Compare · The Dark Universe

Primordial Black Holes vs WIMPs

Dark Matter Candidates· within family

	Primordial Black Holes Frontier	WIMPs Frontier
Proposed	1974 / 1975	1977 / 1996
Key figures	Bernard Carr, Stephen Hawking	Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone
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 particles with masses around the weak scale that interact with ordinary matter via the weak nuclear force and gravity but not light. Their thermal abundance from the early universe naturally matches the observed dark matter density. Direct-detection experiments have been searching for decades and have not seen them.
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	Nuclear recoils with characteristic energy spectrum in xenon or argon detectors at rates set by WIMP-nucleon cross-section, WIMP mass, and local halo density Gamma rays, antimatter, and neutrinos from WIMP-WIMP annihilation in the Galactic center, dwarf spheroidals, and the Sun Missing transverse momentum in LHC events with one or more visible particles (mono-jet, mono-photon, mono-Z) A leftover abundance from the early universe that naturally matches today's dark matter density, the 'WIMP miracle', set by thermal freeze-out (Ωh² ≈ 0.12 for an annihilation rate ⟨σv⟩ ≈ 3×10^-26 cm³/s)
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	LHC has produced no evidence of weak-scale supersymmetric WIMPs (neutralinos); natural SUSY models with TeV-scale masses now require fine-tuning Direct-detection limits exclude the simplest weak-cross-section WIMPs for masses where the WIMP miracle was strongest The WIMP miracle is less compelling post-LHC: many of the natural models that gave the coincidence are now constrained or ruled out Critics argue WIMPs are becoming unfalsifiable: every null result is met with a more elaborate model variant
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 neutrino fog problem: detectors are now so sensitive they pick up a steady drizzle of natural neutrinos, and it is unclear whether a real WIMP signal could ever be told apart from this unavoidable background.
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Primordial Black Holes

1974 / 1975 · Frontier

WIMPs

1977 / 1996 · Frontier

Proposed

1974 / 1975

1977 / 1996

Key figures

Bernard Carr, Stephen Hawking

Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone

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 particles with masses around the weak scale that interact with ordinary matter via the weak nuclear force and gravity but not light. Their thermal abundance from the early universe naturally matches the observed dark matter density. Direct-detection experiments have been searching for decades and have not seen them.

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

Nuclear recoils with characteristic energy spectrum in xenon or argon detectors at rates set by WIMP-nucleon cross-section, WIMP mass, and local halo density
Gamma rays, antimatter, and neutrinos from WIMP-WIMP annihilation in the Galactic center, dwarf spheroidals, and the Sun
Missing transverse momentum in LHC events with one or more visible particles (mono-jet, mono-photon, mono-Z)
A leftover abundance from the early universe that naturally matches today's dark matter density, the 'WIMP miracle', set by thermal freeze-out (Ωh² ≈ 0.12 for an annihilation rate ⟨σv⟩ ≈ 3×10^-26 cm³/s)

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

LHC has produced no evidence of weak-scale supersymmetric WIMPs (neutralinos); natural SUSY models with TeV-scale masses now require fine-tuning
Direct-detection limits exclude the simplest weak-cross-section WIMPs for masses where the WIMP miracle was strongest
The WIMP miracle is less compelling post-LHC: many of the natural models that gave the coincidence are now constrained or ruled out
Critics argue WIMPs are becoming unfalsifiable: every null result is met with a more elaborate model variant

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 neutrino fog problem: detectors are now so sensitive they pick up a steady drizzle of natural neutrinos, and it is unclear whether a real WIMP signal could ever be told apart from this unavoidable background.

Reader vote

No votes yet