Are There Other Universes?

This is a two part question: 1) How big is the Universe we live in, and how do we know this? 2) Are we the only Universe, and do YOU believe there are other Universes, apart from ours, if not? - Question posed by Sammy.

I covered the first part of Sammy's question in this post, so you might want to read that first!

As for the second part... In all honesty, you could do any entire university course on this topic! I'll try to pick out the best bits.


What is a universe?

Before we start to talk about possible answers to this question we have to make sure that we know what a 'universe' actually is. Classical definitions of 'the universe' can generally be summed up as 'everything'. Indeed, mediaeval philosopher Johannes Scotus Eriugena defined it exactly that way.

The difficult bit is, then, what do we mean by everything? There are certain places in our universe that we can never know anything about, for reasons discussed in the post mentioned at the beginning of this one, so do these count as being outside of our universe?


What might other universes be like?

There are various multiverse theories in existence. The 'multiverse' is the hypothetical set of possible universes, including the one which we call our own. Cosmologist Max Tegmark has developed a four-level taxonomy of possible universes outside our own.

Level 1 universes include areas of what we might term our own 'universe' but are unreachable to us because of the speed of light and the expansion of space. Part of this idea, assuming that the universe is infinite and that the matter contained within it is also infinite (and fairly evenly spread), is that if we travelled far enough* we would eventually encounter an identical copy of our own universe, right down to you and your pet frog.

Level 2 universes have different physical constants to our own: maybe space is expanding at a different rate, or maybe even contracting instead. This level also contains ideas such as those of oscillatory universes (those which form from a big bang, and eventually collapse in a 'big crunch', which sets off a  new big bang, and the whole process repeats) and fecund universes (which suggests new universes with slightly different properties may be born inside black holes, which then go on to produce black holes and baby universes of their own).

Level 3 contains the 'many worlds' interpretation of quantum mechanics. This is similar on basic levels to the classic sci-fi idea that each outcome to a decision is played out in different 'parallel' universes**. Level 4 includes any universe that can be described mathematically. This includes many weird and wonderful possibilities that it can be impossible for a human being to properly comprehend.


Are we the only universe?

We don't know, and we may never know. There has, however, been research into patterns in the background radiation that we can detect from the Big Bang that some scientists think could possibly be an indication of another universe's gravity interfering with ours. That's by no means confirmed though, and could all come to nothing.


Do you believe there are other universes?

I don't believe in believing things without evidence, and so far there is no evidence beyond mathematical models that say they are possible. Just because something is possible does not mean that it definitely exists, so to answer this question honestly, I have to say that there isn't enough information available to make a decision!

So I'm going to answer a slightly different question; one that you may have asked me in a parallel universe:

What would you like to be true?


I'd go for the most interesting answer: the one that says yes, there are other universes! The idea of universes beyond our own provides inspiration for our imaginations, whether that goes into creating scientific hypotheses or writing stories. It would be amazing to think that there are other universes out there, some wildly different from ours, some relatively close cousins, all with their own histories and futures. We probably wouldn't ever be able to visit them, but it's good to dream.




* This is, of course, impossible, but an estimate for how far you'd have to travel has been calculated. Your identical copy should be about 10¹⁰¹¹⁵ metres away. Before you get your hiking boots on, 10¹¹⁵ is a 1 with 115 zeroes after it. 10¹⁰¹¹⁵, then, could be written like this:
10^{1000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000}. To write that out as a proper number would take up more space than I have available in this post, even if I deleted everything else I've already written.
** i.e., you flip a coin and it lands on heads, but a parallel universe is created in which a parallel version of you flips a coin and gets tails. There's more to it than that, but the analogy works for now!