Avi Loeb

instead of expanding with the rest of the universe, collapsing to a black hole.

And back then, in the fraction of a second after the Big Bang, those black holes would have the mass of an asteroid or smaller than that, the mass of a dust particle down to that.

The smallest black hole we can imagine has a mass of 10 to the power minus five of a gram.

That's called the Planck mass, the smallest.

Because what happens for those black holes

the horizon size is equal to the wavelength of the radiation emitted by them.

So it's sort of an object, the smallest black hole you can imagine because it will immediately evaporate.

But if the universe had dense regions that collapse to make black holes, this could be the dark matter as long as they don't evaporate.

For them not to evaporate within the age of the universe,

They need to be more massive than a kilometer-sized asteroid.

So about 10 to the 15 grams and higher would not evaporate.

It's a viable dark matter candidate.

It was ruled out by microlensing experiments, the method that I helped develop.

They were ruled out.

down to the mass of the moon or so, or even smaller.

So basically, the allowed range for dark matter, being primordial black holes that do not evaporate, is in the mass range of asteroids right now, and we don't know.

It's possible that dark matter is these primordial black holes.

If there were smaller ones, we would see them exploding, and we would see photons coming out of them, and we haven't so far.

When they explode or evaporate, do we get all that...

potential energy release, that information?