The title of this article may not sound like a clever joke. According to the generally accepted cosmological concept, the Big Bang theory, our Universe emerged from an extreme state of a physical vacuum generated by a quantum fluctuation. In this state, neither time nor space existed (or they were entangled in space-time foam), and all fundamental physical interactions were fused together. Later they separated and acquired an independent existence - first gravity, then strong interaction, and only then - weak and electromagnetic.
The moment that preceded these changes is usually denoted as zero time, t = 0, but this is pure convention, a tribute to mathematical formalism. According to the standard theory, the continuous flow of time began only after the force of gravity became independent. This moment is usually attributed to the value t = 10-43 s (more precisely, 5, 4x10-44 s), which is called the Planck time. Modern physical theories are simply not able to meaningfully work with shorter periods of time (it is believed that this requires a quantum theory of gravity, which has not yet been created). In the context of traditional cosmology, it makes no sense to talk about what happened before the initial moment of time, since time in our understanding simply did not exist at that time.
The Big Bang theory is trusted by the absolute majority of scientists studying the early history of our Universe. It really explains a lot and does not contradict the experimental data in any way. Recently, however, it has a competitor in the face of a new, cyclical theory, the foundations of which were developed by two extra-class physicists - the director of the Institute of Theoretical Science at Princeton University Paul Steinhardt and the laureate of the Maxwell Medal and the prestigious international TED Prize Neil Turok, director of the Canadian Institute for Advanced Study in the field of theoretical Physics (Perimeter Institute for Theoretical Physics). With the help of Professor Steinhardt, Popular Mechanics tried to talk about the cyclical theory and the reasons for its appearance.
An indispensable part of standard cosmological theory is the concept of inflation (see sidebar). After the end of inflation, gravity came into its own, and the Universe continued to expand, but at a decreasing rate. This evolution lasted for 9 billion years, after which another antigravitational field of a still unknown nature, which is called dark energy, entered into action. It again brought the Universe into a mode of exponential expansion, which seems to be preserved in future times. It should be noted that these conclusions are based on astrophysical discoveries made at the end of the last century, almost 20 years after the appearance of inflationary cosmology.
The inflationary interpretation of the Big Bang was first proposed about 40 years ago and has been refined many times since then. This theory allowed to resolve several fundamental problems that previous cosmology failed to cope with. For example, she explained why we live in a universe with flat Euclidean geometry - according to the classical Friedmann equations, this is exactly what it should do with exponential expansion. The inflationary theory explained why cosmic matter is granular on a scale not exceeding hundreds of millions of light years, and is evenly distributed over long distances. She also gave an interpretation of the failure of any attempts to detect magnetic monopoles, very massive particles with a single magnetic pole, which are believed to have been born in abundance before the onset of inflation (inflation stretched space so much that the initially high density of monopoles was reduced to almost zero, and therefore our instruments cannot detect them).
Soon after the emergence of the inflationary model, several theorists realized that its internal logic did not contradict the idea of permanent multiple birth of more and more new universes. Indeed, quantum fluctuations, like those to which we owe our world to exist, can occur in any quantity if the conditions are right. It is possible that our universe has left the fluctuation zone formed in the predecessor world. In the same way, it can be assumed that sometime and somewhere in our own Universe a fluctuation will form, which will “blow out” a young universe of a completely different kind, also capable of cosmological “procreation”. There are models in which such child universes emerge continuously, branch off from their parents, and find their own place. Moreover, it is not at all necessary that the same physical laws are established in such worlds. All these worlds are "nested" in a single space-time continuum, but they are so spaced apart that they do not feel the presence of each other in any way. In general, the concept of inflation allows - moreover, compels! - to believe that in the gigantic megacosmos there are many isolated universes with different arrangements.
Theoretical physicists love to come up with alternatives to even the most generally accepted theories. The inflationary model of the Big Bang also has competitors. They did not receive wide support, but they did and have their own followers. The theory of Steinhardt and Turok is not the first among them, and certainly not the last. However, to date, it has been developed in more detail than the others and better explains the observed properties of our world. It has several versions, some of which are based on quantum string theory and multidimensional spaces, while others rely on traditional quantum field theory. The first approach gives more vivid pictures of cosmological processes, so we will dwell on it.
The most advanced version of string theory is known as M-theory. She claims that the physical world has 11 dimensions - ten spatial and one temporal. Spaces of lower dimensions, the so-called branes, float in it. Our universe is just one such brane, with three spatial dimensions. It is filled with various quantum particles (electrons, quarks, photons, etc.), which are actually open vibrating strings with only one spatial dimension - length. The ends of each string are firmly fixed inside a three-dimensional brane, and the string cannot leave the brane. But there are also closed strings that can migrate outside the branes - these are gravitons, quanta of the gravitational field.
How does cyclical theory explain the past and future of the universe? Let's start with the current era. First place now belongs to dark energy, which is causing our universe to expand exponentially, periodically doubling in size. As a result, the density of matter and radiation constantly decreases, the gravitational curvature of space weakens, and its geometry becomes more and more flat. Over the next trillion years, the size of the universe will double about a hundred times and it will turn into an almost empty world, completely devoid of material structures. Next to us is another three-dimensional brane, separated from us by an insignificant distance in the fourth dimension, and it also undergoes a similar exponential expansion and flattening. All this time, the distance between the branes remains practically unchanged.
And then these parallel branes begin to converge. They are pushed towards each other by a force field, the energy of which depends on the distance between the branes. Now the energy density of such a field is positive, so the space of both branes is expanding exponentially - hence, it is this field that provides the effect that is explained by the presence of dark energy! However, this parameter is gradually decreasing and in a trillion years will fall to zero. Both branes will continue to expand anyway, but not exponentially, but at a very slow pace. Consequently, in our world, the density of particles and radiation will remain almost zero, and the geometry will remain flat.
But the end of the old story is just a prelude to the next cycle. The branes move towards each other and eventually collide. At this stage, the energy density of the interbranch field drops below zero, and it begins to act like gravity (let me remind you that the potential energy of gravity is negative!). When the branes are very close, the inter-brane field begins to amplify quantum fluctuations at every point in our world and transforms them into macroscopic deformations of spatial geometry (for example, in a millionth of a second before the collision, the calculated size of such deformations reaches several meters). After the collision, it is in these zones that the lion's share of the kinetic energy released during the impact is released. As a result, it is there that most of all hot plasma with a temperature of about 1023 degrees occurs. It is these regions that become local nodes of gravitation and turn into the embryos of future galaxies.
Such a collision replaces the Big Bang of inflationary cosmology. It is very important that all the newly formed matter with positive energy appears due to the accumulated negative energy of the interbranch field, therefore the law of conservation of energy is not violated.
Inflationary theory allows for the formation of multiple daughter universes that continually sprout from existing ones.
And how does such a field behave at this decisive moment? Before the collision, the density of its energy reaches a minimum (and negative), then begins to increase, and upon collision it becomes zero. The branes then repel each other and begin to disperse. The density of the interbranch energy goes through the reverse evolution - again it becomes negative, zero, positive. The brane, enriched with matter and radiation, first expands at a decreasing speed under the braking effect of its own gravitation, and then again goes over to exponential expansion. The new cycle ends like the previous one - and so on ad infinitum. The cycles preceding ours took place in the past - in this model, time is continuous, so the past exists beyond the 13.7 billion years that have passed since the last enrichment of our brane with matter and radiation! Whether they had any beginning at all, the theory is silent.
Cyclic theory explains the properties of our world in a new way. It has a flat geometry, since at the end of each cycle it stretches excessively and only slightly deforms before starting a new cycle. Quantum fluctuations, which become the precursors of galaxies, arise chaotically, but on average evenly - therefore, outer space is filled with clumps of matter, but at very large distances it is quite homogeneous. We cannot detect magnetic monopoles simply because the maximum temperature of the newborn plasma did not exceed 1023 K, and much higher energies are required for the appearance of such particles - on the order of 1027 K.
The moment of the Big Bang is the collision of the branes. A huge amount of energy is released, the branes scatter, there is a slowing expansion, matter and radiation cool down, and galaxies are formed. The expansion is again accelerated due to the positive inter-branched energy density, and then it slows down, the geometry becomes flat. Branes are attracted to each other, before the collision, quantum fluctuations are amplified and transform into deformations of spatial geometry, which in the future will become the seeds of galaxies. A collision occurs and the cycle starts over.
A world without beginning or end
The cyclical theory exists in several versions, as does the inflation theory. However, according to Paul Steinhardt, the differences between them are purely technical and interesting only to specialists, the general concept remains unchanged: “First, in our theory there is no moment of the beginning of the world, no singularity. There are periodic phases of intense creation of matter and radiation, each of which, if desired, can be called the Big Bang. But any of these phases does not mark the emergence of a new universe, but only a transition from one cycle to another. Both space and time exist both before and after any of these cataclysms. Therefore, it is quite natural to ask what was the state of affairs 10 billion years before the last Big Bang, from which the history of the universe is counted.
The second key difference is the nature and role of dark energy. Inflationary cosmology did not predict the transition of a decelerating expansion of the Universe to an accelerated one. And when astrophysicists discovered this phenomenon by observing the explosions of distant supernovae, standard cosmology did not even know what to do about it. The hypothesis of dark energy was put forward simply in order to somehow tie the paradoxical results of these observations to the theory. And our approach is much better sealed by internal logic, since we have dark energy from the very beginning and it is this energy that ensures the alternation of cosmological cycles. " However, as Paul Steinhardt notes, the cyclic theory also has weak points: “We have not yet been able to convincingly describe the collision and rebound process of parallel branes that takes place at the beginning of each cycle. Other aspects of the cyclical theory are much better developed, and there are still many ambiguities to be cleared."
But even the most beautiful theoretical models need experimental verification. Can cyclic cosmology be confirmed or disproved by observation? “Both inflationary and cyclical theories predict the existence of relic gravitational waves,” explains Paul Steinhardt. - In the first case, they arise from primary quantum fluctuations, which are smeared over space during inflation and generate periodic oscillations of its geometry - and these, according to the general theory of relativity, are gravitational waves. In our scenario, quantum fluctuations are also the root cause of such waves - the same ones that are amplified by collisions of branes. Calculations have shown that each mechanism generates waves with a specific spectrum and specific polarization. These waves were required to leave imprints on cosmic microwave radiation, which is an invaluable source of information about early space. So far, no such traces have been found, but, most likely, this will be done within the next decade. In addition, physicists are already thinking about the direct registration of relic gravitational waves using spacecraft, which will appear in two to three decades."
A radical alternative
In the 1980s, Professor Steinhardt made a significant contribution to the development of the standard theory of the Big Bang. However, this did not stop him from looking for a radical alternative to the theory, in which so much work had been invested. As Paul Steinhardt himself told Popular Mechanics, the inflation hypothesis does reveal many cosmological mysteries, but this does not mean that there is no point in looking for other explanations: “At first I was just interested in trying to understand the basic properties of our world without resorting to inflation. Later, when I delved into this issue, I became convinced that the inflationary theory is not at all as perfect as its proponents claim. When inflationary cosmology was just being created, we hoped that it would explain the transition from the original chaotic state of matter to the current ordered Universe. She did just that - but she went a lot further. The internal logic of the theory demanded to admit that inflation constantly creates an infinite number of worlds. This would not be a big deal if their physical device was copying our own, but this just does not work. For example, with the help of the inflationary hypothesis, it was possible to explain why we live in a flat Euclidean world, but after all, most of the other universes will certainly not have the same geometry. In short, we were building a theory to explain our own world, and it got out of hand and spawned an endless variety of exotic worlds. This state of affairs ceased to suit me. In addition, the standard theory is unable to explain the nature of the earlier state, which preceded the exponential expansion. In this sense, it is as incomplete as the pre-inflationary cosmology. Finally, it is unable to say anything about the nature of dark energy, which has been driving the expansion of our universe for 5 billion years."
Another difference, according to Professor Steinhardt, is the temperature distribution of the background microwave radiation: “This radiation coming from different parts of the sky is not completely uniform in temperature, it has more and less heated zones. At the level of measurement accuracy provided by modern equipment, the number of hot and cold zones is approximately the same, which coincides with the conclusions of both theories - both inflationary and cyclical. However, these theories predict more subtle differences between zones. In principle, they can be detected by the European space observatory 'Planck' launched last year and other newest spacecraft. I hope that the results of these experiments will help to make a choice between inflationary and cyclical theories. But it may also happen that the situation remains uncertain and none of the theories will receive unequivocal experimental support. Well, then I'll have to come up with something new."