Section 1.1
How primordial black holes differ from stellar black holes
Two very different origins
Both kinds of object end up being called "black holes" because both are regions of space where gravity has become strong enough that nothing, not even light, can escape once it falls past the event horizon. But the routes that lead to that end state, and the eras of the universe in which they happen, are almost completely different.
A black hole formed by stellar collapse begins its life as an ordinary star.The final stages of stellar evolution. Gravitational collapse. The formation of a black hole.Only highly massive stars with an initial mass
greater than about 20 M¤ will have a residual mass M' >2 M¤ and
will succumb to complete gravitational collapse with the
formation of a black hole.
Open source For most of a massive star's life, the outward push of energy released by nuclear fusion in its core balances the inward pull of its own gravity. Once the star runs out of fuel it can fuse, that support disappears, and if enough mass is left in the core, nothing can stop it from collapsing all the way down to a black hole. This is a process tied intimately to the life and death of individual stars, and it can only happen once stars exist. Primordial black holes are proposed to have formed by a completely different route, one that does not involve stars at all.Primordial black hole - WikipediaIn cosmology, primordial black holes (PBHs) are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating prim
Open source
The idea is that in the first fraction of a second after the Big Bang, the universe's density was not perfectly smooth. In places where a random fluctuation packed matter or radiation unusually densely into a small region, that region could have collapsed directly under its own gravity into a black hole, long before any star, or even any atom, had formed. No supernova, no dying star, no nuclear fusion is required for this process, just gravity acting on an overdense patch of the extremely young, hot universe.
| Primordial black holes | Stellar-collapse black holes | |
|---|---|---|
| When they form | Within the first second after the Big Bang, long before any stars existed | Only after a massive star has lived out its life and exhausted its nuclear fuel, so billions of years into cosmic history at the earliest |
| Trigger | Direct gravitational collapse of an unusually dense patch of the early universe | Core collapse once a massive star's fusion-generated pressure can no longer support it against gravity |
| Possible masses | According to Hawking and Carr's original theoretical estimates, primordial black holes could in principle span an enormous range of masses, from as small as about 10 micrograms up to roughly 10,000 times the Sun's mass, unlike the comparatively narrow range produced by stellar collapseConcerning primordial black holes – Physics WorldHawking and Carr predicted that PBHs could have formed with a wide range of possible sizes, from as small as 10 μg to as large as 10,000 solar masses.Open source | Typically 5 to 20 times the Sun's mass, set by how much mass the collapsing core retains|The mass difference is one of the clearest fingerprints separating the two populations. A star has to have a minimum mass to end up as a black hole rather than a neutron star or white dwarf, and however much of its outer layers it blows off in the process, what's left behind still has to exceed a few times the Sun's mass to collapse the rest of the way. That physics puts a floor on stellar black hole masses, and observations of stellar-collapse black holes typically show masses in the 5-to-20-solar-mass range.How Black Holes Form: Stellar Collapse, Mergers & More | Cosmic HorizonsThe black hole left behind contains typically 5 to 20 solar masses, concentrated in a point of infinite density according to general relativity, though quantum effects are expected to modify this picture at the smallest scales.Open source |
| Spin | Not required to be spinning fast, since it need not have formed from a rotating bodyFrontiers | A Brief Review on Primordial Black Holes as Dark MatterIn contrast, astrophysical BHs are expected to have substantially larger spins, since angular momentum must be conserved during the collapse of their stars of origin, which are often rotating.Open source | |
| Observational status | Still hypothetical; none has been confirmed to exist|Directly and indirectly observed many times, including through X-ray binaries and gravitational-wave mergers| So far, though, every black hole astronomers have actually confirmed traces back to stellar collapse or to mergers of such objects: primordial black holes remain hypothetical, with no confirmed primordial black hole yet identified. |
Why the mass range matters
Primordial black holes carry no such restriction, because they aren't produced from stellar material at all: whatever mass happened to be enclosed within the collapsing patch of the early universe becomes the black hole's mass, so calculations allow for primordial black holes far smaller than an atom's nucleus or far larger than any star.
This has one striking consequence: some of the very lightest primordial black holes that theory allows would already have evaporated away by now through the slow quantum process known as Hawking radiation, something that would never happen to a stellar black hole over the age of the universe.