Equivalent neutrinos, light WIMPs, and the chimera of dark radiation

Abstract

According to conventional wisdom, in the standard model (SM) of particle physics and cosmology the "effective number of neutrinos" measured in the late Universe is N eff = 3 (more precisely, 3.046). For extensions of the standard model allowing for the presence of ∆Nν "equivalent neutrinos" (or "dark radiation"), it is generally the case that N eff > 3. These canonical results are reconsidered, demonstrating that a measurement of N eff > 3 can be consistent with ∆Nν = 0 ("dark radiation without dark radiation"). Conversely, a measurement consistent with N eff = 3 is not inconsistent with the presence of dark radiation (∆Nν > 0). In particular, if there is a light WIMP that annihilates to photons after the SM neutrinos have decoupled, the photons are heated beyond their usual heating from e ± annihilation, reducing the late time ratio of neutrino and photon temperatures (and number densities), leading to N eff < 3. This opens the window for one or more equivalent neutrinos, including "sterile neutrinos", to be consistent with N eff = 3. By reducing the neutrino number density in the present Universe, this also allows for more massive neutrinos, relaxing the current constraints on the sum of the neutrino masses. In contrast, if the light WIMP couples only to the SM neutrinos and not to the photons and e ± pairs, its late time annihilation heats the neutrinos but not the photons, resulting in N eff > 3 even in the absence of equivalent neutrinos or dark radiation. A measurement of N eff > 3 is no guarantee of the presence of equivalent neutrinos or dark radiation. In the presence of a light WIMP and/or equivalent neutrinos there are degeneracies among the light WIMP mass and its nature (fermion or boson, as well as its couplings to neutrinos and photons), the number and nature (fermion or boson) of the equivalent neutrinos, and their decoupling temperature (the strength of their interactions with the SM particles). As the analysis here reveals, there's more to a measurement of N eff than meets the eye. * steigman.1@asc.ohio-state.edu 2

BBN and the CMB constrain light, electromagnetically coupled WIMPs

“…Recently, one of us (G.S.) has explored the effect on the cosmic microwave background (CMB) measurement of the effective number of neutrinos, N eff , of a sufficiently light WIMP that its late-time annihilation heats either the photons or the SM neutrinos beyond the usual heating from e ± annihilation [13]. This analysis had some overlap with the earlier work of Kolb et al [1] and of Serpico and Raffelt [2], and with the more recent analyses of Ho and Scherrer [14,15] and Boehm et al [16].…”

Combining Planck data with large scale structure information gives a strong neutrino mass constraint

“…Even though such large masses are ruled out by structure formation if the neutrinos are thermalized [20][21][22][23][24][25], those constraints can be circumvented by nonstandard physics mechanisms [26][27][28]. We have analyzed one such short baselineinspired scenario called ΛCDM 3þ1ν .…”

Dark matter-neutrino interactions through the lens of their cosmological implications

“…However, N eff cannot be arbitrarily large as this would impact the formation of light elements at the time of big bang nucleosynthesis (BBN) [38][39][40][41][42][43] and the CMB angular power spectrum at decoupling [44][45][46][47][48][49][50][51][52][53][54]. This condition eventually rules out DM candidates much lighter than a few MeVs [52,55].…”