Theorists and cosmologists have to searchanswers 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 them all with its interest: “What was before the Big Bang? ”
Frankly: we cannot answer this question. Nobody can. But no one forbids to speculate on this topic and consider several interesting assumptions? I agree with this, for example, Sean Carroll of the California Institute of Technology. Last month, Carroll took part in a biannual meeting of the American Astronomical Community, where he proposed several "pre-explosive" scenarios, whose "chord" could be the appearance of our universe. Again, this is just a discussion, not a 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 whatthen it happened, they say it in all seriousness. This stretch of history is in utter darkness, ”agrees Peter Voight, a theoretical physicist at Columbia University.
One of the strangest properties of our universeis 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 fly apart - in front of you is essentially a model of the Big Bang.
“Only instead of the expected chaotic expansion, thesethe grains of sand representing the matter of our Universe immediately turn into many ready-made “sand castles” that were formed in a way that was not possible without outside help, ”says Stefan Countryman, a graduate student at Columbia University.
The result of the Big Bang could (and possiblyshould) the appearance of a high level of entropy of the mass in the form of unevenly distributed matter. However, instead we see stellar systems, galaxies and entire galactic clusters, interconnected. We see order.
In addition, it is important to understand that entropy, ordisorder, with time they can only increase - the same sand castle sooner or later and without outside help will fall apart into many grains of sand. Moreover, as Carroll points out, our observation of time is directly correlated with the level of entropy since the advent of the universe. At the same time, entropy itself can be regarded as a kind of time-dependent physical property that has only one direction of progress - into the future.
So, entropy, according to the laws of physics, canonly 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, that is, the Universe should have been even more organized and orderly. And this, in turn, may suggest the idea of what happened to our Universe before the Big Bang itself.
“There are many people who think that earlyThe universe was a very simple, uninteresting, and expressionless system. However, as soon as you connect entropy to this issue, the perspective changes immediately, and you understand that in this case things appear that need to be explained, ”Carroll continues.
Even if we put aside the entropy, then beforewe will remain with other equally important aspects that need to be somehow adjusted to our current Universe in which we live. Moreover, in some cases, a low level of entropy seems less significant than in others. Therefore, we will try to consider the three most popular assumptions about what could happen to the Universe before the Big Bang.
The Big Bounce Model
According to one hypothesis, lowthe entropy of our Universe is due to the fact that its appearance in itself was the result of the collapse of some “previous” Universe. This hypothesis says that our Universe could be formed as a result of rapid compression (“rebound”), controlled by the complex effects of quantum gravity (singularity), which in turn generated the Big Bang. In turn, this may indicate that we can equally well live at any point in an infinite sequence of emerging Universes, and, conversely, in the “first iteration” of the Universe.
This hypothetical model of the appearance of the universesometimes called the "Big Bounce" model. The first mention of this term sounds back in the 60s, however, this model turned into a more or less formed hypothesis only in the 80s - early 90s.
Among the less significant controversial issues, the modelThere are obvious shortcomings of the Big Bounce. For example, the idea of collapse into a singularity contradicts Einstein's general theory of relativity - the rules according to which gravity works. Physicists believe that the effect of a singularity can exist inside black holes, but the physical laws known to us cannot provide us with a mechanism to explain why the “other Universe”, having reached a singularity, should give rise to the Big Bang.
“There is nothing in the general theory of relativity that would indicate a“ rebound ”in 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 known to us, the appearance of a bouncing universe is impossible.
Further development of the model led tohypotheses that time in the universe may be cyclical. But at the same time, the model is still not able to explain how the ongoing expansion of the Universe will be replaced by its compression. And yet this does not necessarily mean that the Big Bounce model is completely wrong. It is possible that our current theories about it are simply imperfect and not fully thought out. In the end, the physical laws that we now have were deduced from the limit, according to which we are able to observe the universe.
Model of the "Sleeping" Universe
"Perhaps, before the Big Bang, the Universeit was a kind of very compact, slowly evolving static space, ”theorists such as Kurt Hinterbichler, Austin Joyce and Justin Hury are theorizing.
This "pre-explosive" universe was supposed to possessmetastable state, that is, to be stable until an even more stable state appears. By analogy, imagine a precipice, on the edge of which a boulder is in a state of vibration. Any touch to the boulder will cause it to fall 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 form and state of what we see now.
“The sleeping model of the universe, however, also has its own problems,” says Carroll.
"It also suggests the presence of a low level of entropy in our Universe and does not explain why this is so."
However, Hinterbichler, a theoretical physicist at the Case Western Reserve University, does not consider the appearance of a low level of entropy a problem.
“We are just looking for an explanation of the dynamics,which happened before the Big Bang, which explains why we see what we see now. So far this is just the only thing left for us, ”says Hinterbichler.
Carroll, however, believes that there is another theory of the “pre-explosive” universe that can explain the low level of entropy in our universe.
The emergence of new universes from the "parent Universe"
The hypothetical multiverse model avoidsthe omissions associated with a decrease in entropy, as is the case with the Big Bounce model, provide 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 of this model were proposed in 1981 by physicist Alan Gut, who is currently working at the Massachusetts Institute of Technology. According to this model, the space after the Big Bang expanded dramatically. So abruptly that the speed of this expansion turned out to be higher than the speed of light. According to quantum mechanics, random, barely noticeable vibrations of energy constantly occur in space. At some point in the inflationary period, the peaks of these fluctuations reached their maximum and caused the appearance of galaxies, voids, and large-scale low-entropy structures, which we observe today in the Universe.
The inflation model itself was developed on the basis ofobservations of cosmic relic microwave radiation - the most ancient type of radiation that appeared only a few hundred thousand years after the Big Bang. Scientists believe that the inflationary model perfectly predicts its existence.
According to one assumption, the multiversemay result from inflation. The assumption suggests that there is one very, very large Universe, from time to time generating more compact universes. Moreover, no form of communication between these universes is possible. Marcus Wu from PBS Nova explains:
“In the early 80s, physicists came to the conclusion thatinflation 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 proceeds faster than the speed of light. In turn, “pockets” isolated from each other become universes over time. ”
Carroll is most impressed with this particular model, although his own proposed model is somewhat different from what is described above:
“This is just one version of the theory ofa multiverse, however, the main difference here is that the “parent Universe” can have a high level of entropy and generates universes with a low level, ”Carroll says.
According to this model, before the Big Bang there wassome large expanding space from which our and infinite many 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 oneof the most popular models. Nevertheless, scientists, of course, perceive it differently. Some support this idea, while others, on the contrary, completely disagree with it. But if you take Peter Voight from Columbia University as an example, the Multiverse theory, although it looks very attractive from a popular scientific point of view, can make physicists lazy and make them stop searching for answers to the most basic questions, for example, why physical constants in our Universe exactly as they are - writing off everything to variability.
“Theorists speculate about the possibilitythe existence of an infinite number of universes, and ultimately we can come up with clear models that can explain why the values (like the fundamental properties of the particles we observe) can differ from each other in every single universe, ”says Voight.
Voight fears that once the main issue forscience in this area will be the discussion on the topic “how fortunate we are to be in this random Universe, where everything happens this way and not differently, despite the endless variety of possibilities, so let's give up this venture with theories.”
What can be summed up? Many physicists receive money for arguing and writing books in which they try to describe how the Big Bang and the model of the “pre-explosive” Universe are able to explain what we see today, although they themselves do not know and actually cannot know why is this so. The fact is that even despite serious simplifications in both mathematical models and explanations, we have not come close to the correct answer, and we still have to carry out a lot of discussion on this topic until we reach 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 reality 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 can find the right answer that will suit everyone, ”says Carroll.