Adam Brown

towards the high density stuff.

So if you have a small inhomogeneity, they naturally grow under that effect where they just gravitationally fall towards the denser thing.

If you start seeded with small inhomogeneities, that will grow large inhomogeneities.

And that's well understood.

The thing that we now understand much better than we did is where those small inhomogeneities come from.

Like why, just after the Big Bang, was the universe not perfectly homogeneous?

Because if it was perfectly homogeneous, there's no opportunity for it to

for anything to grow.

And we now understand with a high degree of confidence something that we didn't understand, which is that those inhomogeneities were seeded by quantum fluctuations.

That when the universe, just after the Big Bang, was considerably smaller than it is today, the effects of quantum mechanics were correspondingly more important.

And those quantum fluctuations produced tiny little fluctuations

in the density of matter in the universe.

And all of those tiny little one part in a million fluctuations grew into all of the structures you see in the universe, all the galaxies, you, me, everything else.

We believe that these were generated during the period we call inflation, very poorly understood, very early in the universe.

there were fluctuations made not just at one scale in those days, but at all scales, or many, many scales.

So there were fluctuations made at a scale that nowadays corresponds to 10% of the distance across the visible universe, all the way down to...

structures that were, you know, inhomogeneities that were much, much smaller scale that crossed onto a galaxy today, all the way down to, now this is speculation, but in some models of inflation, there were tiny inhomogeneities, very small scale inhomogeneities that would give rise to primordial black holes, like tiny little black holes left over from the Big Bang.

There's no actual evidence in terms of observational evidence, no strong observational evidence for those, but those are a possibility that's allowed by our theory and people think about them and look for them.

I think general relativity is really an extraordinary story.

It's pretty unusual in the history of physics that you, to first approximation, just have one guy who sits down and thinks really, really hard with lots of thought experiments about jumping up and down in elevators and beetles moving on the surface of planets and all the rest of it.