Compare · The Dark Universe

Primordial Black Holes vs Self-Interacting Dark Matter (SIDM)

Dark Matter Candidates· within family

	Primordial Black Holes Frontier	Self-Interacting Dark Matter (SIDM) Frontier
Proposed	1974 / 1975	2000 / 2018
Key figures	Bernard Carr, Stephen Hawking	David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu
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.	Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.
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	Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path) Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section
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	Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle
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 mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.
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Primordial Black Holes

1974 / 1975 · Frontier

Self-Interacting Dark Matter (SIDM)

2000 / 2018 · Frontier

Proposed

1974 / 1975

2000 / 2018

Key figures

Bernard Carr, Stephen Hawking

David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu

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.

Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.

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

Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion
Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path)
Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section

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

Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM
Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all
Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes
Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle

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 mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.

Reader vote

No votes yet