Neutrino Interactions in the Late Universe

TL;DR

This paper investigates how light right-handed neutrinos coupled to dark matter affect late-time cosmological observations, potentially mimicking increased neutrino mass and offering a new probe of neutrino physics beyond the standard model.

Contribution

It introduces a novel scenario where neutrino-dark matter coupling influences late universe cosmology, providing a new observational signature for physics beyond the standard model.

Findings

Coupling affects structure growth at late times.

Signature mimics increased neutrino mass in surveys.

Limited to 0.5% of dark matter can be coupled under current constraints.

Abstract

The cosmic neutrino background is both a dramatic prediction of the hot Big Bang and a compelling target for current and future observations. The impact of relativistic neutrinos in the early universe has been observed at high significance in a number of cosmological probes. In addition, the non-zero mass of neutrinos alters the growth of structure at late times, and this signature is a target for a number of upcoming surveys. These measurements are sensitive to the physics of the neutrino and could be used to probe physics beyond the standard model in the neutrino sector. We explore an intriguing possibility where light right-handed neutrinos are coupled to all, or a fraction of, the dark matter through a mediator. In a wide range of parameter space, this interaction only becomes important at late times and is uniquely probed by late-time cosmological observables. Due to this coupling, the dark matter and neutrinos behave as a single fluid with a non-trivial sound speed, leading to a suppression of power on small scales. In current and near-term cosmological surveys, this signature is equivalent to an increase in the sum of the neutrino masses. Given current limits, we show that at most 0.5% of the dark matter could be coupled to neutrinos in this way.