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

TL;DR

The paper addresses the apparent need for extra radiation in the early universe suggested by CMB measurements. It proposes that a small fraction of dark matter produced via late-time, non-thermal decays can mimic an additional light neutrino species in the pre-recombination expansion history, characterized by $ΔN_{Eff}^{ν}$. A key result relates the decay parameters to the effective neutrino count via $ΔN^{ν}_{Eff} ≈ 4.8×10^{-3} (τ/10^{6}\,s)^{1/2} (m_{X'}/m_X + m_X/m_{X'} - 2) f$, with $f<0.01$, and shows that certain decay scenarios reproduce the expansion history of $N_{Eff}^{ν}=4$. Constraints from large-scale structure and BBN limit the viable region (e.g., $f<0.01$ and decays without photons can have longer lifetimes), but they still permit acceptable parameter space, making Planck data crucial for testing this degeneracy.

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.

Figures (2)

• Figure 1: The effect on the universe's expansion history of additional neutrino species, and of late-time decays. In the upper frame we show the fractional change of the Hubble constant as a function of scale factor. In the lower frame, we show the fractional change in the scale factor as a function of time. In each case, we show results for three late-time decay scenarios (solid curves). We compare these results to that predicted from additional neutrino species (dashed curves) and find that these three scenarios can each effectively mimic the presence of approximately one additional light neutrino species in the early universe. Note that in each case shown, the relativistic decay products, $X$, only make up a fraction $f=0.01$ of the total dark matter density.
• Figure 2: Constraints on the late-time decay scenario discussed in this paper from measurements of the light element abundances bbn. Shown for comparison are the contours which correspond to the parameter space which can mimic 1, 0.5, or 0.1 additional effective neutrino species. These constraints apply specifically to decays of the form $X' \rightarrow X + \gamma$, and can be evaded in other cases ($X' \rightarrow X + \nu$, for example). Results here are shown assuming $m_{X'} \gg m_{X}$.