Sean Carroll

And so therefore, if you can imagine a finite dimensional quantum mechanical system that has a spacetime interpretation as universe expands, recontracts, crunches, bounces, and then expands, and the cycle returns again and again, you can number one, avoid the Boltzmann brain problem,

because the period is much, much less than what you had in the typical case, the generic case.

And because the period is much, much less, there's just not enough time to make that many Boltzmann brains.

And number two, you're not explaining the arrow of time, but you're accommodating the arrow of time because your cycles, unlike the traditional semi-classical cosmological cyclic universes,

in this model the cycles are exactly repeating each other okay so in this model which is the model we proposed in our paper um what is happening to you right now just like friedrich nietzsche warned you about has already happened to someone like you an infinite number of times in the past and will happen to someone like you an infinite number of times in the future in our model the universe expands from a big bang it branches because it's ever ready in quantum mechanics so there's different branches where different things are going on

It then sort of approaches thermal equilibrium, which looks like empty space, just a sitter space with a positive cosmological constant.

It lingers there for what to you and I would count as a very long time, but still a very short time compared to the naive recurrence time.

And then it starts to…

Matter starts to appear, and it looks like the universe is re-collapsing, but of course it's just the expansion played backward in time, statistically speaking.

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.