Neutrino interactions in the late universe

Abstract

The cosmic neutrino background is both a dramatic prediction of the hot Big Bang and a compelling target for current and future observations. The impact of relativistic neutrinos in the early universe has been observed at high significance in a number of cosmological probes. In addition, the non-zero mass of neutrinos alters the growth of structure at late times, and this signature is a target for a number of upcoming surveys. These measurements are sensitive to the physics of the neutrino and could be used to probe physics beyond the standard model in the neutrino sector. We explore an intriguing possibility where light right-handed neutrinos are coupled to all, or a fraction of, the dark matter through a mediator. In a wide range of parameter space, this interaction only becomes important at late times and is uniquely probed by late-time cosmological observables. Due to this coupling, the dark matter and neutrinos behave as a single fluid with a non-trivial sound speed, leading to a suppression of power on small scales. In current and near-term cosmological surveys, this signature is equivalent to an increase in the sum of the neutrino masses. Given current limits, we show that at most 0.5% of the dark matter could be coupled to neutrinos in this way.

Dark matter produced from neutrinos

“…figure 3). 16 Hence, for points below the orange line, χ cannot account for all of DM, and we instead assume that the remaining abundance is accounted for by standard cold DM. Also, in our analysis, we explicitly set N ν = 1 unlike before, as PTOLEMY is only sensitive to electron neutrinos.…”

“…Since both neutrinos and dark matter (DM) are two of the most compelling pieces of evidence for new physics beyond the Standard Model (BSM), it is conceivable that both particles are indeed connected and thus feature non-trivial interactions among each other. Such a scenario could then potentially induce noticeable changes to the evolution of the Universe [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], as DM-neutrino interactions are capable of changing the anisotropies in the cosmic microwave background (CMB) [1,3,12,13], modifying big bang nucleosynthesis (BBN) [10,11], and/or affecting structure formation at small scales [2,4]. Additionally, one might ask the question if interactions between DM and neutrinos could also be responsible for setting the correct relic abundance, e.g.…”

The Physics of Light Relics

“…Similarly, models in which a fraction of the dark matter acquires a large velocity after recombination (e.g. via DM-neutrino scattering [92]) will lead to a suppression of clustering in a manner similar to that described above. Despite the different physical mechanism, the observable consequences for cosmology are also similar to a change to 𝑚 𝜈 [91].…”

Neutrino bound states and bound systems

“…In particular, neutrino interactions with new and very light bosons have been actively explored, with considerations covering e.g. oscillation phenomena [1][2][3][4][5][6] and various astrophysical and cosmological consequences [7][8][9][10][11][12]. Yet another possible consequence of this interaction (the subject of this paper) is the formation of bound states and bound systems of neutrinos.…”