Neutrinoless Universe

TL;DR

The relic neutrinos would annihilate to bosons at late times and thus make a negligible contribution to the matter density today, which evades the neutrino mass limits arising from large scale structure.

Abstract

We consider the consequences for the relic neutrino abundance if extra neutrino interactions are allowed, e.g., the coupling of neutrinos to a light (compared to $m_ν$) boson. For a wide range of couplings not excluded by other considerations, the relic neutrinos would annihilate to bosons at late times, and thus make a negligible contribution to the matter density today. This mechanism evades the neutrino mass limits arising from large scale structure.

Figures (2)

• Figure 1: Evolution of the energy density as a function of the scale factor $a$. Heavy curves at top are total energy density including matter, photons, and neutrinos; light curves at bottom are energy density in the neutrino sector (including $\phi$'s in the interacting case). Three different scenarios are depicted, differing in neutrino content: three massless neutrinos (solid), three degenerate standard model neutrinos with $\sum m_\nu = 1$ eV (dotted); and three interacting degenerate neutrinos plus massless $\phi$ (dashed). We use the same total matter density, $\Omega_m =0.3$, throughout; $\rho_{\text{c}r}$ denotes the critial debsity today.
• Figure 2: The ratio of power spectra $P/P(m_\nu=0)$ where $P(m_\nu=0)$ is the power spectrum for the standard scenario with massless neutrinos. The solid curves show this ratio for various (degenerate) neutrino masses in the interacting scenario. Dashed curves show the ratio in the standard scenario, for which the current limit is $\sum m_\nu < 1-2$ eV. Note that the tritium bound, $\sum m_\nu <$ 6.6 eV, always applies.