Let Me Blow Your Mind

Here’s something trippy to think about.

The universe is big. Like, really big. There are a lot of particles in it, and a ton of space.

Let’s pretend as though three things are true. This is a thought experiment, so we’ll have to go with the flow and imagine these things really are the case, even though we have some evidence that at least makes us raise a skeptical eyebrow.

First, we need for the universe to be deterministic. That means that there is a very clear, predetermined set of rules that everything follows, all the time. It also means that this set of rules is something that can be used to predict any future outcome, so if you follow a particle and you know its position and momentum right now, you’ll know it at any arbitrary future point, provided it doesn’t collide with another particle.

If it collides with another particle, that’s totally cool too, because we know exactly where that other particle will be, and when. We know the position and momentum of every particle in the universe. We are Laplace’s demon writ large.

Second, we need to have infinite time. Maybe we have infinite time, and maybe we don’t—I really don’t know. It’s clear that spacetime is the fabric upon which the entire universe seems to exist, and space and time are inseparable: if you don’t have any space, then you also don’t have any time.

Regardless, we need to have our time set to infinity while playing this universe on cheat mode.

Finally, we need to assume that the universe has a finite number of particles in it. This number can be so huge as to seem almost meaningless to us, but it still needs to be finite.

If you have all of these things, then something funny happens.

Actually, everything happens, whether it’s funny or not. Then, it eventually happens again and again. Remember: if we know the position and momentum of every particle, we can predict the outcome. At some point, the position and momentum of every particle will be in just the right state, so that every single one of those particles will end up exactly where it is right now.

Eventually, everything will happen again.

This is one way to describe how a cyclical universe might come to be, but there are other ideas out there that might seem even wilder than this one. Pardon my laziness, but here’s how I described one version of this about fifteen years ago:

In a brief moment of contemplation, one of the possible models of the universe Einstein considered, among others, was that of a "cyclic model", featuring what is called an "oscillating universe." The idea is that gravity, eventually, will overcome the expansion of space (which is only logical when you think about it; after all, gravity constantly attracts, so everything should be slowing down over time). As the mutual gravity of all matter pulls collectively inward, the universe must eventually collapse in a "Big Crunch", the opposite complement of the "Big Bang."

Einstein's thought process continued from here. What if the "Crunch" was just a moment when everything crossed the paths of the universal center, and it just kept flying away in the opposite direction? Consider one particle from way, way over there (say, 10 billion light years) accelerating until it crosses the collective center of gravity of the whole universe. Unless said particle was then annihilated in the "Big Crunch", the particle would simply keep going, just like an eternal pendulum, and the process of the "Big Bang" would thus begin again.

I don’t remember when this tantalizing concept was first introduced to me, but I know it blew my mind right away. Could the universe be bouncing back and forth out there? Could it be recreating exact copies of itself over and over again, inevitably repeating the same process forever?

There are even more wild ideas out there about how a cyclical universe might work, even some that are consistent with an accelerating expansion, which all signs show us to be the case. I’m going to leave some space for you to contribute your own wild idea—do you know of any cyclical universe models? What’s the most intriguing version for you?