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The physically infinite universe
We will look into some ways in which this can be true. Firstly, though, we should understand that there is a difference between “universe” and “visible universe”.
The Big Bang that started everything we see in motion happened about 13.8 billion years ago. Ever since that time, space itself has been expanding so rapidly that light that came out of stars billions of years ago is still only just arriving in our telescopes today, and has been stretched by the expanding space over those billions of years as it travelled.
There was a period of expansion in the beginning where space had not yet cooled enough for light to freely travel. The light we measure from the “beginning” of the visible universe is actually from the beginning of when light could freely travel.
By that time, the universe had already expanded so much that objects that had been right next to us at the very moment of the Big Bang are now so far away from us that we only just see their light now.
The “visible universe” is that tiny part of reality that was right next to us in the Big Bang that was close enough that light from it could reach us since it has been possible for light to travel.
We can see objects up to about 38 billion light years away.
This does not mean that there is nothing further away than that. Just that light from further objects has not yet had time to reach us.
And, there is absolutely no reason at all to think that the universe simply stops right at that distance. To even imagine that it simply stops completely at a distance of 38 billion light years away from us is to make a hugely self-centred statement – that we, humans, are literally at the centre of the universe.
It has been known for almost a century that every galaxy is moving away from every other galaxy, in the same way that all dots painted on a balloon appear to move away from every other dot as the balloon grows. If you imagine yourself looking outwards from any one of those galaxies, you will find that the universe looks almost exactly the same in all directions no matter where you are.
Even the idea that the universe simply stops suddenly is not easy to imagine. If you are in a spaceship and you fly to the edge of the universe and then fly one more kilometre, are you now “outside” the universe?
There are four possible solutions to this problem.
First solution: the universe is truly infinite in size. There is no reason I know of that this cannot be true. In fact, the idea that the universe is infinite in size is actually more reasonable than the idea that the universe has a precise non-zero size (or mass) and is no larger than that. Either the universe is infinite in size, or it is zero in size. Everything else throws up very difficult to answer questions.
Illustration 8: we can only see 46 billion light years in any direction, but there's no reason to think that the universe simply stops right there
Second solution: the universe is finite, but not “flat” - it curves back on itself at some point. In this idea, if you travel in a straight line in any direction, you will eventually come back to where you started because the universe “loops”. This is not as crazy as it sounds. However, there is also no evidence that it is true. As far as we can tell by measuring the “flatness” of space, we are not in a curved universe.
Third solution: there are an infinite number of “universes” out there in the empty space beyond our own visible universe. The idea here is that if you travel far enough to reach the end of all visible matter in the universe, and then keep travelling, you might eventually find yourself entering another universe. This is based on the idea that “universe” means “everything that originated in the Big Bang”, and that the Big Bang is an event that happened in an even larger space-time that can contain other Big Bangs. One thing to keep in mind, though, is that what we call “space” is actually a part of the universe itself. As far as I know, if there are multiple physical universes sharing a common “reality”, there is no way for them to actually be linked together, so travelling far in any direction won’t really get you from one to another.
Fourth solution: we are in a universe that really is finite. If we travel far enough, we will reach a point where there simply is nothing more in that direction, forever. The immediate flaw in this is that it states straight out that our Big Bang was a special one of a kind event that can never be repeated at any point ever, anywhere. But that’s obviously wrong – if anything can be done once, it can be done twice. If a universe can begin in one place, another universe can begin somewhere else. We don’t know for a fact how to create a universe, but that certainly does not mean that it cannot happen twice. If, for example, the universe was created by a god, then who are we to say that this god cannot do it again? And if the universe emerged from a quantum void, how can we say it won’t happen again?
Of the four solutions above, one is flawed (the universe is finite and unique), one is very probably wrong (the universe loops around on itself and is finite), and the other two solutions suggest that the universe really is infinite in size, or that there may be infinite baby universes out there, not linked together by space-time, but an infinite many of them.