Compare · The Dark Universe

Sterile Neutrinos vs Axions and axion-like particles

Dark Matter Candidates· within family

	Sterile Neutrinos Frontier	Axions and axion-like particles Frontier
Proposed	1994 / 1999	1977 / 1983
Key figures	Scott Dodelson, Lawrence Widrow, Xiang-Dong Shi, George Fuller	Roberto Peccei, Helen Quinn, Steven Weinberg, Frank Wilczek, John Preskill, Pierre Sikivie
In one sentence	Hypothetical neutrinos with masses around a few keV that don't feel the weak force at all, only gravity and a tiny mixing with the three Standard Model neutrinos. They would warm-dark-matter the universe and decay into X-rays, producing a detectable line in galaxy spectra. A tentative 3.5 keV X-ray line has been the subject of a decade-long debate.	Very light particles originally proposed to solve a fine-tuning problem in QCD (the strong-CP problem), with a tiny coupling to photons that makes them invisible to most experiments but also makes them a natural cold-dark-matter candidate.
Predictions	Radiative decay line at E ≈ m_s / 2 in galaxy and galaxy-cluster X-ray spectra, with line strength scaling with dark-matter column density Small-scale structure suppression: altered Lyman-α forest at z ~ 3-5, fewer dwarf satellites of the Milky Way, lower-density halo cores If an X-ray decay line is detected, its energy and brightness would pin down both the sterile neutrino's mass and how strongly it mixes with ordinary neutrinos (the mixing angle θ), since the production rate scales as sin²(2θ) × (m_s)²	A narrow radio signal from microwave cavity haloscopes (resonant chambers that amplify axion-to-photon conversion in a strong magnetic field), tuned to a frequency set by the unknown axion mass (f = m_a c² / h), with peak power fixed by the axion-photon coupling Stellar cooling anomalies: helium-burning stars and SN1987A would lose energy faster than observed if axion-photon couplings were too strong, bounding the coupling Time-varying signals in precision atomic clocks, NMR experiments, and interferometers from coherent oscillation of an axion dark matter field Spectral features from axion-photon conversion in galactic, stellar, and laboratory magnetic fields
Where it breaks	Hitomi (2017) failed to confirm the 3.5 keV line in the Perseus cluster (the satellite was destined for deeper sensitivity but lost before extended observations) Dessert et al. (2020) found the line absent in XMM blank-sky observations, arguing the original signal is inconsistent with a dark-matter origin Lyman-α forest and dwarf-galaxy structure constrain the simplest Dodelson-Widrow production, disfavoring sterile neutrinos as 100% of dark matter unless production is tuned The active-sterile mixing parameter space is heavily constrained by X-ray searches (XMM, Chandra, NuSTAR) over a wide mass range	Theoretically well-motivated but the allowed parameter space is vast: several orders of magnitude in mass and in coupling Some axion-production scenarios would leave a patchy imprint in the ancient light of the CMB (an isocurvature signal) that we do not see, so they survive only if the conditions during cosmic inflation are tuned No direct collider or laboratory hint; all motivation is theoretical and cosmological Critics argue axions are easy to 'rescue' with parameter tuning whenever an experiment finds nothing
Key unresolved problem	The 3.5 keV line ambiguity: a faint X-ray glow that could be sterile neutrinos decaying has neither been confirmed nor ruled out, and researchers are waiting on sharper XRISM data to settle whether it is real or an instrument artifact.	The unknown-mass problem: the axion's mass could fall anywhere across a vast range, and no experiment can yet sweep the whole plausible window in any reasonable amount of time.
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Sterile Neutrinos

1994 / 1999 · Frontier

Axions and axion-like particles

1977 / 1983 · Frontier

Proposed

1994 / 1999

1977 / 1983

Key figures

Scott Dodelson, Lawrence Widrow, Xiang-Dong Shi, George Fuller

Roberto Peccei, Helen Quinn, Steven Weinberg, Frank Wilczek, John Preskill, Pierre Sikivie

In one sentence

Hypothetical neutrinos with masses around a few keV that don't feel the weak force at all, only gravity and a tiny mixing with the three Standard Model neutrinos. They would warm-dark-matter the universe and decay into X-rays, producing a detectable line in galaxy spectra. A tentative 3.5 keV X-ray line has been the subject of a decade-long debate.

Very light particles originally proposed to solve a fine-tuning problem in QCD (the strong-CP problem), with a tiny coupling to photons that makes them invisible to most experiments but also makes them a natural cold-dark-matter candidate.

Predictions

Radiative decay line at E ≈ m_s / 2 in galaxy and galaxy-cluster X-ray spectra, with line strength scaling with dark-matter column density
Small-scale structure suppression: altered Lyman-α forest at z ~ 3-5, fewer dwarf satellites of the Milky Way, lower-density halo cores
If an X-ray decay line is detected, its energy and brightness would pin down both the sterile neutrino's mass and how strongly it mixes with ordinary neutrinos (the mixing angle θ), since the production rate scales as sin²(2θ) × (m_s)²

A narrow radio signal from microwave cavity haloscopes (resonant chambers that amplify axion-to-photon conversion in a strong magnetic field), tuned to a frequency set by the unknown axion mass (f = m_a c² / h), with peak power fixed by the axion-photon coupling
Stellar cooling anomalies: helium-burning stars and SN1987A would lose energy faster than observed if axion-photon couplings were too strong, bounding the coupling
Time-varying signals in precision atomic clocks, NMR experiments, and interferometers from coherent oscillation of an axion dark matter field
Spectral features from axion-photon conversion in galactic, stellar, and laboratory magnetic fields

Where it breaks

Hitomi (2017) failed to confirm the 3.5 keV line in the Perseus cluster (the satellite was destined for deeper sensitivity but lost before extended observations)
Dessert et al. (2020) found the line absent in XMM blank-sky observations, arguing the original signal is inconsistent with a dark-matter origin
Lyman-α forest and dwarf-galaxy structure constrain the simplest Dodelson-Widrow production, disfavoring sterile neutrinos as 100% of dark matter unless production is tuned
The active-sterile mixing parameter space is heavily constrained by X-ray searches (XMM, Chandra, NuSTAR) over a wide mass range

Theoretically well-motivated but the allowed parameter space is vast: several orders of magnitude in mass and in coupling
Some axion-production scenarios would leave a patchy imprint in the ancient light of the CMB (an isocurvature signal) that we do not see, so they survive only if the conditions during cosmic inflation are tuned
No direct collider or laboratory hint; all motivation is theoretical and cosmological
Critics argue axions are easy to 'rescue' with parameter tuning whenever an experiment finds nothing

Key unresolved problem

The 3.5 keV line ambiguity: a faint X-ray glow that could be sterile neutrinos decaying has neither been confirmed nor ruled out, and researchers are waiting on sharper XRISM data to settle whether it is real or an instrument artifact.

The unknown-mass problem: the axion's mass could fall anywhere across a vast range, and no experiment can yet sweep the whole plausible window in any reasonable amount of time.

Reader vote

No votes yet