Primordial Black Holes
Black holes formed in the very early universe from large density fluctuations, not from collapsing stars. Compact, dark, gravitating, and tightly constrained across most mass ranges.
Placeholder for a 3D visualisation of Dark Matter Candidates. The interactive scene will land in Phase 3. Roughly 26% of the universe's energy density behaves like cold, non-baryonic, gravitationally clustering matter. Plain ΛCDM asserts that dark matter exists; this family asks what it is. Five candidates dominate the literature: WIMPs, axions, primordial black holes, sterile neutrinos, and self-interacting dark matter. Each has a different theoretical motivation, a different production mechanism in the early universe, and a different experimental signature today. None has produced a confirmed detection after decades of dedicated searches.
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.
The claim
Standard black holes form from stellar collapse, but Carr and Hawking showed in 1974 that the very early universe could produce black holes directly from large primordial density fluctuations, without any star needing to form first. A region with density contrast above a critical threshold collapses directly to a black hole. The mass of the resulting PBH depends on when this happened: PBHs forming earlier have smaller masses, as small as 10^15 grams (just heavy enough to survive Hawking evaporation until today).
PBHs are non-luminous, gravitating, and (for masses above 10^15 g) long-lived. They behave gravitationally exactly like cold dark matter. The asteroid-mass through stellar-mass range was once a wide-open candidate window for explaining dark matter without invoking any new particle physics.
That window has been largely closed by observations. Microlensing surveys (OGLE, EROS, Subaru HSC) have placed strong limits on PBHs across most mass windows from asteroid to several solar masses. Current consensus from 2024 reviews: PBHs cannot make up 100% of dark matter in most windows, but sub-fractional contributions remain viable in tightly constrained ranges, notably the asteroid-mass window around 10^17 grams. LIGO/Virgo's discovery of stellar-mass black hole mergers has revived interest: if some fraction of those black holes are primordial rather than stellar in origin, this would be a smoking gun.
The family stance
Dark matter is some form of non-baryonic, gravitationally clustering matter that is not in the Standard Model. Multiple specific candidate particles or objects are seriously researched. The case for 'something' beyond ordinary matter is overwhelming; the case for any one specific candidate is not.
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
Evidence
- LIGO/Virgo/KAGRA black hole merger events include systems with masses falling in stellar-evolution mass gaps, consistent with at least a sub-population of primordial origin
- The 2024 review by Arbey identifies the asteroid-mass window (around 10^17 g) as remaining viable for ~100% PBH dark matter
- PBHs would behave gravitationally exactly like cold dark matter, with no need for new particle physics
Counterpoints
- 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
Variants in this family
▸Go deeperTechnical detail with proper terminology
Formation: in the radiation-dominated era, a region with density contrast δρ/ρ above the threshold δ_c ≈ 0.42 collapses to a black hole with mass approximately equal to the horizon mass at that time. M_BH ~ (10^15 g) × (10^-23 s / formation time). Earlier formation gives smaller PBHs.
Hawking evaporation: the lifetime scales as τ_evap ≈ 5×10^60 yr × (M / M_sun)³. PBHs with M < 10^15 g have already evaporated; only heavier PBHs can still be dark matter today.
Mass windows: the asteroid-mass window (M ~ 10^17 to 10^21 g) is the leading candidate for ~100% PBH dark matter. The sub-solar-mass and solar-mass windows are largely excluded by microlensing surveys. The stellar-mass window (~10 to 100 solar masses) is constrained by LIGO/Virgo merger rates and CMB observations.
LIGO/Virgo gap regions: mass gap ~3 to 5 solar masses (between neutron stars and the lightest stellar BHs) and the pair-instability gap ~50 to 100 solar masses are regions where stellar formation predicts no black holes should exist. Detection of mergers in these gaps is potential evidence for primordial origin.
References
- EstablishedCarr & Hawking (1974). Black holes in the early Universe. Mon. Not. Roy. Astron. Soc. 168, 399
- EstablishedCarr (1975). The Primordial Black Hole Mass Spectrum. Astrophys. J. 201, 1
- EstablishedArbey (2024). Primordial black holes, a small review
Last reviewed May 17, 2026
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