Sean Carroll

Is it an exact repeat?

Is it only approximate?

Et cetera, et cetera.

Okay.

I've never been especially fond of these ideas, of the cyclic universe idea, mostly because I care about the arrow of time.

So, you know, the arrow of time problem has to do with the fact that our observed universe has a very low entropy initial condition near the Big Bang.

So the options are either number one, that is the beginning of the universe and you have to explain why it had low entropy.

Or number two, it's not the beginning of the universe and you put it into some bigger context and try to explain why it seemed to have low entropy.

So the problem, as I see it, with most cyclic cosmologies is that they are cyclic but not periodic.

So they are cycles.

The universe expands, contracts, bounces, expands, contracts, bounces, and does that a very long number of times.

But the cycles are not exact copies of each other.

And this is just a feature of...

most models, okay?

We're not talking about what is necessarily true, we're talking about ideas that scientists have actually explored.

And so in particular, this goes back to Tolman, I think, Richard Tolman is his name, like many years ago,

thinking about the possibility of cyclic cosmologies and pointing out that entropy will increase from cycle to cycle.

Again, not just as a matter of necessity, but as a matter of what is overwhelmingly likely.

And so Tolman said that the cycles will take longer and longer each time.

and from the point of view of the arrow of time many many of the modern versions of cyclic universes have a similar feature that entropy it may or may not go up and down a little bit but it's increasing from cycle to cycle and what that means is that if you think about going backward in time the entropy is increasingly finely tuned right and that same thing for every other fine-tuning feature of the universe like if you think that it's weird