Compare · The Dark Universe

Primordial Black Holes vs Sterile Neutrinos

Dark Matter Candidates· within family

	Primordial Black Holes Frontier	Sterile Neutrinos Frontier
Proposed	1974 / 1975	1994 / 1999
Key figures	Bernard Carr, Stephen Hawking	Scott Dodelson, Lawrence Widrow, Xiang-Dong Shi, George Fuller
In one sentence	Black holes that formed in the first fraction of a second after the Big Bang, from regions where the matter density was unusually high. They gravitate exactly like dark matter would. Strict constraints from microlensing and gravitational waves allow only a sub-fraction of dark matter to be PBHs in most mass windows.	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.
Predictions	Microlensing events: temporary brightening of background stars as a PBH passes in front, with event rate and timescale set by PBH mass and abundance Gravitational wave signals from PBH-PBH binary mergers, especially in mass gaps where stellar evolution predicts no black holes (~3-5 solar masses, ~50-100 solar masses pair-instability gap) CMB and reionization constraints: gas accretion onto PBHs in the early universe would deposit energy, affecting the CMB temperature and reionization history Diffuse gamma-ray background from [[Hawking radiation]] of very-low-mass PBHs (M < 10^14 g) that would have evaporated by now	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
Where it breaks	Microlensing surveys (OGLE, EROS, Subaru HSC) and dynamical limits exclude PBHs as 100% of dark matter across the asteroid-to-solar-mass range almost everywhere Most LIGO/Virgo events have plausible astrophysical alternative explanations (stellar BH binary formation in dense globular clusters or galactic nuclei) Many formation models require fine-tuned primordial power spectra or specific inflationary features The asteroid-mass window is hard to probe directly: no current microlensing experiment is sensitive at that scale, leaving the most-allowed window also the least-tested	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
Key unresolved problem	The untestable window problem: the only mass range where primordial black holes could still be all the dark matter, roughly asteroid-sized, is exactly the one no current star-brightening (microlensing) survey is sensitive enough to check.	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.
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Primordial Black Holes

1974 / 1975 · Frontier

Sterile Neutrinos

1994 / 1999 · Frontier

Proposed

1974 / 1975

1994 / 1999

Key figures

Bernard Carr, Stephen Hawking

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

In one sentence

Black holes that formed in the first fraction of a second after the Big Bang, from regions where the matter density was unusually high. They gravitate exactly like dark matter would. Strict constraints from microlensing and gravitational waves allow only a sub-fraction of dark matter to be PBHs in most mass windows.

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.

Predictions

Microlensing events: temporary brightening of background stars as a PBH passes in front, with event rate and timescale set by PBH mass and abundance
Gravitational wave signals from PBH-PBH binary mergers, especially in mass gaps where stellar evolution predicts no black holes (~3-5 solar masses, ~50-100 solar masses pair-instability gap)
CMB and reionization constraints: gas accretion onto PBHs in the early universe would deposit energy, affecting the CMB temperature and reionization history
Diffuse gamma-ray background from [[Hawking radiation]] of very-low-mass PBHs (M < 10^14 g) that would have evaporated by now

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

Where it breaks

Microlensing surveys (OGLE, EROS, Subaru HSC) and dynamical limits exclude PBHs as 100% of dark matter across the asteroid-to-solar-mass range almost everywhere
Most LIGO/Virgo events have plausible astrophysical alternative explanations (stellar BH binary formation in dense globular clusters or galactic nuclei)
Many formation models require fine-tuned primordial power spectra or specific inflationary features
The asteroid-mass window is hard to probe directly: no current microlensing experiment is sensitive at that scale, leaving the most-allowed window also the least-tested

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

Key unresolved problem

The untestable window problem: the only mass range where primordial black holes could still be all the dark matter, roughly asteroid-sized, is exactly the one no current star-brightening (microlensing) survey is sensitive enough to check.

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.

Reader vote

No votes yet