In simple words: what is a multiverse?

Thinking about what the Universe is,most people imagine the unlimited depths of space, limited by our ability to observe, and everything that has ever been or will be. But even with such a universe, which:

If we were anywhere else in thisUniverse, we could see the same amount of the Universe. On the largest scales, the Universe is more than 99.99% homogeneous, and variations in its density do not exceed 0.01%. This means that if we were lucky to be somewhere 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 details.

Of all that we can observe, and of allof the theoretical conjectures that the Universe throws at us on the topic of topology, form, curvature, and origin, we fully expect that somewhere there is much more of the Universe - identical in properties to the one we observe - but we do not see it. And only due to the fact that the Universe existed for a certain period of time, we can see its specific part. In fact, this is the simplest definition of the multiverse: beyond what we see, there is much more to the unobservable universe.

Most scientists take it for granted,since otherwise we would see that the Universe is much more curved, or we would see repeating patterns in a cosmic microwave background. The lack of evidence for this clearly indicates that beyond the limits of the known Universe, there is much more than anything else. The absence of strong curvature indicates that we are not visible hundreds of times larger than the universe; the unobservable universe is much larger than our own. But no matter how big it may be, it must have come from one cosmic event - that same Big Bang - billions of years ago.

But the Big Bang was not just a “start”The universe. There was a state before the Big Bang, from which it all began: cosmic inflation. This exponentially rapid expansion of space itself in the young Universe created more and more space while continuing. And if inflation has definitely come to an end where we are, perhaps something else is also possible: the rate at which inflation creates a new space in almost all models is higher than the rate at which it comes to an end 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 bigger than we thoughtbefore, and if the inflationary state was eternal (and it could be like that), then the number of universes is infinite, and not finite. Which is strange, because in these other universes formed by other big explosions, there may be completely different physical laws and constants. In other words, there may be not just areas with worlds like ours, but with worlds that are completely different from ours.

What is a multiverse? It can be understood as one of three things:

The multiverse can be finite in size andthe number of universes or infinite. If you accept the Big Bang and modern cosmology, then the first will certainly be true. If you accept cosmic inflation (and there are good reasons for this), the second will be true. If you accept certain models of string theory or other unification theories, a third may be true. As for the question of finiteness or infinity, then here we do not know for sure. There is a theorem that inflation could not go on forever, but there are loopholes in it that allow inflation to go on forever.

One thing is certain: a multiverse exists, and you do not need to be a scientist to acknowledge it. The question is which version of the multiverse is hiding from us, and this we may never know.