Multiverse
The multiverse (meta-universe) is a hypothetical set of all possible real parallel universes (including the one we are in). Representations about the structure of the Multiverse, the nature of each universe that is part of its composition, and the relationships between these universes depend on the chosen hypothesis. Universes entering the Multiverse are called alternative universes, alternative realities, parallel universes or parallel worlds.
Various hypotheses about the existence of the multiverse were expressed by cosmologists, physicists, philosophers, religious figures and science fiction writers. The possibility of the existence of a multiverse generates various scientific, philosophical and theological questions. The term "multiverse" was created in 1895 by philosopher and psychologist William James (however, in a different context)
In simple words: what is the multiverse?
Thinking about what the universe is, most people imagine the boundless depths of the cosmos, limited by our observation capabilities, and everything that ever was or will be. But even with such a universe that:
contains hundreds of billions of galaxies;
in each of which billions or even trillions of stars;
which exists 13.8 billion years from the time of the Big Bang;
and extends to 46 billion light years, as far as we can see;
and we are available about 1091 particles in it,
it is still finite and limited. This is our observable universe, which began from the moment of the hot Big Bang and which contains everything that can only be comprehended. And yet, perhaps there is much more of this.
If we were anywhere else in this universe, we would be able to see the same amount of the universe. On the largest scale, the universe is homogeneous by more than 99.99%, 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 by 46 billion light years. We would simply see another set of galaxies and particles, a bit different in detail.
From all that we can observe, and from all the theoretical conjectures that the Universe throws at us on 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 because of 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 a multiverse: beyond what we see, there is much more to the unobserved universe.
Most scientists accept this as a given, because otherwise we would have seen that the universe is much more curved, or have seen repeated patterns in the cosmic microwave background. The lack of evidence for this obviously indicates that beyond the known universe we have much more than anything else. The absence of strong curvature indicates that we can not see hundreds of times more than the universe; The unobserved universe is much larger than our own. But no matter how big it is, it must have originated from a single cosmic event - that same Big Bang - billions of years ago.
But the Big Bang was not just the "beginning" of the universe. There was a state before the Big Bang, from which everything began: cosmic inflation. This exponential rapid expansion of space itself in the young universe created more and more space while it continued. And if inflation has definitely come to an end where we are, maybe another: the speed with which inflation creates a new space in almost all models is higher than the speed with which it comes to an end and the Big Bang begins. In other words, inflation predicts an unusually large number of severed Big Bangs, each of which gave rise to its own universe.
This multiverse is even bigger than we thought before, and if the inflationary state was eternal (and it could be so), then the number of universes is infinite, not finite. What is strange, since in these other universes, formed by other large explosions, there can be completely different physical laws and constants. In other words, there may not be simply areas with worlds like ours, but with worlds that are completely different from ours.
What is a multiverse? Under it you can understand one of three things:
More "Universe", like ours, which came out of the same Big Bang, but not observable.
More Universes like ours that came from other Big Bangs, but were born in the same inflationary state.
Or there may be many more universes - some as ours, some as not - with different constants and even laws.
The multiverse can be finite in the sizes and 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 compelling reasons), the second will be true. If you accept certain models of string theory or other unification theories, the third may also be true. As for the question of finiteness or infinity, here we do not know for sure. There is a theorem that inflation could not last forever, but there are loopholes in it that allow inflation to last forever.
One thing is certain: a multiverse exists, and you do not need to be a scientist to admit it. The question is which particular version of the multiverse is hidden from us, and we probably never will know it.
