Sterile Neutrino Cosmology Overview

• Sterile neutrino cosmologies are theoretical frameworks that introduce additional neutrino species, influencing early-universe radiation and dark matter properties.
• They employ varied production mechanisms—non-resonant, resonant, and freeze-in—that yield distinct momentum distributions and observable cosmological signatures.
• Observations from CMB, BBN, LSS, and X-ray searches, alongside secret interactions and modified expansion histories, constrain the sterile neutrino parameter space.

Sterile neutrino cosmologies constitute a research field examining the roles, observational consequences, and theoretical frameworks of neutrino sectors that extend beyond the three active neutrinos of the Standard Model. Sterile neutrinos, defined as fermions that are singlets under the Standard Model gauge group and interact with ordinary matter only via mixing with active neutrinos, have been hypothesized in response to laboratory anomalies, cosmological observations, and theoretical considerations related to neutrino mass. Their cosmological implications range from subtle changes in the radiation content of the early universe and the growth of cosmic structure to possible explanations for the nature of dark matter and interpretations of X-ray line emissions. The exploration of sterile neutrino cosmologies involves the intersection of particle physics, observational cosmology (CMB, large scale structure, BBN), and laboratory neutrino oscillation data.

1. Parameterization and Phenomenology of Sterile Neutrinos

Sterile neutrino cosmologies introduce new parameters to standard cosmological models, most importantly:

• The effective number of neutrino species, $N_{\mathrm{eff}}$, where the Standard Model predicts $N_{\mathrm{eff}} = 3.046$. Additional sterile neutrinos contribute $\Delta N_{\mathrm{eff}}$, so that $N_{\mathrm{eff}} = 3.046 + N_s$, where $N_s$ quantifies the number of thermally excited sterile species (Hamann et al., 2010).
• The sterile neutrino mass $m_s$, which may range from sub-eV (as motivated by oscillation experiments) to keV–MeV scales (as considered for dark matter).
• The mixing angle(s) with active neutrinos, typically parameterized through $\sin^2 2\theta$.

In cosmological analyses, two limiting scenarios are commonly employed for sterile neutrino injection into radiation and matter content: a) "3+$N_s$" scenario: All active neutrinos are taken as massless and sterile states have mass $m_s$, contributing to the matter density via $$\Omega_{\nu} h^2 = (N_s \cdot m_s)/(93\ \mathrm{eV})$$. b) "$N_s$+3" scenario: The sterile neutrinos are massless and all three active neutrinos are massive and degenerate.

The impact of sterile species on cosmology is dual: (i) as extra radiation ($N_{\mathrm{eff}}$), increasing the expansion rate before recombination and (ii) as hot/warm dark matter, suppressing power in the matter power spectrum on small scales (Hamann et al., 2010, Jacques et al., 2013, Kristiansen et al., 2013).

2. Production Mechanisms and Thermal Histories

Sterile neutrinos can be generated in the early universe via a variety of mechanisms:

• Non-resonant (Dodelson–Widrow) production, wherein sterile neutrinos are produced via active–sterile oscillations in a thermal active neutrino background. This mechanism predicts a quasi-thermal momentum distribution and an abundance proportional to the mixing squared (Abazajian, 2017).
• Resonant (Shi–Fuller) mechanisms, operative when there exists a primordial lepton asymmetry, such that MSW-like resonance enhances the effective mixing for select momentum ranges. The resonance condition depends on the chemical potential and thermal background; the resulting sterile neutrino population is "colder" (lower average momentum) than in the non-resonant case, and the abundance is fixed by the available lepton asymmetry (Gelmini et al., 2019).
• Freeze-in scenarios involving decays of heavier scalar or vector parents that are either in equilibrium or freeze-in themselves. In these cases, the resultant sterile neutrino abundance and spectra are highly model-dependent, leading to cold, warm, or hot dark matter scenarios, possibly yielding nontrivial features in velocity distributions (Roland et al., 2016).

Production and eventual relic abundance are sensitive to:

• The expansion rate in the pre-BBN Universe. Non-standard cosmologies, such as kination or scalar-tensor modifications, alter the temperature–time relation and thus the efficiency of sterile neutrino production (Rehagen et al., 2014, Gelmini et al., 2019, Chichiri et al., 2021).
• Self-interactions among sterile or active neutrinos, which can block mixing and delay/inhibit production (see below).

3. Cosmological Constraints and Observational Signatures

Sterile neutrino cosmologies are constrained through their impact on:

• Big Bang Nucleosynthesis (BBN): The energy density in radiation ($N_{\mathrm{eff}}$ at BBN) affects the primordial neutron-to-proton freeze-out and thus light element abundances. Fully thermalized sterile species of eV-scale mass are strongly constrained, especially when more than one additional species is present (Jacques et al., 2013, Hamann et al., 2010).
• Cosmic Microwave Background (CMB): $N_{\mathrm{eff}}$ modifies the expansion rate at recombination, affecting both acoustic peak positions and damping tails. The sum of neutrino masses, $\Sigma m_\nu$, impacts CMB lensing power spectra. Precision CMB, CMB lensing, and baryon acoustic oscillation (BAO) measurements tightly constrain the allowed parameter space for sterile neutrinos, especially in the eV-mass regime (Escudero et al., 29 Sep 2025).
• Large Scale Structure (LSS): Non-relativistic sterile neutrinos free stream and suppress small-scale power. Constraints arise from galaxy clustering, weak lensing, Lyman-$\alpha$ forest, and counts of dwarf satellites (Kristiansen et al., 2013, Vegetti et al., 2018, Bose et al., 2016).

Astrophysical observations also search for X-ray lines (decay channels $\nu_s \to \nu + \gamma$) from dark matter sterile neutrinos, notably the 3.5 keV line interpreted as a 7 keV sterile neutrino decay (Bose et al., 2016, Abazajian, 2017).

4. The Role of New Physics: Secret Interactions and Modified Cosmologies

To reconcile laboratory hints for eV-scale sterile neutrinos with cosmological constraints, several extensions are explored:

• Secret/self-interactions: Introducing new forces (e.g., "secret" $U(1)_s$ gauge interactions mediated by an $A'$ boson or pseudoscalar coupling) block efficient active–sterile mixing in the early Universe by modifying the in-medium potential. This can suppress thermalization and delay production until after effective cosmological decoupling, thus avoiding the BBN and CMB $N_{\mathrm{eff}}$ bounds. Such interactions can also yield late-time sterile–dark matter scattering, affecting small-scale structure (Hannestad et al., 2013, Archidiacono et al., 2014, Chu et al., 2015, Pires, 2019).
• Non-standard pre-BBN expansion: Enhanced or suppressed expansion rates increase or dilute sterile neutrino production, respectively, shifting the required mixing for the same relic abundance and altering tension with cosmological data (Rehagen et al., 2014, Gelmini et al., 2019, Chichiri et al., 2021).
• Invisible decays: Sterile neutrinos may decay into lighter invisible states before matter–radiation equality, transitioning their energy density from matter to radiation and circumventing structure formation suppression (Gariazzo et al., 2014).
• Scalar–tensor cosmologies and freeze-in: Kination and scalar–tensor epochs further reduce the thermalization window, opening parameter space otherwise excluded in standard cosmology (Rehagen et al., 2014, Gelmini et al., 2019).

5. Sterile Neutrinos and Experimental Anomalies

Persistent anomalies in short-baseline (SBL) oscillation experiments, including LSND, MiniBooNE, reactor, and Gallium experiments, suggest the existence of eV-scale sterile neutrinos with substantial mixing [$\Delta m^2 \sim 1~\mathrm{eV}^2$, $\sin^2 2\theta \sim 0.08$]. While these anomalies generate strong interest, global fits reveal severe tension—especially the so-called "appearance–disappearance" tension—between observed electron flavor appearance and muon flavor disappearance channels, as well as constraints from cosmological data (Hamann et al., 2010, Jacques et al., 2013, Kang, 2019). The preferred cosmological parameter region for sterile neutrinos is sensitive to the degree of thermalization and the expansion history of the Universe. Standard cosmology disfavors fully thermalized eV sterile neutrinos, while models with partial thermalization or non-standard cosmological histories can mitigate the tension (Gelmini et al., 2019, Escudero et al., 29 Sep 2025).

The table below summarizes the interplay between SBL results, cosmological probes, and mechanisms for evasion of constraints:

Mechanism/Scenario	Evasion of Cosmological Limits	Status per Cited Work
Secret/self-interactions	Block early thermalization via modified in-medium mixing	Viable for some parameter ranges (Hannestad et al., 2013, Chu et al., 2015)
Non-standard expansion (pre-BBN)	Dilute relic abundance by enhancing Hubble rate	Allows larger mixing, relaxes bounds (Rehagen et al., 2014, Chichiri et al., 2021)
Invisible decay	Relativize late-time impact on matter density fluctuation	Compatible fit to CMB and LSS data (Gariazzo et al., 2014)
Pseudoscalar-sterile interactions	Suppress mixing and facilitate late annihilation	Addresses Hubble tension and LSS (Archidiacono et al., 2014)
Standard cosmology, full thermalization	Strongly constrained ($m_s < 0.17$ eV at 95% CL w/ $N_{\mathrm{eff}}=3.5$ (Escudero et al., 29 Sep 2025))	Disfavors eV-mass sterile states

6. Impact on Cosmological Tensions and Future Directions

Sterile neutrinos, particularly in scenarios with partially thermalized sub-eV masses ($m_s \sim 0.1$–$0.2$ eV, $N_{\mathrm{eff}} \simeq 3.5$), have been shown to alleviate the tension between the local measurement of the Hubble constant ($H_0$, e.g., from SH0ES) and the values inferred from CMB data under $\Lambda$CDM. By increasing early-universe radiation density, the sound horizon at recombination is reduced, allowing for a larger $H_0$ when fitting CMB and BAO data. The most recent analyses indicate that, with SH0ES data included, light sterile neutrinos are favored at more than $3\sigma$, while eV-scale masses remain strongly excluded (Escudero et al., 29 Sep 2025).

The limits on the sum of neutrino masses, $\Sigma m_\nu$, and $m_s$ in such cosmologies are substantially weaker than in standard analyses (e.g., $m_s < 0.17$ eV at 95% CL for $N_{\mathrm{eff}} = 3.5$). However, the preferred sterile neutrino mass scale from cosmology overlaps but does not coincide with that inferred from SBL anomalies.

Ongoing and future cosmological surveys (such as continued DESI BAO releases and CMB-S4), improved laboratory oscillation results, X-ray searches for sterile neutrino decay lines, and laboratory $\beta$-decay search experiments (KATRIN/TRISTAN, HUNTER, PTOLEMY) will further constrain the allowed parameter space and clarify the role of sterile neutrinos in cosmology (Gelmini et al., 2019).

7. Synthesis and Open Issues

Sterile neutrino cosmologies are at an intersection of particle physics and precision cosmology where theoretical extensions, laboratory anomalies, and a diverse suite of cosmological datasets converge. While sub-eV partially thermalized sterile species are currently preferred as a solution to the Hubble tension, the region favored by oscillation anomalies (eV-mass, large mixing, full thermalization) is largely excluded in standard cosmologies. Mechanisms invoking new secret interactions, modified expansion histories, or nontrivial decay channels expand the viable parameter space, but often require tuning or additional fields. The ongoing challenge is to identify a consistent framework that can simultaneously accommodate laboratory anomalies, cosmological observations, and astrophysical probes without introducing new conflicts or parametric tensions.

The theoretical and observational status remains dynamic, with several unresolved questions:

• Are SBL anomalies evidence for sterile neutrinos or for more exotic physics?
• How generic are cosmological mechanisms that evade standard bounds? Do they introduce new observational signatures?
• Is there a unique cosmological probe that can decisively confirm or rule out the existence of sterile neutrino cosmologies, particularly in the sub-eV to keV regime?

Future laboratory and cosmological experiments are positioned to test these foundational questions in the coming decade.