Question: do you believe in the theory of our universe in a bubble of another universe, or do you think that this universe just goes on and on forever? Why?

Laurence Perreault Levasseur answered on 17 Jun 2014:

That is a very good question.
First, there is something I’d like to define: the observable Universe.
The observable Universe is a fancy word to say ‘everything we can see in the Universe today’. We can’t see everything there is in the Universe. Instead, there is a furthest distance past which we can’t see. This is maximal distance is the distance light has had time to travel since the Big Bang. Everything beyond that is so far that light hasn’t had time to reach us yet. This also mean that the observable Universe is constantly becoming bigger. Finally, this means that we have no experimental way to know anything about what lies beyond our observable Universe.

The actual total Universe, however, can be completely infinite all along, since the beginning of times – and we think it is.

Ok. To answer your question now, just after the Big Bang there was a period of inflation, when space was expanding exponentially fast (that means really CRAZY fast). During that time, tiny pieces of space were stretched well beyond the size of the observable Universe today.

If all of space stopped inflating at basically the same time, then the Universe would just go on and on forever: it would be infinity and look the same everywhere. Inside our observable Universe, but also everywhere beyond it.

Instead of that, let us now imagine that inflation ended in some parts of the Universe, but not everywhere at the same time. Then, say, here, in a little ‘bubble’ of space, inflation has ended, and our normal Universe as we see it starts evolving. But everywhere outside that bubble, the Universe is still expanding like crazy and inflation is still going on!

Then in a little bubble somewhere else inflation ends at a different time, and another universe start evolving, with similar properties as our own, but maybe slightly different, because, after all, it’s not the same place, nor the same Universe!!

Even with those two bubble universes, there is still a lot of space in inflation, and the amount of space in inflation is MUCH bigger (and constantly becomes bigger) than the space where inflation has stopped. So I can basically create an infinite amount of ‘little’ (I mean, they are still bigger than our observable universe today!!!) bubble Universes, and still have even more space in inflation, outside of those bubbles.

They will all be different, be at different places, and have different stuff in it. This theory where some part of space is always in inflation is called ‘eternal inflation’ and the ensemble of all the bubble universes is called the ‘multiverse’.

This theory has some problems to it, though. Since to test it we’d have to go outside of our observable universe, and it’s impossible, it doesn’t yet make any testable prediction. This means that many scientists think it’s not a scientific theory, because it cannot be falsified by experiment.