How and how did the universe begin? Almost all religions, creeds and cults offer answers to this question, as old as the world. But science has taken it seriously quite recently - only in the 20th century.
The simplest answer will be the shortest - it all started with the Big Bang. This is evidenced by the solutions of all reasonable models of the evolution of the Universe, built on the basis of the general theory of relativity. If we scroll them back in time, we will inevitably hit the moment when the density and temperature of matter become infinite. It also has to be taken as the origin, the zero time point. It is impossible to continue solutions to the area of previous times: mathematics does not allow.
The only way out
Physicists never liked this situation. Ever since they learned to calculate rigorously world models, the hopes of getting rid of infinities and looking, so to speak, into the past of the Big Bang, have not disappeared. But all attempts to find reasonable models of the "beginningless", in other words, the eternal Universe, were unsuccessful. This state of affairs persisted even after the models of inflationary expansion of the early Universe were developed in the early 1980s, which relied not only on general relativity, but also on the false vacuum hypothesis borrowed from quantum field theory.
Inflation is a super-fast expansion of the Universe at the very beginning of its existence. It arises due to the fact that the vacuum at this moment is in a state with a very high positive energy density, immeasurably exceeding its minimum value. The vacuum with the lowest energy density is called true, with a higher one - false. Any positive vacuum acts as anti-gravity, that is, it makes space expand. A false vacuum with an extremely high energy density is also extremely unstable, it quickly disintegrates, and its energy is spent on the formation of radiation and particles heated to extremely high temperatures. This vacuum decay is what is called the Big Bang. It leaves behind ordinary space filled with gravitating matter, which expands at a moderate rate.
However, there is one scenario that overcomes the dead end of mathematical infinities. According to this scenario, the Universe arose from nothing, more precisely, from a state where there is neither time, nor space, nor matter in the classical sense of these terms. At first glance, this idea seems ridiculous - how can nothing give rise to something? Or, moving from metaphors to physics, how can you get around the fundamental conservation laws? Let's say the law of conservation of energy, which is considered absolute. The energies of matter and radiation are always positive, so how could they arise from a state with zero energy?
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Fortunately, this difficulty is completely solvable - however, not for any universes, but only for closed ones. It can be proved that the total energy of any closed universe is exactly zero. How can this be, since the universe is filled with matter and radiation? However, there is also the energy of gravity, which is known to be negative. It turns out that in a closed universe, the positive energy contribution of particles and electromagnetic fields is exactly compensated by the equal in magnitude and opposite in sign contribution of the gravitational field, so that the total energy is always zero. This conclusion applies not only to energy, but also to electrical charge. In a closed universe, any positive charge is necessarily accompanied by the same charge with a minus sign, so that the total sum of all charges again turns out to be zero. The same can be said about other physical quantities obeying strict conservation laws.
What follows from this? If a closed universe arises from absolute emptiness, all conserved quantities are as they were and remain zero. It turns out that the fundamental conservation laws do not prohibit such a birth at all. Now let us remember that any quantum mechanical process not prohibited by these laws can occur, even with a very low probability. So the birth of a closed universe from nothing is in principle possible. This is how quantum mechanics differs from classical mechanics, where emptiness itself cannot give rise to anything.
To the beginning of time
The chances of spontaneous birth of different universes according to this scenario can be calculated: physics has a mathematical apparatus for this. It is intuitively obvious that they fall as the size of the universe increases, and the equations confirm this: Lilliputian universes are more likely to arise than larger universes. At the same time, the size of the universe is associated with the properties of the false vacuum that fills it: the higher the density of its energy, the smaller the universe. So, the maximum chances of spontaneous birth are given to closed micro-universes filled with a high-energy vacuum.
Now let's say that probability worked in favor of this scenario and a closed universe was born out of nothing. The false vacuum creates negative gravity, which forces the newborn universe to expand rather than contract. As a result, she will evolve from the initial moment that fixes her spontaneous birth. When approaching this moment from the perspective of the future, we do not run into infinity. But the question of what happened before this moment does not make sense, since then there was neither time nor space.
Must have a start
Several years ago, I, together with two co-authors, proved a theorem that is directly related to our problem. Roughly speaking, she argues that any universe that expands on average has a beginning. Clarification "on average" has the meaning that at some stages the universe can contract, but throughout its existence it is still mainly expanding. And the conclusion about the existence of the beginning means that this universe has stories that, when continued into the past, break off, their world lines have certain starting points. On the contrary, any universe that exists eternally cannot have such world lines, all of its stories continuously recede into the past to an infinite depth. And since universes that are born as a result of inflationary processes satisfy the conditions of the theorem,they must have a beginning.
You can also mathematically simulate a closed universe that was in a static state for an infinitely long time, and then began to expand. It is clear that our theorem does not apply to it, because the time-averaged rate of its expansion is zero. However, such a universe will always have a chance to collapse: this is required by quantum mechanics. The probability of collapse can be very small, but since the universe is in a static state for an infinite time, it will certainly happen, and such a universe simply will not live to expand. So we again come to the conclusion that the expanding universe must have a beginning. Naturally, it also applies to our own universe.
Alexander Vilenkin, director of the Institute of Cosmology at Tufts University, author of The World of Many Worlds. Physicists in search of other universes”.
Interviewed by: Alexey Levin, Oleg Makarov, Dmitry Mamontov