Compare · The Dark Universe

Primordial Black Holes vs Axions and axion-like particles

Dark Matter Candidates· within family

	Primordial Black Holes Frontier	Axions and axion-like particles Frontier
Proposed	1974 / 1975	1977 / 1983
Key figures	Bernard Carr, Stephen Hawking	Roberto Peccei, Helen Quinn, Steven Weinberg, Frank Wilczek, John Preskill, Pierre Sikivie
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.	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	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	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	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	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 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 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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Primordial Black Holes

1974 / 1975 · Frontier

Axions and axion-like particles

1977 / 1983 · Frontier

Proposed

1974 / 1975

1977 / 1983

Key figures

Bernard Carr, Stephen Hawking

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

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.

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

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

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

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

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 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 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