Primordial black holes (PBHs) are hypothetical black holes that may have formed in the universe’s earliest moments, rather than from the later collapse of stars. One possible route is that an unusually dense region of the hot early universe collapsed under its own gravity. No PBH has been definitively confirmed, and the details of how they might form remain dependent on the model.
What makes a black hole primordial?
“Primordial” means belonging to the universe’s earliest stages. NASA describes PBHs as objects theorized to have formed within the first second after the Big Bang. Unlike familiar black holes produced by the collapse of massive stars, they would have originated in conditions present in the early universe. Their existence remains unconfirmed: NASA says scientists have not found definitive proof that they ever existed. NASA’s overview of black-hole types explains the distinction.
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How could primordial black holes have formed?
Gravity may have collapsed unusually dense regions
In a broad possible scenario, some regions of the early universe were denser than their surroundings. If a region was sufficiently concentrated under the conditions then prevailing, gravity could have caused it to collapse into a black hole. NASA describes this density-contrast route in its Roman mission explainer.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsNot every density fluctuation would become a black hole
A density difference alone is not enough to establish that a black hole formed. The conditions for collapse, and the mass of any resulting PBH, depend on the early-universe model and the particular disturbance being considered. The available sources do not establish one universal collapse threshold or provide a basis for comparing every proposed formation mechanism, so a single numerical threshold would be misleading. The 2021 Annual Review of Nuclear and Particle Science review treats formation and abundance as model-dependent questions.
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What masses could they have, and would they still exist?
NASA gives an illustrative theoretical range extending from roughly 100,000 times less massive than a paperclip to 100,000 times the Sun’s mass. This describes a broad range of possibilities, not an observed population or a prediction from one specific model. NASA’s overview provides that illustration.
In the standard Hawking-radiation picture, black holes lose mass over time, and the smaller they are, the faster they evaporate. The 2021 Annual Reviews treatment gives an approximate initial-mass threshold of 5 × 1014 g: under the standard assumptions it reviews, a PBH below that mass would have evaporated within the present age of the universe. The review discusses this survival limit alongside other constraints.
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How do astronomers search for them?
A PBH would not necessarily shine by itself. Researchers instead look for ways its gravity or other effects might reveal its presence. Different techniques test different mass ranges and rely on different astrophysical or cosmological assumptions.
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Microlensing: watch for a foreground object bending starlight
If a compact object passes close to the line of sight between Earth and a distant star, its gravity can bend and focus the star’s light. That temporary brightening can reveal an otherwise dark object. NASA discusses microlensing as a way to investigate possible isolated objects around Earth mass, but such objects are candidates, not confirmed PBHs. NASA’s Roman mission explainer describes the search.
Gravitational waves: look for compact-object binaries
When compact objects orbit and merge, they can produce gravitational waves. Searches can test for binaries in mass ranges where ordinary stellar black holes are not expected to dominate. The cited LIGO summaries report no detections in the specific searches they describe, and their limits apply to the mass ranges and formation assumptions in those analyses—not to every possible PBH population.
- The LIGO Scientific Collaboration’s O3a subsolar-mass search summary (2023) reports an upper limit below 5% on PBH abundance for the analysis described. It is not a universal limit across all masses or models.
- The Collaboration’s O4a planetary-mass search summary (2026) reports constraints below a dark-matter fraction of unity for PBHs in the interval 10−6 to 10−4 solar masses under specified formation assumptions.
Other observations constrain different possibilities
Researchers also examine possible evaporation products, gravitational lensing, effects on the motions of astronomical systems, accretion, and large-scale structure. Each method constrains a different part of the picture and depends on its own astrophysical or cosmological assumptions. The 2021 review catalogs these approaches.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Could primordial black holes make up dark matter?
PBHs have been proposed as one possible component of dark matter, but the cited sources do not establish that they account for all dark matter or determine a particular fraction. Observational constraints restrict how abundant PBHs could be in different mass ranges, but the strength of those limits depends on the assumed mass distribution, formation scenario, and observational method. A result for one mass interval or search cannot be treated as an overall limit on every PBH population. The Annual Reviews review, LIGO’s O3a summary, and LIGO’s O4a summary illustrate how those constraints differ.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11NASA quotes astronomer Kailash Sahu of the Space Telescope Science Institute on the significance of confirming Earth-mass candidates: “It would affect everything from galaxy formation to the universe’s dark matter content to cosmic history.” The remark concerns the potential significance of confirming those candidates, not evidence that PBHs have already been found. NASA’s explainer gives the attribution and context.
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