Non-Thermal Dark Matter Mimicking An Additional Neutrino Species In The Early Universe

TL;DR

This paper proposes that non-thermal dark matter produced by late decays can mimic the effects of an additional neutrino species in the early universe, aligning with CMB observations without conflicting with nucleosynthesis data.

Contribution

It introduces a scenario where a small fraction of dark matter from late decays explains the extra radiation density, offering an alternative to extra neutrino species.

Findings

Dark matter decays can replicate the effects of an extra neutrino in the CMB.

This scenario is consistent with nucleosynthesis constraints.

Less than 1% of dark matter from decays suffices to match observations.

Abstract

The South Pole Telescope (SPT), Atacama Cosmology Telescope (ACT), and Wilkinson Microwave Anisotropy Probe (WMAP) have each reported measurements of the cosmic microwave background's (CMB) angular power spectrum which favor the existence of roughly one additional neutrino species, in addition to the three contained in the standard model of particle physics. Neutrinos influence the CMB by contributing to the radiation density, which alters the expansion rate of the universe during the epoch leading up to recombination. In this paper, we consider an alternative possibility that the excess kinetic energy implied by these measurements was possessed by dark matter particles that were produced through a non-thermal mechanism, such as late-time decays. In particular, we find that if a small fraction (<1%) of the dark matter in the universe today were produced through the decays of a heavy and relatively long-lived state, the expansion history of the universe can be indistinguishable from that predicted in the standard cosmological model with an additional neutrino. Furthermore, if these decays take place after the completion of big bang nucleosynthesis, this scenario can avoid tension with the value of three neutrino species preferred by measurements of the light element abundances.