Definition
Primordial black holes are hypothetical black holes that formed in the very early universe from large density fluctuations, rather than from the collapse of massive stars.
Their possible masses span an extraordinary range — from less than a microgram to thousands of solar masses — depending on when in the early universe they formed.
Why it matters
How it works
In the very early universe, density fluctuations on small scales were rapidly stretched by cosmic expansion but, in rare regions, could be large enough to overcome that expansion and collapse. If a region exceeded the critical density on the scale of the cosmic horizon, it would collapse to a black hole whose mass is roughly the horizon mass at that time. Earlier collapse means smaller masses: a primordial black hole formed when the universe was 10⁻²³ seconds old has mass around 10⁵ kg; formed at 10⁻⁵ seconds, around one solar mass.
This formation channel is independent of the stellar-collapse route, removing the lower mass limit. Combined with Hawking's 1974 prediction, it makes small primordial black holes especially interesting. A black hole of mass M evaporates in a time roughly (M / 10¹² kg)³ × 10¹⁰ years. So a black hole of about 10¹² kg formed in the early universe should be evaporating today, ending its life in a flash of high-energy gamma rays. Searches by EGRET, Fermi-LAT, and air-shower detectors have not found such bursts, placing upper limits on primordial black-hole abundance in that mass range.
Larger primordial holes — asteroid-mass or larger — would not be evaporating and remain candidates for dark matter. Various constraints from microlensing surveys, dynamical friction in stellar clusters, and CMB anisotropies have closed off many mass windows, but several gaps remain open and remain active research targets.