Origin of Delta N_eff as a Result of an Interaction between Dark Radiation and Dark Matter

TL;DR

This paper investigates how interactions between dark radiation and dark matter could explain the observed excess of effective neutrino species, proposing a decay mechanism that increases dark radiation after Big Bang Nucleosynthesis and fits cosmological data.

Contribution

It introduces a generic decay scenario where dark matter produces dark radiation, linking it to the observed Delta N_eff and providing constraints from cosmological observations.

Findings

Dark radiation produced by dark matter decay can account for Delta N_eff.

The model fits WMAP, ACT, and SPT data within certain decay fraction limits.

Dark matter decay into dark radiation can relax tensions with light element abundance measurements.

Abstract

Results from the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama Cosmology Telescope (ACT) and recently from the South Pole Telescope (SPT) have indicated the possible existence of an extra radiation component in addition to the well known three neutrino species predicted by the Standard Model of particle physics. In this paper, we explore the possibility of the apparent extra dark radiation being linked directly to the physics of cold dark matter (CDM). In particular, we consider a generic scenario where dark radiation, as a result of an interaction, is produced directly by a fraction of the dark matter density effectively decaying into dark radiation. At an early epoch when the dark matter density is negligible, as an obvious consequence, the density of dark radiation is also very small. As the Universe approaches matter radiation equality, the dark matter density starts to dominate thereby increasing the content of dark radiation and changing the expansion rate of the Universe. As this increase in dark radiation content happens naturally after Big Bang Nucleosynthesis (BBN), it can relax the possible tension with lower values of radiation degrees of freedom measured from light element abundances compared to that of the CMB. We numerically confront this scenario with WMAP+ACT and WMAP+SPT data and derive an upper limit on the allowed fraction of dark matter decaying into dark radiation.