Trying to understand what is the nature of the Universe, how it originated and what awaits it in the future, scientists sometimes construct unusual hypotheses and models, such as, for example, the Big Bounce theory.
Did the universe start with an explosion, or with a bounce, or with something else? The question of our origins is one of the most thorny problems in physics, with only a few answers and a lot of speculation. The most popular and generally accepted theory today is cosmic inflation, according to which, in the first few fractions of a second after the Big Bang, all space-time went through a period of incredibly rapid expansion. However, there are other competing ideas as well. For example, according to the cyclic model of the Universe, our space-time was preceded by another, which survived the period of the Great Compression and then exploded again - we can observe it today. In addition, there is the Big Bounce theory that follows from the cyclical pattern.
The inflationary model has many fans, as the rapid expansion it postulates explains many of the properties of the universe - such as why it appears relatively flat (rather than curved when speaking of large scales) and uniform in all directions (everywhere in space, in all directions of matter are approximately the same). Both conditions develop when areas of space that are far from each other were initially located very close. However, the latest versions of the theory seem to suggest - or even require - that inflation has created not only our universe, but an endless landscape of universes, on which all possible types of universes with all sets of physical laws and properties were formed. Some scientists like this assumption, since it can explain the existence of our Universe with seemingly random, but ideally tuned conditions for the existence of life. If on such a landscape there are all imaginable and unimaginable types of space, then there is nothing surprising that ours is among them. At the same time, other physicists consider the idea of a multiverse repulsive - in part because if a theory predicts the occurrence of all possible events, it will not uniquely predict our universe.
The Big Rebound theories also predict a flat and evenly filled space due to the smoothing effects that can occur when it contracts. However, the stumbling block of the rebound idea has long been considered the transition from contraction to expansion, which requires the hated idea of "singularity" - the time when the universe was a point of infinite density - which many consider a mathematically meaningless assumption indicating that the theory train got off rails. More recently, physicists have begun to claim that they have found bounce equations that have no singularities. In 2016, Neil Turok and Steffen Giehlen published their calculations in Physical Review Letters. Then Turok commented on this work as follows:
“We found that we could accurately describe the quantum evolution of the universe, and we found that the universe smoothly transitions through the singularity to the other side. We hoped for it, but we haven’t received such results before”.
So, many consider 2016 to be the birth of the Big Rebound, although the concept itself dates back to the works of such scientists as, in particular, Willem de Sitter and Georgy Gamow.The breakthrough in theory development came from two techniques used by Turok and Ghilen. The first was to use the still incomplete theory of quantum cosmology - a mixture of quantum mechanics and General Relativity - instead of describing the universe with the classical General Theory. The second technique assumed that when space was very young, matter behaved like light - in the sense that the laws of physics that describe it were independent of scale. For example, light acts the same regardless of its wavelength. However, the physics of matter usually differs depending on the scale in question. According to modern models, for about the first 50 thousand years, the universe was filled with radiation, and there was little common matter that is observed everywhere in space today. Recent models of the Big Rebound Universe indicate that it was scaleless in its early stages.
Turok and Ghilen found that a contracting universe under such conditions would never go into a state of actual singularity. In fact, it “tunnels” through this point, “jumping” from the state before it to the state after it. While this may seem like a gimmick at first, it is a proven phenomenon in quantum mechanics (quantum tunneling). Since particles do not exist in absolute states, but rather are clouds of probability, there is a small but real possibility that they "tunnel" through physical obstacles to get to inaccessible places. It's like walking through walls, only at a microscopic level. Turok notes that inaccuracies in space, time and matter indicate that it is impossible to say exactly where the universe is at a particular moment, which allows it to pass through the singularity.
However, in 2016, Paul Steinhardt and Anna Ijas worked on another way to mathematically demonstrate the possibility of rebound. They introduced a special kind of field into the model of the universe, in which compression can go into expansion before space becomes small enough to go into a state of singularity. In their research, they used the classical theory of general relativity. In other words, with this work, they showed that a rebound is possible not only from the point of view of quantum mechanics, but also from the point of view of the theory of relativity.
Like other hypotheses about the origin and evolution of the universe, the Big Bounce theory attempts to reveal why the universe is exactly as we observe it. Physicists' models represent only idealized, absolutely smooth universes, in which there are no small density fluctuations that lead to the formation of stars, galaxies and real space. So scientists have yet to develop Big Bounce models to more complex systems.
If the Universe has already "bounced" once, then a logical question arises: will it happen again? Be that as it may, not all rebound theories assume that the cycle of contractions and expansions will be infinite, as the cyclic model of the universe claims. For example, even if our Universe has gone through such a rebound, there is still no hint that it is going to the next compression. Moreover, observations show that dark energy is increasingly stretching space, carrying galaxies that are not gravitationally bound to each other, further and further from each other. Science does not have an unequivocal answer to the question of what awaits us in the future. However, it is directly related to how it all started.