Sean Carroll

In the early universe, it was even smoother than it is now.

It was very, very smooth.

There was only a difference in one part in 100,000 as you went from place to place.

Number one, why was it not perfectly smooth?

But number two, why was it pretty darn smooth?

And number three, how did it evolve, using the word evolve again, from

that condition 100,000 years after the Big Bang to our conditions now.

The last one, how it evolved, is the one we have the best handle on.

It was gravity doing the work.

Gravity turns up the contrast knob on the universe.

So if you have a slightly emptier region, it empties out.

If you have a slightly heavier region, it collects matter onto it.

And so we went from very faint ripples, if you look at the cosmic background radiation, to these very vivid,

voids and galaxy clusters that we see today.

We still don't know where those first ripples came from.

Inflationary cosmology is a favorite thing to talk about, but that's a whole other episode.

There you go.

Well, it's once again a reflection of the fact that the early universe had low entropy because gravity was so strong in the early universe.

A more common generic random configuration would have been wild fluctuations like black holes here and empty space there.

And so the fact that it was so smooth does kind of demand an explanation.