Sean Carroll

What if you're thinking about the whole universe all at once?

And someone says, okay, I have two cosmological models, two theories that describe all of the universe at once.

And they predict statistically more or less the same local conditions that we observe.

So they are compatible with the data that we already have.

But here's the difference.

In one theory, the universe is bigger.

than in the other one.

Like maybe in one theory, the universe is a closed universe, a sphere or a torus or something like that, and it doesn't actually extend very far beyond the universe that we can see today.

In the other theory, the universe is open, it goes on forever, and there's just an infinite number of things going on.

And this person says, so I think that the theory where the universe is bigger is much more likely.

You say, well, why is that?

Is it because there's some mechanism that gives you that or whatever?

And they say, no, it's from updating on the data.

And you say, what is that data?

And they say, well, the data that I exist.

Because in the bigger universe, it is just much more likely that someone like me would exist than in the smaller universe, just because there's random fluctuations because of quantum mechanics.

It's unlikely in any one small universe that I would exist.

But as the universe becomes bigger and bigger, the chances of someone just like me get larger and larger.

Is that kind of reasoning correct in the cosmological context?

The answer is we don't know, or at least we don't have an agreed upon procedure for dealing with these kinds of puzzles.