Our universe is not eternal. Our universe began nearly 14 billion years ago with the expansion from a singularity (Big Bang to its friends). Our universe doesn’t just happen to be finite (i.e. not eternal). Our universe couldn’t be eternal. It had to have a beginning. There is a very good reason for this, and it has to do with the way time works in our little space/time bubble.
First of all, that is what a universe is: a bubble of space/time. If you take out all the planets and the asteroids and the life you are left with a bubble of space/time, a universe. Second, the reason that our universe couldn’t exist forever is this: how many seconds would it take before we got to now? It would take an infinite number of seconds, it would take forever. In fact, this moment would never come if there were an infinite number of seconds before it. It doesn’t make sense for our space/time bubble to last forever.
That’s not really proof that the universe couldn’t be eternal; we don’t know enough about time. But for the sake of this post I’ll accept it, it is convincing.
At the singularity that birthed our universe there was no space/time. The bubble of space/time began at the moment the universe began to expand from the singularity. What came before that? Nothing. In time as we know it, there was no before the Big Bang, time began there. And no one really knows where that singularity came from (although there are some plausible scientific and mathematical models). That singularity could have come from the collapse of a previous space/time bubble. That space/time bubble may have a completely different function of time, it may not. It may also have come from a singularity which was the result of a third space/time bubble collapsing. And that pattern could go back infinitely. And at each singularity space/time is collapsed and reborn.
Having an infinite series of independent space/time bubbles separated by a singularity doesn’t have the logical conundrum that a single, infinite space/time bubble has. Each bubble is finite and has no continuum or connection with the other bubbles. They are independent. Two consecutive space/time bubbles may last for 1 million seconds each, but to say that the two bubbles lasted for 2 million seconds is totally meaningless.
So, whatever makes up universe at its core, its elemental ingredient, could well be eternal. The only thing that cannot be eternal is any one bubble of space/time.
The model of many universes I have discussed here is called an oscillating universe. There are many universes that exist in series. But this same principle could easily be expanded to a bubble-foam universe; a model where many universes exist out of series with each other, and simultaneously. So long as no one space/time bubble is infinite the model is plausible and the ‘foam’ is eternal even though the bubbles aren’t.