Neutrinos as Dark Matter

TL;DR

This paper proposes that enhanced densities of cold active neutrinos, produced by late-time decays of a light scalar, could serve as dark matter, offering testable predictions via the Cosmic Neutrino Background.

Contribution

It introduces a novel mechanism for neutrinos to act as dark matter through late-time scalar decays, avoiding previous constraints and suggesting observable signatures.

Findings

Enhanced neutrino density could be 100-200 times larger than in ΛCDM.

Detectable neutrino background signals are predicted due to this mechanism.

A Majoron scalar with specific Yukawa couplings is a viable candidate.

Abstract

Active neutrinos in standard cosmology were ruled out as a dark matter candidate in the 1980's. The reason is twofold: they are too light to account for the observed energy density of dark matter in the Universe, and their relativistic nature would spoil structure formation. In this note we suggest that an enhanced density of cold Standard Model active neutrinos today could behave effectively as dark matter, avoiding constraints from recombination and structure formation. Such an enhancement could be produced, for instance, by late-time decays of a light scalar field that is not in thermal equilibrium with the plasma. This mechanism is testable through the detection of the Cosmic Neutrino Background (C$\nu$B), which could have an average cosmological energy density a factor of $\sim 100-200$ times larger than expected in $\Lambda$CDM. The postulated light neutrinophilic scalar field may be observable, with Yukawa couplings in the range $y \sim 5 \times 10^{-16}-10^{-12}$. A scenario preferred by structure formation constraints is that the scalar is a Majoron, and the neutrinos have an inverted mass hierarchy.