Did the very young universe make swarms of tiny black holes?
The UniverseFridays
March 27, 2026
5 min read
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Did the very young universe make swarms of tiny black holes?
Long ago, the cosmos might have been a black hole factory—and these primordial objects are even weirder than you think
By Phil Plait edited by Lee Billings
Milatoo/Getty Images
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Black holes are weird. You heard it here first.
But really, they’re even more bizarre than most of us realize. They warp space. They warp time. They can spin so rapidly they wrap the fabric of spacetime around them like a warm blanket on a winter’s day. Despite greedily pulling in everything around them, they are the engines that power some of the most luminous objects in the entire cosmos.
And yet, if you stand back and squint a little, the task of making one is quite simple: just squeeze enough matter into a small enough volume. As you do so, the gravity of the resulting object gets stronger and stronger until eventually the escape velocity equals the speed of light—that is, the fastest anything can move through space. At that point whatever falls in can never get back out, and voilà! Black hole.
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In today’s universe, there aren’t too many ways to do this. The classic method is to blow up a massive star at the end of its life. The outer layers explode away as a supernova, but the star’s core collapses, and, if it’s massive enough (about three times as massive as the sun), it collapses all the way down, becoming a black hole.
I may have skipped a few steps here, but that’s the general picture. There are other ways of making these mindless eating machines, as well, including smashing neutron stars together, letting two black holes merge into a bigger black hole (though, in the spirit of this discussion, I consider this cheating) or, back in the day some 12 billion years ago, streaming matter into a single spot via the influence of dark matter and letting it pile up enough to directly create a supermassive black hole, which just grows from there. Astronomers still argue over this last method.
But in the very early universe—and I do mean “very”—there was another possible method to manufacture these monsters.
We know the universe is expanding, dragging galaxies along with it. That means if you run the clock backward, you’ll see space getting smaller and galaxies closer together in the past. If you go back far enough, before galaxies formed, even before the normal matter that we see around us now took shape, the cosmos was quite dense. It was a thick soup of subatomic particles and energy, getting denser and denser the farther back we look.
In the first teeny fraction of a second after that expansion started, the densities were far higher than in the cores of stars, higher than in neutron stars—actually, very close to what’s needed to create a black hole! Any strong fluctuation in the density of the material making up the universe at that point could create a small overdense region, a small pocket of extra matter, which could then collapse to form what’s called a primordial black hole (or PBH). There are a few theoretical ways these overstuffed spots themselves could arise, but in the end, what’s created is a black hole.
Unlike black holes that form today, which have that three-sun minimum mass, these first black holes could have an extraordinarily wide range of masses, from a few millionths of a gram (far less than the mass of a mosquito) up to ones as beefy as the supermassive black holes we see now.
Mind you, this is all theoretical. No primordial black hole has as yet ever been unequivocally detected. Still, they’re fun to think about.
For example, their sizes are surprising. A black hole’s event horizon—its Point of No Return—depends mostly on its mass. A decent rule of thumb is that it’s about six kilometers in diameter for every sun’s worth of mass; a small stellar-mass black hole would then have an event horizon roughly 18 km wide.
It’s possible, though, that in those early fractions of a femtosecond of the universe’s existence, a primordial black hole with only the mass of Earth formed. If so, its event horizon would b