Thinking about what the universe is, most people imagine the limitless depths of space, limited by our observation capabilities, and everything that has ever been or will be. But even with a universe that:
- contains hundreds of billions of galaxies;
- in each of which there are billions or even trillions of stars;
- which has existed for 13.8 billion years since the Big Bang;
- and extends over 46 billion light years as far as we can see;
- and about 1091 particles in it are available to us, it is still finite and limited. This is our observable universe, which began with the hot Big Bang and which contains everything that can be comprehended. And yet, perhaps there is much more than that.
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If we were anywhere else in this universe, we would be able to see the same amount of the universe. On the largest scales, the Universe is more than 99.99% homogeneous, and its density variations do not exceed 0.01%. This means that if we were lucky enough to be anywhere else, we would still see hundreds of billions of galaxies, about 1091 particles, scattered over 46 billion light years. We would just see a different set of galaxies and particles, slightly different in detail.
From everything that we can observe, and of all the theoretical guesses that the Universe throws at us on the topic of topology, shape, curvature and origin, we fully expect that somewhere there is much more of the Universe - identical in properties to the one that we observe - but we do not see it. And only due to the fact that the universe has existed for a certain period of time, we can see its specific part. Basically, this is the simplest definition of a multiverse: beyond what we see, there is much more of the unobservable universe.
Most scientists take this for granted, because otherwise we would see the universe much more curved, or see repeating patterns in the cosmic microwave background. The lack of evidence for this clearly indicates that there is much more than anything else outside of the known universe. The absence of strong curvature indicates that we cannot see hundreds of times more of the universe; the unobservable universe is much larger than our own. But no matter how big it may be, it probably originated from one cosmic event - that same Big Bang - billions of years ago.
But the Big Bang was not only the "beginning" of the universe. There was a state before the Big Bang that started it all: cosmic inflation. This exponential rapid expansion of space itself in the young universe created more and more space as it lasted. And if inflation has definitely come to an end where we are, something else is possible: the speed with which inflation creates new space in almost all models is higher than the speed with which it ends and the Big Bang begins. In other words, inflation predicts an unusually large number of disconnected Big Bangs, each of which gave rise to its own universe.
This multiverse is even larger than we previously thought, and if the inflationary state was eternal (and it could be), then the number of universes is infinite, not finite. Which is strange, because in these other universes formed by other big bangs, there may be completely different physical laws and constants. In other words, there may be not just areas with worlds similar to ours, but with worlds that are completely different from ours.
What is the multiverse? It can be understood as one of three:
1. More "Universe" similar to ours, which came out of the same Big Bang, but is not observable.
2. More Universes like ours that came out of other Big Bangs, but were born in the same inflationary state.
3. Or there may be many more universes - some like ours, and some like not - with different constant and even laws.
The multiverse can be finite in size and number of universes, or infinite. If you accept the Big Bang and modern cosmology, then the former will certainly be true. If you accept cosmic inflation (and for good reason), the latter is true. If you accept certain models of string theory or other unification theories, the third may be true. As for the question of finiteness or infinity, here we do not yet know for sure. There is a theorem that inflation could not go on forever, but it also has loopholes that allow inflation to go on forever.
One thing is certain: the multiverse exists and you don't need to be a scientist to admit it. The question is which version of the multiverse is hiding from us, and this we may never know.
ILYA KHEL