Towards Neutrino Mass from Cosmology without Optical Depth Information

TL;DR

This paper demonstrates that combining low-redshift galaxy surveys and CMB lensing can constrain neutrino mass independently of optical depth measurements, potentially surpassing current limits.

Contribution

It introduces a method to measure neutrino mass using low-redshift data without relying on optical depth, achieving competitive precision.

Findings

Neutrino mass constraints of 25 meV are possible without optical depth info.

Combining LSST and CMB-S4 data yields competitive neutrino mass limits.

Structure growth and galaxy power spectrum shape are key to these constraints.

Abstract

With low redshift probes reaching unprecedented precision, uncertainty of the CMB optical depth is expected to be the limiting factor for future cosmological neutrino mass constraints. In this paper, we discuss to what extent combinations of CMB lensing and galaxy surveys measurements at low redshifts $z\sim 0.5-5$ will be able to make competitive neutrino mass measurements without relying on any optical depth constraints. We find that the combination of LSST galaxies and CMB-S4 lensing should be able to achieve constraints on the neutrino mass sum of 25meV without optical depth information, an independent measurement that is competitive with or slightly better than the constraint of 30meV possible with CMB-S4 and present-day optical depth measurements. These constraints originate both in structure growth probed by cross-correlation tomography over a wide redshift range as well as, most importantly, the shape of the galaxy power spectrum measured over a large volume. We caution that possible complications such as higher-order biasing and systematic errors in the analysis of high redshift galaxy clustering are only briefly discussed and may be non-negligible. Nevertheless, our results show that new kinds of high-precision neutrino mass measurements at and beyond the present-day optical depth limit may be possible.