DodelsonwidrowEdit

The Dodelson–Widrow mechanism, named after Scott Dodelson and Lisa Widrow, is a theoretical framework in particle cosmology for producing a population of sterile neutrinos in the early universe through non-resonant mixing with active neutrinos. In this scenario, a gauge-singlet neutrino with a mass in the keV range interacts only feebly with the standard model through mixing with the active species. As the hot plasma of the early cosmos cooled, active neutrinos could oscillate into sterile neutrinos, generating a relic population that contributes to the total dark matter density. The resulting momentum distribution is non-thermal, and the produced sterile neutrinos behave as a warm dark matter candidate, with implications for structure formation and observational signatures.

The Dodelson–Widrow mechanism is one of the principal non-thermal production channels discussed for sterile neutrino dark matter. It relies on physics that is well-mounded within the standard cosmological framework, but it also sits at the center of several observational debates, especially related to how much of the dark matter can be accounted for by sterile neutrinos without conflicting with X-ray, Lyman-alpha, and other cosmological constraints. Researchers compare this mechanism with alternative production channels, such as resonant production in the presence of a lepton asymmetry (often associated with the Shi–Fuller mechanism), to understand the full range of viable parameter space for sterile neutrino dark matter. sterile neutrino neutrino dark matter neutrino oscillation

Dodelson–Widrow mechanism

Overview

The mechanism envisions a sterile neutrino species that mixes with the active neutrinos of the standard model. Because of this mixing, active neutrinos in the primordial plasma can coherently oscillate into the sterile state, producing a population of sterile neutrinos as the universe expands and cools. The produced relic density depends on the sterile-neutrino mass and the mixing angle with the active species; larger masses and larger mixings produce more sterile neutrinos, up to limits set by observations. The resulting distribution in momentum space is typically broader than a thermal distribution, which translates into a characteristic imprint on how these particles affect the growth of cosmic structure. For a mass in the keV range, the produced sterile neutrinos behave as warm dark matter, affecting small-scale structures differently from cold dark matter. See for example discussions in cosmology and structure formation literature. sterile neutrino neutrino oscillation early universe dark matter Lyman-alpha forest

Mechanism details

In the hot, dense early universe, active neutrinos were abundant and in approximate thermal contact with the plasma. Through quantum-mechanical mixing with a sterile state, active neutrinos can convert into sterile neutrinos without requiring any new interactions beyond gravity and the existing electroweak sector. The production rate is sensitive to the temperature, the mass of the sterile neutrino, and the mixing angle. As the universe expands and cools, production naturally shuts off, leaving a relic population whose abundance tracks these parameters. The process is non-resonant, meaning it does not rely on a resonance in the medium to enhance production; instead, it proceeds through standard flavor mixing.

Parameter dependence and relic density

The viable range of sterile-neutrino masses and mixing angles is constrained by the requirement that the sterile component accounts for roughly the observed dark matter density, while also complying with indirect constraints. In broad terms: - Mass in the keV range is discussed as the natural window for a warm dark matter candidate. - Mixing angles must be small enough to avoid conflict with X-ray searches, because sterile neutrinos decay radiatively with a photon carrying roughly half the sterile-neutrino mass energy. - The resulting relic density scales with a combination of mass and mixing angle; for a given mass, there is a limited band of mixings that can yield the correct dark matter abundance.

Observational constraints and status

• X-ray observations place upper limits on the mixing angle for a given sterile-neutrino mass, since sterile neutrinos decay into an active neutrino plus a photon. The non-detection of a corresponding line in X-ray spectra from galaxies and clusters translates into restrictions on the DW parameter space. X-ray astronomy sterile neutrino decay
• The Lyman-alpha forest, which probes the suppression of small-scale matter clustering, constrains the free-streaming scale of dark matter. For DW-produced sterile neutrinos, these data typically push the allowed mass range upward, because warmer dark matter erases small-scale structures more readily. Depending on modeling choices, this can imply lower bounds of a few keV to several keV for the sterile-neutrino mass. Lyman-alpha forest structure formation
• The potential 3.5 keV line reported in some X-ray observations sparked discussion about a possible 7 keV sterile neutrino decay, but the interpretation as a universal DW signal remains contentious, with many follow-up observations yielding inconsistent results. This illustrates the broader issue of how robust such indirect signals are as evidence for any particular dark matter model. 3.5 keV line

Controversies and debates

• Viability under current constraints: A recurring topic is whether the non-resonant DW production, by itself, can account for all of the dark matter without violating X-ray and Lyman-alpha bounds. A number of analyses suggest that the simplest DW scenario struggles to meet all constraints while simultaneously delivering the observed dark-matter density, prompting consideration of alternative or hybrid production channels. sterile neutrino Lyman-alpha forest X-ray astronomy
• Comparison with resonant production: The Shi–Fuller mechanism, which leverages a lepton asymmetry to enhance production at lower momenta, can yield the same sterile-neutrino mass with different momentum distribution, potentially easing some structure-formation constraints. This has generated a substantial debate about which production channel, or combination thereof, best fits the data. Shi–Fuller mechanism lepton asymmetry neutrino oscillation
• Interpretational volatility of signals: The existence or non-existence of claimed indirect signals (e.g., the 3.5 keV line) has a direct bearing on how compelling the DW scenario remains in the scientific dialogue. Critics emphasize the fragility of such signals and the need for independent confirmation, while proponents stress that multiple observational channels must be weighed together. X-ray astronomy 3.5 keV line

See also

• sterile neutrino
• neutrino
• dark matter
• neutrino oscillation
• Lyman-alpha forest
• Shi–Fuller mechanism
• lepton asymmetry
• structure formation