Relic neutrino decay solution to the excess radio background

Abstract

The excess radio background detected by ARCADE 2 represents a puzzle within the standard cosmological model. There is no clear viable astrophysical solution, and therefore, it might indicate the presence of new physics. Radiative decays of a relic neutrino ${\nu }_i$ (either $i=1$, or $i=2$, or $i=3$) into a sterile neutrino ${\nu }_{\mathrm{s}}$, assumed to be quasi-degenerate, provide a solution that currently evades all constraints posed by different cosmological observations and reproduces very well the ARCADE 2 data. We find a very good fit to the ARCADE 2 data with best fit values ${\tau }_i=1.46\times 10^{21}\mathrm{s}$ and $\Delta m_i=4.0\times 10^{-5}\mathrm{eV}$, where ${\tau }_i$ is the lifetime and $\Delta m_i$ is the mass difference between the decaying active neutrino and the sterile neutrino. On the other hand, if relic neutrino decays do not explain ARCADE 2 data, then these place a stringent constraint $\Delta m_i^{3/2}{\tau }_i\gtrsim 2\times 10^{14}{\mathrm{eV}}^{3/2}\mathrm{s}$ in the range $1.4\times 10^{-5}\mathrm{eV}<\Delta m_i<2.5\times 10^{-4}\mathrm{eV}$. The solution also predicts a stronger 21 cm absorption global signal than the predicted one from the $\Lambda$CDM model, with a contrast brightness temperature $T_{21}=-238_{-20}^{+21}\mathrm{mK}$ ($99\%$ C.L.) at redshift $z\simeq 17$. This is in mild tension with the even stronger signal found by the EDGES collaboration, $T_{21}=-500_{-500}^{+200}\mathrm{mK}$, suggesting that this might have been overestimated, possibly receiving a contribution from some unidentified foreground source.

Comments: 21 pages, 3 figures, 3 tables, v2: it matches version to be published in JCAP