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 be just under two centimeters across, the size of a grape! Think about that the next time you’re grabbing fruit at your local grocer.
But far smaller ones could have been created as well. A mosquito-mass PBH would be just 3 × 10–33 meters in size: a proton is about a billion billion times bigger. Such an object would be bizarre in the extreme; it could pass through solid matter without ever touching a single atom, and even though its gravitational field would be incredibly grabby, the force dies off with distance so rapidly that this mosquito-mass PBH might never get close enough to another bit of matter to draw it in! It would, in effect, be isolated from the much, much, much larger universe.
Even if such minuscule PBHs were created, most of them might be gone by now. This is because of an idea that’s bizarre even for black holes: they have a temperature. Well, sort of—in the 1970s physicists started working out the ridiculously complicated effects of quantum mechanics on black holes and discovered that their gravity is so intense that it has an unexpected effect on spacetime. Quantum mechanics essentially causes a black hole to emit energy in the form of light just outside the event horizon. This is called Hawking radiation, after Stephen Hawking, who first did the temperature calculations.
The effect for stellar-mass black holes is vanishingly small but actually gets larger for less massive black holes. This means that the smallest PBHs will be the hottest, which means they emit the most photons. That energy has to come from somewhere, and the source is the mass of the black hole itself. Because the energy flies away, this means the black hole must be losing mass. Astrophysicists call this evaporation, but it’s not quite the same as boiling water disappearing into steam.
(Yes, this is completely contrary to the idea that once something falls into a black hole it’s there for keeps, but it turns out when quantum mechanics is involved, black holes aren’t like Las Vegas.)
There’s more, too. Because the black hole is losing mass, it winds up emitting radiation faster and faster as it does so. This vicious cycle increases until the energy is released essentially in an explosion. Pop! Gone.
It’s possible to calculate how long that takes, and it turns out any PBH with less than about a billion tons of mass would have already evaporated over the age of the universe. That’s very roughly equivalent to a mountain about 750 meters high—though a black hole with that mass would be less than a trillionth of a centimeter across, far smaller than a hydrogen atom.
Could we detect such a catastrophic evaporative PBH explosion? In theory, yes, because the radiation emitted in the last few milliseconds would be in the form of extremely high-energy gamma rays. NASA’s Fermi Gamma-ray Space Telescope could potentially detect these intense explosions, but (so far) it hasn’t found any.
PBHs have also been considered as a possible constituent of dark matter, the invisible matter permeating the universe that outweighs normal matter by a factor of about five. This is highly contentious, and physicists argue over how big these PBHs could be and how much dark matter they’d constitute, but no conclusion has been reached yet.
We don’t know if these first black holes even exist or ever did, but it’s a fascinating topic, even with the high bar set by the “normal” black holes—if you can consider anything about black holes normal.