Neutrinos as Dark Matter

Abstract

Active neutrinos in standard cosmology were ruled out as a dark matter candidate in the 1980's. The reason is twofold: they are too light to account for the observed energy density of dark matter in the Universe, and their relativistic nature would spoil structure formation. In this note we suggest that an enhanced density of cold Standard Model active neutrinos today could behave effectively as dark matter, avoiding constraints from recombination and structure formation. Such an enhancement could be produced, for instance, by late-time decays of a light scalar field that is not in thermal equilibrium with the plasma. This mechanism is testable through the detection of the Cosmic Neutrino Background (C$ν$B), which could have an average cosmological energy density a factor of $\sim 100-200$ times larger than expected in $Λ$CDM. The postulated light neutrinophilic scalar field may be observable, with Yukawa couplings in the range $y \sim 5 \times 10^{-16}-10^{-12}$. A scenario preferred by structure formation constraints is that the scalar is a Majoron, and the neutrinos have an inverted mass hierarchy.

Figures (6)

• Figure 1: Yukawa coupling of light scalar $\phi$ to neutrinos $\nu$ versus $m_{\phi}$, with $m_{\nu}=0.1$ eV representing the heaviest active neutrino mass. Shaded regions are ruled out as labeled on the figure. Horizontal dotted lines indicate decays occuring at redshifts 2 and 20.
• Figure 2: Left: Predicted enhancement relative to the $\Lambda$CDM cosmic neutrino background, as a function of the lightest neutrino mass. Solid black line: overdensity if active neutrinos are all of the dark matter. Shaded grey band: a fraction of the DM is in neutrinos on diffuse cosmological scales, while $\phi$ is rest of DM at small scales. Red region: excluded by KATRIN KATRIN:2024cdt. Colored curves: limits from IceCube on the Diffuse Boosted Cosmic Neutrino Background Herrera:2026pzj, for normal (NO) and inverted (IO) neutrino mass ordering, assuming star-formation–rate (SFR) and quasar (QSO) source evolution models. Dotted curves: projected sensitivity of IceCube-Gen2. Right: Illustration of the high-energy cut-off of cosmogenic neutrinos versus cosmic-ray boosted cosmic neutrino background, for an assumed maximum cosmic-ray energy of $E^{\rm max}_{\rm CR}=10^{12}$GeV. The resulting fluxes for an enhanced cosmic neutrino background as dark matter as predicted in this work are also shown, for $m_{\nu}=0.1$ eV. A different maximum cosmic-ray energy would shift the cut-off energy of the cosmogenic and boosted cosmic neutrino background fluxes, but the cross-over is fixed by particle physics, given by the different inelasticities of both processes. Estimated projected sensitivities from PUEO, POEMMA, GRAND200k and IceCube-Gen2 are shown for comparison Ackermann:2022rqc.
• Figure 3: Effective post-recombination radiation fraction from scalar decays into neutrinos as a function of scalar mass over twice the neutrino mass. Normal Ordering (NO) fails to satisfy the bound from Poulin:2016natBringmann:2018jpr for any mass, but inverted ordering (IO) does satisfy the bound.
• Figure 4: Maximally allowed fraction of dark matter that can decay into free active neutrinos without violating the Pauli exclusion principle (Tremaine–Gunn bound), as a function of the escape velocity of the system. Curves correspond to different neutrino masses, while shaded bands indicate characteristic velocities of dwarf galaxies, Milky-Way–like halos, and galaxy clusters. Neutrino dark matter is strongly suppressed in galactic halos but allowed on large, diffuse cosmological scales.
• Figure 5: Dwarf-coldness constraint on late $\phi \to \nu\bar{\nu}$ decays for $m_\nu = 0.4~\mathrm{eV}$. The blue curve shows the boundary obtained by requiring the neutrino velocity at dwarf-galaxy formation ($z_f \simeq 20$) to satisfy $v_\nu(z_f) = v_{\rm esc}^{\rm dwarf}=80~\mathrm{km\,s^{-1}}$. Parameter space above the curve correspond to earlier decays and sufficient cosmological redshifting of the decay products, such that neutrinos behave as cold matter on dwarf scales, while the region below corresponds to too warm neutrinos. The vertical dashed line shows the independent post-recombination constraint $m_\phi \le 2.039\,m_\nu$.