Dark Radiation Emerging After Big Bang Nucleosynthesis?

TL;DR

The paper addresses a potential mismatch between the effective number of neutrino species inferred during big bang nucleosynthesis ($N_{eff}^{BBN}$) and at the time of the cosmic microwave background ($N_{eff}^{CMB}$). It proposes a general mechanism in which a subdominant non-relativistic, SM-singlet component decays after BBN into dark radiation, yielding a calculable increase in $N_{eff}$ given by $\Delta N_{eff} = \left(\frac{8}{7}\right)\left(\frac{11}{4}\right)^{4/3}\left(\frac{\tau}{10^{-4}\mathrm{s}}\right)^{1/2}\frac{\rho_X[t=10^{-4}\mathrm{s}]}{\rho_0}$, while satisfying $N_{eff}<3.2$ during BBN and ensuring decay before horizon reentry. The authors provide two concrete realizations—an invisible photino in a SUSY framework and an oscillating (pseudo)scalar decaying via a dimension-5 operator—to illustrate how such dark radiation could arise and discuss observational implications for Planck-era data. This mechanism offers a natural way to reconcile a possible difference between $N_{eff}^{BBN}$ and $N_{eff}^{CMB}$ and predicts gravitational signatures that could be tested by future CMB measurements. The work thus links early-universe nucleosynthesis, late-time radiation content, and particle-physics model-building into a coherent framework for additional dark radiation.

Abstract

We show how recent data from observations of the cosmic microwave background may suggest the presence of additional radiation density which appeared after big bang nucleosynthesis. We propose a general scheme by which this radiation could be produced from the decay of non-relativistic matter, we place constraints on the properties of such matter, and we give specific examples of scenarios in which this general scheme may be realized.

Figures (1)

• Figure 1: These figures illustrate the constraints on energy density and lifetime of non-relativistic matter decaying into radiation. The vertical axis gives the ratio of the energy density with $\rho_0 \equiv a_{\mathcal{B}}(1.334\times10^{12}\rm{K})^4=1.149\times10^{-4} \mathrm{GeV}^4$ evaluated at $10^{-4}$ seconds, and the horizontal axis shows the lifetime reported in seconds. The blue shaded region near the top of the figure is excluded due to the constraint that $N_{eff}<3.2$ during big bang nucleosynthesis while the shaded region near the right is excluded by requiring that the decay into radiation occurs before the highest $l$ modes observable in the cosmic microwave background reenter the horizon. The dark and light orange shaded regions give the 1- and 2-$\sigma$ constraints, and the black dashed line corresponds to the current observed central value of $N_{eff}^{CMB}=4.34$ from WMAP. The constraints from WMAP 7-year data are consistent with no extra energy density, while the projected constraints from Planck require a nonzero energy density and fix the lifetime of the decaying matter.