, thereby enhancing the understanding of neutrino properties through cosmological data.

Contribution

It introduces new methods for calibrating galaxy bias affected by massive neutrinos and quantifies the preference for the normal neutrino mass ordering using cosmological data.

Findings

Best upper limit on neutrino mass sum: 0.12 eV

Weak preference for normal mass ordering

Galaxy bias calibration method using CMB lensing-galaxy cross-correlations

Abstract

In this thesis, I describe a number of recent important developments in neutrino cosmology on three fronts. Firstly, focusing on Large-Scale Structure (LSS) data, I will show that current cosmological probes contain a wealth of information on the sum of the neutrino masses. I report on the analysis leading to the currently best upper limit on the sum of the neutrino masses of $0.12\,{\rm eV}$. I show how cosmological data exhibits a weak preference for the normal neutrino mass ordering because of parameter space volume effects, and propose a simple method to quantify this preference. Secondly, I will discuss how galaxy bias represents a severe limitation towards fully capitalizing on the neutrino information hidden in LSS data. I propose a method for calibrating the scale-dependent galaxy bias using CMB lensing-galaxy cross-correlations. Moreover, in the presence of massive neutrinos, the usual definition of bias becomes inadequate, as it leads to a scale-dependence on large scales which has never been accounted for. I show that failure to define the bias appropriately will be a problem for future LSS surveys, and propose a simple recipe to account for the effect of massive neutrinos on galaxy bias. Finally, I discuss implications of correlations between neutrino parameters and other cosmological parameters. In non-phantom dynamical dark energy models, the upper limit on the sum of the neutrino masses becomes tighter than the $\Lambda$CDM limit. Therefore, such models exhibit an even stronger preference for the normal ordering, and their viability could be jeopardized should near-future laboratory experiments determine that the mass ordering is inverted. I then discuss correlations between neutrino and inflationary parameters. I find that our determination of inflationary parameters is stable against assumptions about the neutrino sector. (abridged)