A trillion years before the Big Bang
The title of this article may not seem like a smart joke. According to the generally accepted cosmological concept, the Big Bang theory, our Universe arose from the extreme state of a physical vacuum generated by quantum fluctuation. In this state, neither time nor space existed (or they were confused in the space-time foam), and all fundamental physical interactions were merged together. Later they split up and gained an independent being - first gravity, then strong interaction, and only then - weak and electromagnetic.
The moment preceding these changes is usually designated as zero time, t = 0, however this is pure convention, a tribute to mathematical formalism. According to standard theory, a continuous flow of time began only after gravity gained independence. The value t = 10-43 s (more precisely, 5.4x10-44 s), which is called Planck time, is usually attributed to this moment. Modern physical theories simply are 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, because time in our understanding simply did not exist then.
An indispensable part of the standard cosmological theory is the concept of inflation (see box). After inflation ended, gravity came into its own, and the Universe continued to expand, but at a decreasing rate. This evolution stretched for 9 billion years, after which another anti-gravity field of an still unknown nature came into play, which is called dark energy. It again brought the Universe into an exponential expansion mode, 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 advent of inflationary cosmology.
The inflationary interpretation of the Big Bang was first proposed about 30 years ago and has since been polished many times. This theory allowed us to solve several fundamental problems that the previous cosmology could not cope with. For example, she explained why we live in a Universe with flat Euclidean geometry - in accordance with the classical Friedman equations, this is exactly what it should become with exponential expansion. The inflationary theory has explained why cosmic matter has granularity on a scale not exceeding hundreds of millions of light years, and is evenly distributed over long distances. It also interpreted the failure of any attempts to detect magnetic monopoles, very massive particles with a single magnetic pole, which are believed to have been produced in abundance before inflation began (inflation stretched outer space so much that the initially high density of monopoles was reduced to almost zero, and therefore our devices cannot detect them).
Shortly after the advent of the inflationary model, several theorists realized that its internal logic does not contradict the idea of permanent multiple birth of ever new universes. In fact, quantum fluctuations, such as those to which we owe the existence of our world, can occur in any quantity, if there are suitable conditions for this. It is possible that our universe emerged from the fluctuation zone that had formed in the predecessor world. In the same way, it can be assumed that someday 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 daughter universes arise continuously, budding 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 "embedded" in a single space-time continuum, but are so separated in it that they do not feel the presence of each other. In general, the concept of inflation allows - moreover, it compels! - to believe that in the gigantic megacosmos there are many isolated universes with different devices.
Theoretical physicists like to come up with alternatives to even the most generally accepted theories. Competitors also appeared in the inflationary Big Bang model. They did not receive wide support, but they had and have their followers. The theory of Steinhardt and Turok among them is not the first and certainly not the last. However, to date it has been developed in more detail than the rest and better explains the observed properties of our world. It has several versions, some of which are based on the theory of quantum strings and multidimensional spaces, while others rely on traditional quantum field theory. The first approach gives more visual 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 smaller dimensions, the so-called branes, float in it. Our Universe is just one of these branes with three spatial dimensions. It is filled with various quantum particles (electrons, quarks, photons, etc.), which are actually open vibrating strings with a single spatial dimension - length. The ends of each string are tightly fixed inside the three-dimensional brane, and the string cannot leave the brane. But there are also closed strings that can migrate beyond the limits of branes - these are gravitons, quanta of the gravitational field.
How does cyclic theory explain the past and future of the universe? Let's start with the current era. The first place now belongs to dark energy, which makes our universe expand exponentially, periodically doubling its size. As a result, the density of matter and radiation is constantly falling, the gravitational curvature of space is weakening, and its geometry is becoming more and more flat. Over the next trillion years, the dimensions 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 extension and flattening. All this time, the distance between the branes practically does not change.
And then these parallel branes begin to converge. They are pushed to 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, therefore the space of both branes expands exponentially - therefore, it is this field that provides the effect that is explained by the presence of dark energy! However, this parameter is gradually decreasing and after a trillion years it will drop to zero. Both branes will continue to expand anyway, but not at an exponential rate, but at a very slow pace. Therefore, in our world, the density of particles and radiation will remain almost zero, and the geometry will be flat.
But the end of the old story is just a prelude to the next cycle. Branes move towards each other and eventually collide. At this stage, the energy density of the intercranial field drops below zero, and it begins to act like gravity (I recall 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, the estimated size of such deformations reaches several meters in a millionth of a second before a collision). After the collision, it is in these zones that the lion's share of the kinetic energy released upon impact is released. As a result, it is there that the most hot plasma with a temperature of the order of 1023 degrees occurs. It is these areas that become local nodes of gravity and turn into the nuclei of future galaxies.
Such a clash replaces the Big Bang of inflationary cosmology. It is very important that all newly arising matter with positive energy appears due to the accumulated negative energy of the inter-branched field, therefore the law of conservation of energy is not violated.
And how does such a field behave at this crucial moment? Before the collision, its energy density reaches a minimum (and negative), then it begins to increase, and in a collision it becomes zero. Then the branes repel each other and begin to diverge. The density of cross-branched energy undergoes a reverse evolution - again it becomes negative, zero, positive. Enriched with matter and radiation, the brane first expands with decreasing speed under the inhibitory effect of its own gravity, and then again passes to exponential expansion. A new cycle ends like before - and so on ad infinitum. The cycles preceding ours occurred 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 kind of beginning at all, the theory is silent.
The cyclic theory explains in a new way the properties of our world. It has a flat geometry, because at the end of each cycle it stretches excessively and only deforms slightly before the start of a new cycle. Quantum fluctuations, which become the precursors of galaxies, arise randomly, but on average uniformly - therefore, outer space is filled with clumps of matter, but at very large distances it is completely uniform. We cannot detect magnetic monopoles simply because the maximum temperature of the newborn plasma did not exceed 1023 K, and for the appearance of such particles much higher energies are required - about 1027 K.
A world without a beginning and an end
The cyclic theory exists in several versions, as does the theory of inflation. However, according to Paul Steinhardt, the differences between them are purely technical and interesting only to specialists, but 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 the intense birth of matter and radiation, each of which, if desired, can be called the Big Bang. But any of these phases does not signify 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 the state of affairs was 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 slowing expansion of the Universe to an accelerated one. And when astrophysicists discovered this phenomenon, observing the flashes of distant supernovae, standard cosmology did not even know what to do with 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 anchored by internal logic, since dark energy is present in us initially and it provides the alternation of cosmological cycles. ” However, as noted by Paul Steinhardt, cyclic theory has some weaknesses: “So far we have not been able to convincingly describe the process of collision and rebound of parallel branes that takes place at the beginning of each cycle. Other aspects of the cyclic theory are much better developed, but there are still a lot of ambiguities to be addressed.
But even the most beautiful theoretical models need experimental testing. Is it possible to confirm or refute the cyclic cosmology using observations? “Both theories, both inflationary and cyclical, predict the existence of relic gravitational waves, ” explains Paul Steinhardt. “In the first case, they arise from primary quantum fluctuations, which are spread over space during inflation and generate periodic fluctuations in its geometry, ” and this, according to the general theory of relativity, is gravitational waves. In our scenario, quantum fluctuations are also the root cause of such waves - the very ones that amplify in a collision of branes. The calculations showed that each mechanism generates waves with a specific spectrum and specific polarization. These waves were obliged to leave imprints on the cosmic microwave radiation, which serves as an invaluable source of information about early space. So far, no such traces have been found, but most likely it will be done in the next decade. In addition, physicists are already thinking about direct registration of relic gravitational waves with the help of spacecraft, which will appear in two to three decades. ”
In the 1980s, Professor Steinhardt made a significant contribution to the development of the standard theory of the Big Bang. However, this did not prevent him from looking for a radical alternative to the theory, in which so much labor has been invested. As Paul Steinhardt himself told Popular Mechanics, the inflation hypothesis does reveal many cosmological puzzles, but this does not mean that it makes no sense to look 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 inflation 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 initial chaotic state of matter to the current ordered Universe. She did this - but went much further. The internal logic of the theory required to recognize 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 out. So, let’s say, with the help of the inflation hypothesis, it was possible to explain why we live in a flat Euclidean world, but most 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 has ceased to suit me. Moreover, standard theory is not able to explain the nature of an earlier state that preceded exponential expansion. In this sense, it is as incomplete as preinflationary cosmology. Finally, she is unable to say anything about the nature of dark energy, which has been managing 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 quite uniform in temperature, it has more or less heated zones. На том уровне точности измерений, который обеспечивает современная аппаратура, количество горячих и холодных зон примерно одинаково, что совпадает с выводами обеих теорий — и инфляционной, и циклической. Однако эти теории предсказывают более тонкие различия между зонами. В принципе, их сможет выявить запущенная в прошлом году европейская космическая обсерватория 'Планк' и другие новейшие космические аппараты. Я надеюсь, что результаты этих экспериментов помогут сделать выбор между инфляционной и циклической теориями. Но может случиться и так, что ситуация останется неопределенной и ни одна из теорий не получит однозначной экспериментальной поддержки. Ну что ж, тогда придется придумать что-нибудь новое».The article was published in the journal Popular Mechanics (No. 6, June 2010).