The existence of the multiverse is inevitable, and we live in it

An illustration of a multitude of independent universes that are not united by causal relationships in an ever-expanding cosmic ocean is one of the ideas about the idea of ​​the multiverse

Imagine that the Universe, observed by us, from edge to edge - just a drop in the cosmic ocean. That there is still space beyond our field of view, more stars, more galaxies, even more, perhaps a great many billion light-years more than we can ever see. And, although the unobservable universe is large, there are still countless other universes similar to it — some larger and older, some smaller and younger, stretching across the vast expanses of space-time. And while these Universes expand inevitably and quickly, the space-time containing them expands even faster, moving them further apart, ensuring that no pair of Universes will ever meet. Sounds like science fiction: this is the scientific idea of ​​the multiverse. But if science turns out to be correct, then it will not be just a reliable idea, it will be an inevitable sequence of fundamental laws.



The concept of the artist about the logarithmic representation of the observable universe. We are limited to the ability to look into the past in the amount of time that has passed since the time of the hot Big Bang: 13.8 billion years, or, if we take into account the expansion of the Universe, then 46 billion light years.

The idea of ​​the multiverse grows from the physics necessary to describe the Universe that we see and inhabit today. Everywhere we look in the sky, we see stars and galaxies accumulating together in a giant cosmic web. But the further we look into space, the further we look and in time. More distant galaxies are seen by us as younger, and therefore less developed. There are fewer heavy elements in their stars, their apparent size is smaller because they have undergone fewer mergers, there are more spiraling smaller elliptical galaxies (they need time to form after mergers), and so on. If we reach the limits of our ability to see, you will find the very first stars of the Universe, and behind this - the realm of darkness, in which the only light will be the residual glow of the Big Bang.



Looking at more and more distant objects of the Universe, we look into the past, up to the time when there were no atoms yet, up to the Big Bang

But the Big Bang, which occurred simultaneously in the entire Universe 13.8 billion years ago, was not the beginning of space and time, but was the beginning of our observable Universe. Before him, there was an era of cosmic inflation, in which space expanded exponentially, and was filled with the energy inherent in the fabric of space-time. Cosmic inflation is an example of a theory that accompanied the main one, and then eclipsed it, for it:

1. Coordinated with all the successes of the Big Bang and embraced all of modern cosmology.
2. She explained several problems that the Big Bang could not handle, including: why the temperature throughout the Universe is the same, why it is spatially flat, why there are no residual high-energy relics like magnetic monopoles.
3. Made a lot of new predictions that can be checked through observations, and most of them have already been confirmed.

But inflation predicts another consequence, about which we do not know whether we can confirm it, or not: the multiverse.



Inflation causes space to expand exponentially, which can very quickly lead to the fact that any curved space will seem flat

Inflation works in a way that forces space to expand at an exponential rate. Everything that existed before the Big Bang becomes much, much, much larger in size. So far, everything is clear: this explains why we have such a homogeneous and large Universe. At the end of inflation, the Universe is filled with matter and radiation, and this is what we see as the hot Big Bang. But here's what's strange. For inflation to end, the quantum field that is responsible for it must move from a high-energy unstable state that feeds inflation into a low-energy state of equilibrium. This transition and “rolling down” from the hill to the low and causes inflation to end and generate hot Big Bang.



If inflation were a classic field, inflation would have lasted while the field value remained large, but when the value collapsed, it would decrease, inflation would stop and reheat the Universe

But whatever field is responsible for inflation, it, like all other fields obeying the laws of physics, must be quantum in nature. Like all quantum fields, it is described by a wave function, the probability of which is distributed in time. If the field value rolls off the hill slowly enough, then the quantum blurring of the wave function will go faster than the roll, which means that inflation will probably and even possibly slip past the end points and cause the Big Bang over time.



If inflation is a quantum field, then its value is distributed over time, and different realizations of the field value occur in different parts of space. In many areas, the field value will be equal to the bottom of the valley, and inflation will end, and in many others, inflation will continue for an arbitrarily long time.

Since the space expands at an exponential rate during inflation, an exponentially large number of parts of space will emerge over time. In some regions, inflation will end: where the value of the field rolls down to the bottom of the valley. In others, inflation will continue and generate more and more space surrounding all the regions in which inflation has ended. The rate of inflation is much greater than the maximum rate of expansion filled with matter and energy of the Universe, therefore, the expanding regions will rather quickly begin to prevail over everything else. According to plausible mechanisms that give us a sufficiently large value of inflation to generate the observable Universe, there are many more regions of space surrounding ours - where inflation has ended - in which inflation does not end.



Where inflation occurs (blue cubes), it generates an exponentially more number of regions of space with each moment in time. Even if there are many regions in which inflation has ended (marked with red crosses), there will be much more regions in which inflation will continue into the future. And the fact that it never ends makes inflation "endless" after it starts.

Hence the phenomenon known as infinite inflation. Where it ends, we get a hot Big Bang and the Universe, in which we can observe the part where we are (such areas are marked with red crosses in the picture above). But where inflation does not end, even more expanding space appears, as a result of which there appear areas where the hot Big Bang, when it occurs, will not be causally connected to our site, as well as areas where inflation will continue. And so on.



Whatever the huge observable part of the universe, it is only a tiny part of what should exist besides it.

It is this picture, the huge universes, which are much larger than the part that we can observe, constantly appearing in an exponentially expanding space, and describe the multiverse. It is important to understand that the multiverse itself is not a scientific theory. It does not make predictions of any observable phenomena that we could access from our corner of space. Instead, the multiverse is a theoretical prediction that follows from the laws of physics that we understand best today. Perhaps, it is the inevitable consequence of these laws: if you have an inflationary Universe, controlled by quantum physics, then you are destined to come to exactly this conclusion.



Although the existence of a multitude of independent universes is predicted in an expanding space-time, inflation never ends at the same time everywhere, it only happens in certain and independent areas separated by space that continues to expand. Hence the scientific motivation of the idea of ​​the multiverse.

It is possible that our understanding of the state before the Big Bang is wrong, and that our ideas about inflation are completely inapplicable. In this case, the existence of the multiverse is not inevitable. But the prediction of an infinitely expanding state, where countless pocket universes are constantly being born and separated from each other, is a direct consequence of our best theories so far, if they are correct.

What then is the multiverse? It can go beyond the limits of physics and become the first metaphysical phenomenon we have encountered, which follows from physics. For the first time, we understand the limitations of what the Universe can teach us. There is the information we need, but we will never be able to get it in order to raise the idea to the state of science being tested. Until then, we can predict, but neither confirm nor deny the fact that our Universe is only a small part of the grand multiverse.