Sean Carroll

It doesn't need to be an exact copy.

Just to be super-duper clear about this—we're clear in the paper, I'm not being clear right now—

The contracting phase is not exactly the time reverse of the expanding phase.

It's statistically very, very similar.

What is exactly the same is the cycle as a whole.

So one expanding phase is exactly the same as the next expanding phase and so on down the line.

So you're not explaining the arrow of time by making the initial condition natural.

It's exactly the opposite of that.

You're just super-duper fine-tuning it.

But what we're arguing is that there is a way to super-duper fine-tune so you get a phenomenologically acceptable universe.

So you're not ruined by the Boltzmann brain problem.

You have exact cycles forever and ever.

And at the end of the day, it's all just quantum mechanics that is suggesting this.

In the paper, we wave our hands about

how to map this quantum system onto a classical system, but space-time with a metric and things like that.

But that's the part, of course, that we have the least confidence in, in the paper.

When we're just solving the Schrodinger equation, we get it exactly right.

When we're trying to interpret that as an emergent space-time, it's much harder.

So again, just to be super clear, in this model, unlike most other cyclic cosmologies,

the arrow of time reverses during the crunch.