The cosmology of ultralight scalar dark matter coupled to right-handed neutrinos

TL;DR

This paper explores how ultralight scalar dark matter coupled to right-handed neutrinos affects cosmological evolution, revealing new dynamics and implications for interpreting cosmological and laboratory neutrino data.

Contribution

It introduces the impact of high neutrino densities on scalar dark matter evolution, a factor previously neglected, and analyzes its effects on cosmological observations and neutrino experiments.

Findings

Scalar field experiences an asymmetric potential.

Dark matter density and neutrino mass oscillations can vary significantly.

Neutrino measurements complement cosmological constraints.

Abstract

We consider ultralight scalar dark matter that couples to right-handed neutrinos. Due to the high density of neutrinos in the early universe, the background neutrino density dominates the dynamics of the scalar field, and qualitatively alters the field's cosmological evolution. This effect has not been included in previous literature, and changes the interpretation of cosmological data and its interplay with laboratory experiments. To illustrate these points a simplified model of a $1+1$ setup with a single scalar field is analyzed. We find that: {\it i}) The scalar field experiences an asymmetric potential and its energy density redshifts differently than ordinary matter. {\it ii}) Neutrino mass measurements at the CMB and oscillation experiments performed today complement one another (i.e., they constrain different regions of parameter space). {\it iii}) There exists potentially interesting cosmologies with either $O(1)$ variations in the dark matter density between the CMB and today, or $O(1)$ oscillations of neutrino mass.