Theoretical physicists and cosmologists have to look for answers to the most fundamental questions: "Why are we here?", "When did the Universe appear?" and "How did this happen?" However, despite the obvious importance of finding answers to these questions, there is a question that overshadows all of them with its interest: "What happened before the Big Bang?"
Let's be honest: we cannot answer this question. Nobody can. But after all, no one forbids speculating on this topic and considering several interesting assumptions? Sean Carroll of the California Institute of Technology, for example, agrees with this. Last month, Carroll took part in a biannual meeting of the American Astronomical Society, where he proposed several "pre-explosive" scenarios, whose "final chord" could be the emergence of our universe. Again, this is just speculation, not theory, so please keep this in mind.
"At that time, so to speak, the laws of physics that we know were not yet in force, because" then "they did not exist yet," says Carroll.
“When physicists say they have no idea what happened then, they say it in all seriousness. This segment of history is in absolutely impenetrable darkness,”agrees Peter Voight, theoretical physicist at Columbia University.
One of the strangest properties of our universe is that it has a very low level of entropy. This term has many interpretations, but in this case we are talking about the degree of disorder. And in the case of the Universe, there is more order in it than disorder. Imagine a bomb filled with sand. The bomb explodes, and the billions of billions of grains of sand contained in it scatter in different directions - in fact, in front of you is a model of the Big Bang.
“But instead of the expected chaotic scattering, these grains of sand, representing the matter of our universe, immediately turn into many ready-made“sand castles”, formed incomprehensibly in what way and without outside help,” says Stephen Countryman, a graduate student at Columbia University.
The result of the Big Bang could (and perhaps should have) been the emergence of a high level of entropy of mass in the form of unevenly distributed matter. Instead, however, we see star systems, galaxies and entire galaxy clusters interconnected. We see order.
In addition, it is important to understand that entropy, or disorder, can only increase over time - the same sand castle will sooner or later and without outside help again disintegrate into many grains of sand. Moreover, as Carroll points out, our observation of time is directly related to the level of entropy since the beginning of the universe. At the same time, the entropy itself can be considered as a kind of time-dependent physical property with only one direction of movement - to the future.
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So, entropy, according to the laws of physics, can only increase, but its current level in the Universe is very low. According to Carroll, this can only mean one thing: the early Universe had an even lower level of it, that is, the Universe should have been even more organized and ordered. And this, in turn, may give rise to the idea of what happened to our Universe in fact before the Big Bang itself.
“There are many people who believe that the early universe was a very simple, uninteresting and expressionless system. However, as soon as you connect entropy to this question, the perspective immediately changes, and you realize that in this case there are things that need to be explained,”continues Carroll.
Even if we leave aside entropy, then we will have other equally important aspects that need to be somehow adjusted to our current Universe in which we live. Moreover, in some cases, low entropy levels seem less significant than in others. Therefore, we will try to consider the three most popular assumptions about what could have happened to the Universe before the Big Bang.
The Big Rebound Model
According to one of the hypotheses, the low level of entropy of our Universe is due to the fact that its appearance itself was the result of the disintegration of some "previous" Universe. This hypothesis says that our universe could have formed as a result of a rapid compression ("bounce"), driven by complex effects of quantum gravity (singularity), which in turn gave rise to the Big Bang. In turn, this may indicate that we can live with equal success both at any point in the infinite sequence of emerging Universes, and, conversely, in the “first iteration” of the Universe.
This hypothetical model of the appearance of the Universe is sometimes called the "Big Bounce" model. The first mention of this term sounds back in the 60s, but this model turned into a more or less formed hypothesis only in the 80s - early 90s.
Among the less significant controversial points, the Big Bounce model also has clear flaws. For example, the idea of collapse into a singularity contradicts Einstein's theory of general relativity - the rules by which gravity works. Physicists believe that the singularity effect can exist inside black holes, but the physical laws we know cannot provide us with a mechanism to explain why “another universe”, having reached the singularity, should give rise to the Big Bang.
"There is nothing in general relativity that indicates a" bounce "of the new universe as a result of a singularity," says Sean Carroll.
However, this is not the only big controversial point. The fact is that the Big Bounce model implies the presence of a rectilinear course of time with decreasing entropy, however, as mentioned above, entropy only increases with time. In other words, according to the laws of physics we know, the appearance of a bouncing universe is impossible.
Further development of the model led to the emergence of a hypothesis that time in the Universe can be cyclical. But at the same time, the model is still unable to explain how the current expansion of the Universe will be replaced by its contraction. Yet this does not necessarily mean that the Big Bounce pattern is completely wrong. It is possible that our current theories about it are simply imperfect and not fully thought out. After all, the laws of physics that we now have were derived from the limit according to which we are able to observe the universe.
The Sleeping Universe Model
“Perhaps before the Big Bang, the universe was a very compact, slowly evolving static space,” physicists such as Kurt Hinterbichler, Austin Joyce and Justin Khoury theorize.
This "pre-explosive" universe had to have a metastable state, that is, to be stable until an even more stable state appeared. By analogy, imagine a cliff, on the edge of which a boulder is in a state of vibration. Any contact with the boulder will lead to the fact that it falls into the abyss or - which is closer to our case - the Big Bang will occur. According to some theories, the “pre-explosive” universe could exist in a different form, for example, in the form of a flattened and very dense space. As a result, this metastable period came to an end: it expanded dramatically and acquired the shape and state of what we see now.
“The sleeping universe model, however, also has its problems,” says Carroll.
"It also assumes that our Universe has a low level of entropy and does not explain why this is so."
However, Hinterbichler, a theoretical physicist at the Case Western Reserve University, does not see the emergence of low entropy as a problem.
“We're just looking for an explanation of the dynamics that took place before the Big Bang, which explains why we see what we see now. So far, this is only the only thing left for us,”says Hinterbichler.
Carroll, however, believes that there is another theory of a "pre-explosive" universe that can explain the low level of entropy found in our universe.
The Multiverse Model
The emergence of new universes from the "parent universe"
The hypothetical multiverse model avoids the entropy-decreasing reticence of the Big Bounce model and provides an explanation for its low level today, Carroll says. It originates from the idea of "inflation" - a well-accepted but incomplete model of the universe. The term "inflation" and the first explanation for this model was proposed in 1981 by physicist Alan Guth, currently at the Massachusetts Institute of Technology. According to this model, the space after the Big Bang has expanded dramatically. So dramatically that the speed of this expansion was higher than the speed of light. According to quantum mechanics, random, subtle fluctuations of energy constantly occur in space. At some point during the inflationary period, the peaks of these fluctuations reached their maximum and caused the appearance of galaxies,voids and large-scale low-entropy structures that we observe in the Universe today.
The inflationary model itself was developed on the basis of observations of cosmic microwave background radiation - the oldest type of radiation that appeared just a few hundred thousand years after the Big Bang. Scientists believe that the inflationary model predicts its existence perfectly.
One hypothesis is that the multiverse might be the result of inflation. The assumption says that there is some one very, very large Universe, from time to time giving rise to more compact universes. Moreover, no form of communication between these universes is possible. PBS Nova's Markus Wu explains:
“In the early 80s, physicists came to the conclusion that inflation can have the nature of infinity, stopping only in some regions of space, creating some kind of closed“pockets”. However, between these "pockets" inflation continues, and it flows faster than the speed of light. In turn, isolated from each other "pockets" eventually become Universes."
Carroll is most impressed by this model, although his own proposed model is somewhat different from what is described above:
“This is just one version of the multiverse theory, but the main difference here is that the 'parent universe' can have a high level of entropy and spawns universes with a low level of entropy,” says Carroll.
According to this model, before the Big Bang, there was a kind of large expanding space from which our and an infinite number of other universes were born. Other universes are beyond our ability to detect them and could have formed both before and after our universe.
It should be noted that at the moment this is one of the most popular models. Nevertheless, scientists, of course, perceive it differently. Some support this idea, others, on the contrary, completely disagree with it. But if we take Peter Voight from Columbia University as an example, then the theory of the Multiverse, although it looks very attractive from a popular science point of view, can make physicists lazy and make them stop looking for answers to the most basic questions, for example, why are physical constants in our Universe? exactly as they are - writing off all of variability.
“Theorists speculate about the possibility of an infinite number of universes, and ultimately we can come up with clear models that can explain why values (such as the fundamental properties of the particles we observe) can differ from each other in each individual universe,” says Voight …
Voight fears that one day the main question for science in this area will be the reasoning on the topic “how lucky we are to be in this random universe, where everything happens this way, and not differently, despite the infinite variety of possibilities, so let's give up this venture with theories.
What can be summed up? Many physicists get paid for arguing and writing books in which they try to describe how the Big Bang and the model of a "pre-explosive" universe can explain what we see today, although they themselves do not know and really cannot know. why is it so. The fact is, even though there are serious simplifications in both mathematical models and explanations, we have not come close to the correct answer, and we still have a lot of reasoning on this topic until we come to the desired result.
“It is important not only to put forward theories and hypotheses. It is much more important to make it clear to people that in fact we ourselves do not yet understand what we are talking about. All this is only at the level of assumptions, but I hope that sooner or later we will be able to find the right answer that will suit everyone,”says Carroll.
NIKOLAY KHIZHNYAK