Compare · The Dark Universe

Fuzzy Dark Matter vs Primordial Black Holes

Dark Matter Candidates· within family

	Fuzzy Dark Matter Frontier	Primordial Black Holes Frontier
Proposed	2000	1974 / 1975
Key figures	Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui	Bernard Carr, Stephen Hawking
In one sentence	Fuzzy Dark Matter is an ultra-light scalar (mass ~10^-22 eV) whose de Broglie wavelength reaches kpc scales, producing wave-mechanical phenomenology in galactic dynamics. The framework was introduced by Hu, Barkana, and Gruzinov in 2000 (Phys. Rev. Lett. 85, 1158) and substantially developed in the 2017 Hui-Ostriker-Tremaine-Witten paper Ultralight scalars as cosmological dark matter (Phys. Rev. D 95, 043541). Distinct from generic Axions due to its ultra-light mass and the resulting wave-mechanical effects on galactic scales.	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.
Predictions	Galactic dark-matter halos have minimum core size set by the de Broglie wavelength of the FDM particle; sub-kiloparsec cores are predicted for ~10^-22 eV particles Dwarf-galaxy cores are dominated by solitonic structures, the ground-state quantum-wave configurations of the FDM particle in a self-gravitating halo Lyman-alpha forest measurements should detect the wave-mechanical suppression of small-scale structure at masses below the constraint threshold Specific signatures in the matter power spectrum on small scales that distinguish FDM from generic cold dark matter	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
Where it breaks	Lyman-alpha forest constraints place lower bounds on the FDM particle mass around 10^-21 eV or higher; the original ~10^-22 eV proposal is now disfavored Dwarf-galaxy observations are in tension with the FDM predictions for solitonic-core sizes; current best-fit FDM masses produce cores too large for some observed dwarfs Distinguishing FDM from generic CDM observationally requires precise measurements at very small scales; current data places constraints but does not unambiguously favor one over the other The framework requires a specific ultra-light particle mass and self-interaction structure; the deep physical motivation for these values from string compactification or alternative UV physics is not crisp	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
Key unresolved problem	The Lyman-alpha tension: the fine structure seen in distant gas clouds (the Lyman-alpha forest) now rules out the original ultra-light particle mass near 10^-22 eV, leaving only somewhat heavier, still-unconfirmed values in play.	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.
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Fuzzy Dark Matter

2000 · Frontier

Primordial Black Holes

1974 / 1975 · Frontier

Proposed

2000

1974 / 1975

Key figures

Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui

Bernard Carr, Stephen Hawking

In one sentence

Fuzzy Dark Matter is an ultra-light scalar (mass ~10^-22 eV) whose de Broglie wavelength reaches kpc scales, producing wave-mechanical phenomenology in galactic dynamics. The framework was introduced by Hu, Barkana, and Gruzinov in 2000 (Phys. Rev. Lett. 85, 1158) and substantially developed in the 2017 Hui-Ostriker-Tremaine-Witten paper *Ultralight scalars as cosmological dark matter* (Phys. Rev. D 95, 043541). Distinct from generic Axions due to its ultra-light mass and the resulting wave-mechanical effects on galactic scales.

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.

Predictions

Galactic dark-matter halos have minimum core size set by the de Broglie wavelength of the FDM particle; sub-kiloparsec cores are predicted for ~10^-22 eV particles
Dwarf-galaxy cores are dominated by solitonic structures, the ground-state quantum-wave configurations of the FDM particle in a self-gravitating halo
Lyman-alpha forest measurements should detect the wave-mechanical suppression of small-scale structure at masses below the constraint threshold
Specific signatures in the matter power spectrum on small scales that distinguish FDM from generic cold dark matter

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

Where it breaks

Lyman-alpha forest constraints place lower bounds on the FDM particle mass around 10^-21 eV or higher; the original ~10^-22 eV proposal is now disfavored
Dwarf-galaxy observations are in tension with the FDM predictions for solitonic-core sizes; current best-fit FDM masses produce cores too large for some observed dwarfs
Distinguishing FDM from generic CDM observationally requires precise measurements at very small scales; current data places constraints but does not unambiguously favor one over the other
The framework requires a specific ultra-light particle mass and self-interaction structure; the deep physical motivation for these values from string compactification or alternative UV physics is not crisp

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

Key unresolved problem

The Lyman-alpha tension: the fine structure seen in distant gas clouds (the Lyman-alpha forest) now rules out the original ultra-light particle mass near 10^-22 eV, leaving only somewhat heavier, still-unconfirmed values in play.

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.

Reader vote

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