Immediately after its birth, the Universe expanded incredibly quickly.
Since the 30s of the 20th century, astrophysicists already knew that, according to Hubble's law, the Universe is expanding, which means it had its beginning at a certain moment in the past. The task of astrophysicists, thus, outwardly looked simple: to track all the stages of the Hubble expansion in reverse chronology, applying the corresponding physical laws at each stage, and, having gone this way to the end - more precisely, to the very beginning - to understand exactly how everything happened.
In the late 1970s, however, several fundamental problems associated with the early Universe remained unresolved, namely:
- Antimatter problem. According to the laws of physics, matter and antimatter have an equal right to exist in the Universe ( cm. Antiparticles), but the Universe consists almost entirely of matter. Why did it happen?
- Horizon problem. Based on background cosmic radiation ( cm. Big Bang), we can determine that the temperature of the Universe is approximately the same everywhere, but its individual parts (clusters of galaxies) could not be in contact (as they say, they were outside horizon each other). How did it happen that thermal equilibrium was established between them?
- The problem of straightening space. The universe appears to have just the right amount of mass and energy to slow down and stop the Hubble expansion. Of all the possible masses, why does the Universe have just this one?
The key to solving these problems was the idea that immediately after its birth the Universe was very dense and very hot. All the matter in it was a red-hot mass of quarks and leptons ( cm. Standard Model), which had no way of combining into atoms. The various forces acting in the modern Universe (such as electromagnetic and gravitational forces) then corresponded to a single field of force interaction ( cm. Universal theories). But as the Universe expanded and cooled, the hypothetical unified field split into several forces ( cm. Early Universe).
In 1981, American physicist Alan Guth realized that the separation of strong interactions from a unified field, which occurred approximately 10-35 seconds after the birth of the Universe (just think - that's 34 zeros and a 1 after the decimal point!), was a turning point in its development. Happened phase transition substances from one state to another on the scale of the Universe - a phenomenon similar to the transformation of water into ice. And just as when water freezes, its randomly moving molecules suddenly “grab” and form a strict crystalline structure, so under the influence of the released strong interactions, an instant restructuring took place, a kind of “crystallization” of matter in the Universe.
Anyone who has seen how water pipes or car radiator tubes burst in severe frost, as soon as the water in them turns into ice, knows from personal experience that water expands when it freezes. Alan Guth was able to show that when strong and weak interactions separated, something similar happened in the Universe—a jump-like expansion. This is an extension called inflationary, many times faster than the usual Hubble expansion. In about 10 -32 seconds, the Universe expanded by 50 orders of magnitude - it was smaller than a proton, and became the size of a grapefruit (for comparison, water expands by only 10% when it freezes). And this rapid inflationary expansion of the Universe removes two of the three above-mentioned problems, directly explaining them.
Solution space straightening problems The following example demonstrates this most clearly: imagine a coordinate grid drawn on a thin elastic map, which is then crumpled haphazardly. If we now take and strongly shake this elastic map, crumpled into a ball, it will again take a flat appearance, and the coordinate lines on it will be restored, no matter how much we deformed it when we crumpled it. Likewise, it doesn’t matter how curved the space of the Universe was when its inflationary expansion began, the main thing is that at the end of this expansion, space turned out to be completely straightened. And since we know from the theory of relativity that the curvature of space depends on the amount of matter and energy in it, it becomes clear why there is exactly enough matter in the Universe to balance the Hubble expansion.
Explains the inflation model and horizon problem, although not so directly. From the theory of black body radiation, we know that the radiation emitted by a body depends on its temperature. Thus, from the radiation spectra of distant parts of the Universe, we can determine their temperature. Such measurements yielded stunning results: it turned out that at any observable point in the Universe the temperature (with a measurement error of up to four decimal places) is the same. Based on the normal Hubble expansion model, the matter immediately after the Big Bang would have spread too far apart for temperatures to equalize. According to the inflationary model, the matter of the Universe until the moment t = 10 -35 seconds remained much more compact than during the Hubble expansion. This extremely short period was quite enough to establish a thermal equilibrium, which was not disturbed during the stage of inflationary expansion and has been preserved to this day.
American physicist, specialist in the field of elementary particles and cosmology. Born in New Brunswick, New Jersey. He received his doctorate from the Massachusetts Institute of Technology, where he returned in 1986, becoming a professor of physics. Guth developed his theory of inflationary expansion of the Universe while still at Stanford University, working on the theory of elementary particles. His review of the Universe as “an endless self-assembled tablecloth” is known.
In which he briefly describes the emergence and development of the theory of the inflationary universe, which provides a new explanation for the Big Bang and predicts the existence of many other universes along with ours.
Cosmology is in some ways akin to philosophy. Firstly, in terms of the vastness of its subject of research - it is the entire Universe as a whole. Secondly, because some premises in it are accepted by scientists as acceptable without the possibility of conducting any testing experiment. Third, the predictive power of many cosmological theories will only work if we can get to other universes - which is not to be expected.
However, it does not at all follow from all this that modern cosmology is such a hand-waving and not entirely scientific field where you can, like the ancient Greeks, lie in the shade of trees and hypothesize about the number of dimensions of space-time - are there ten or eleven? Cosmological models are based on observational data from astronomy, and the more data there is, the more material there is for cosmological models - which must connect and harmonize these data with each other. The difficulty is that cosmology deals with fundamental issues that require some initial assumptions, which are chosen by the authors of the models based on their personal ideas about the harmony of the universe. In general, there is nothing exceptional in this: when constructing any theory, you need to take some reference points. It’s just that for cosmology, which operates on the largest scales of space and time, it is especially difficult to choose them.
First, a few important definitions.
Cosmology is a science that studies the properties of our Universe as a whole. However, there is still no single theory that would describe everything that is happening and has ever happened. Now there are four main cosmological models that try to describe the origin and evolution of the universe, and each of them has its own pros and cons, its adherents and opponents. The Lambda-CDM model is considered the most authoritative, although not indisputable. It is important to understand that cosmological models are not necessarily in competition with each other. They can simply describe fundamentally different stages of evolution. For example, Labmda-CDM does not consider the Big Bang at all, although it perfectly explains everything that happened after it.
The structure of a multiverse with bubbles of mini-universes inside it.
Drawing: Andrei Linde
The surprising thing about this is that the cosmological constant (that is, the vacuum energy) does not change over time as the universe expands, while the density of matter changes completely predictably and depends on the volume of space. It turns out that in the early universe the density of matter far exceeded the density of vacuum; in the future, as galaxies fly apart, the density of matter will decrease. So why, now that we can measure them, are they so close in value to each other?
The only known way to explain such an incredible coincidence, without involving some unscientific hypotheses, is only with the help of the anthropic principle and the inflationary model - that is, from the many existing universes, life originated in the one where the cosmological constant at a given moment in time turned out to be equal to the density of matter (this in turn determines the time that has passed since inflation began, and provides just enough time for the formation of galaxies, the formation of heavy elements and the development of life).
Another turning point in the development of the inflationary model was the publication in 2000 of a paper by Busso and Polchinski, in which they proposed using string theory to explain a large set of different types of vacuum, in each of which the cosmological constant could take on different values. And when one of the creators of string theory itself, Leonard Susskind, got involved in the work on combining string theory and the inflationary model, it not only helped to create a more complete picture, which is now called the “anthropic landscape of string theory,” but also in some way added weight to the entire model in the scientific world. The number of articles on inflation increased over the year from four to thirty-two.
The inflation model purports to not only explain the fine-tuning of fundamental constants, but also to help discover some of the fundamental parameters that determine the magnitude of these constants. The fact is that in the Standard Model today there are 26 parameters (the cosmological constant was the last to be discovered), which determine the value of all the constants that you have ever encountered in a physics course. This is quite a lot and Einstein already believed that their number could be reduced. He proposed a theorem, which, according to him, cannot currently be more than a belief, that there are no arbitrary constants in the world: it is so wisely structured that there should be some logical connections between seemingly completely different quantities. In an inflationary model, these constants may simply be an environmental parameter that appears to us locally unchanged due to the effect of inflation, although it will be completely different in another part of the universe and is determined by yet to be identified, but certainly existing truly fundamental parameters.
In the conclusion of the article, Linde writes that criticism of the inflationary model is often based on the fact that we will not be able to penetrate into other universes in the foreseeable future. Therefore, it is impossible to test the theory and we still do not have answers to the most basic questions: Why is the universe so big? Why is it homogeneous? Why is it isotropic and does not rotate like our galaxy? However, if we look at these questions from a different angle, it turns out that even without traveling to other mini-universes we have a lot of experimental data. Such as size, flatness, isotropy, homogeneity, the value of the cosmological constant, the ratio of proton and neutron masses, and so on. And the only reasonable explanation to date for these and many other experimental data is given within the framework of the theory of multiverses and, consequently, the model of inflationary cosmology.
, 1990. Andrey Linde
“The Anthropic landscape of string theory” 2003. Leonard Susskind
Marat Musin
Why thirty-three famous scientists of various specializations, led by Stephen Hawking, took up arms against three astrophysicists, what scenarios were used to form our Universe and whether the inflationary theory of its expansion is correct, the site looked into it together with experts.
Standard Big Bang Theory and its problems
The theory of the hot Big Bang was established in the middle of the 20th century, and became generally accepted a couple of decades after the discovery of cosmic microwave background radiation. It explains many properties of the Universe around us and suggests that the Universe arose from some initial singular state (formally infinitely dense) and has been continuously expanding and cooling since then.
The CMB itself - a light "echo" born just 380,000 years after the Big Bang - has proven to be an incredibly valuable source of information. The lion's share of modern observational cosmology is associated with the analysis of various parameters of the cosmic microwave background radiation. It is quite homogeneous, its average temperature in different directions varies on a scale of only 10 –5, and these inhomogeneities are evenly distributed across the sky. In physics, this property is usually called statistical isotropy. This means that locally this value changes, but globally everything looks the same.
Scheme of the expansion of the Universe
NASA/WMAP Science Team/Wikimedia Commons
By studying disturbances in the cosmic microwave background radiation, astronomers accurately calculate many quantities characterizing the Universe as a whole: the ratio of ordinary matter, dark matter and dark energy, the age of the Universe, the global geometry of the Universe, the contribution of neutrinos to the evolution of large-scale structure, and others.
Despite the “generally accepted” theory of the Big Bang, it also had disadvantages: it did not answer some questions about the origin of the Universe. The main ones are called the “horizon problem” and the “flatness problem.”
The first is due to the fact that the speed of light is finite, and the cosmic microwave background radiation is statistically isotropic. The fact is that at the time of the birth of the cosmic microwave background radiation, even the light did not have time to travel the distance between those distant points in the sky from where we catch it today. Therefore, it is not clear why different areas are so identical, because they have not yet had time to exchange signals since the birth of the Universe, their causal horizons do not intersect.
The second problem, the problem of flatness, is associated with the global curvature of space, indistinguishable from zero (at the level of accuracy of modern experiments). Simply put, at large scales the space of the Universe is flat, and the hot Big Bang theory does not imply that flat space is preferable to other curvatures. Therefore, the proximity of this value to zero is at least not obvious.
Thirty three against three
To solve these problems, astronomers created the next generation of cosmological theories, the most successful of which is the theory of inflationary expansion of the Universe (more simply called the theory of inflation). Increasing prices for goods has nothing to do with it, although both terms come from the same Latin word - inflatio- "bloating".
The inflationary model of the Universe assumes that before the hot stage (what is considered the beginning of time in the usual Big Bang theory) there was another era with completely different properties. At that time, space was expanding exponentially quickly thanks to the specific field that filled it. In a tiny fraction of a second, space expanded an incredible number of times. This solved both of the above problems: the Universe turned out to be generally homogeneous, since it arose from an extremely small volume that existed at the previous stage. Moreover, if there were any geometric irregularities in it, they were smoothed out during the inflationary expansion.
Many scientists took part in the development of the theory of inflation. The first models were independently proposed by physicist Alan Guth, PhD at Cornell University, in the USA, and theoretical physicist, specialist in gravity and cosmology, Alexey Starobinsky, in the USSR around 1980. They differed in their mechanisms (Gut considered a false vacuum, and Starobinsky considered a modified general theory of relativity), but led to similar conclusions. Some problems of the original models were solved by a Soviet physicist, Doctor of Physical and Mathematical Sciences, employee of the P.N. Physics Institute. Lebedev Andrey Linde, who introduced the concept of slowly changing potential (slow-roll inflation) and used it to explain the completion of the exponential expansion stage. The next important step was to understand that inflation does not produce a perfectly symmetrical Universe, since quantum fluctuations must be taken into account. This was done by Soviet physicists, MIPT graduates Vyacheslav Mukhanov and Gennady Chibisov.
Norwegian King Harald awards Alan Guth, Andrei Linde and Alexey Starobinsky (from left to right) with the Kavli Prize in Physics. Oslo, September 2014.
Norsk Telegrambyra AS/Reuters
Within the framework of the theory of inflationary expansion, scientists make testable predictions, some of which have already been confirmed, but one of the main ones - the existence of relict gravitational waves - has not yet been confirmed. The first attempts to record them are already being made, but at this stage it remains beyond the technological capabilities of mankind.
However, the inflationary model of the Universe has opponents who believe that it is formulated too generally, to the point that it can be used to obtain any result. For some time, this debate has been going on in the scientific literature, but recently a group of three astrophysicists IS&L (an abbreviation formed by the first letters of the surnames of scientists - Ijjas, Steinhardt and Loeb - Anna Ijjas, Paul Steinhardt and Abraham Loeb) published a popular scientific statement of their claims to inflationary cosmology in Scientific American. In particular, IS&L, citing a map of cosmic microwave background temperatures obtained using the Planck satellite, believe that the theory of inflation cannot be assessed by scientific methods. Instead of the theory of inflation, astrophysicists offer their own version of the development of events: supposedly the Universe began not with the Big Bang, but with the Big Rebound - the rapid compression of a certain “previous” Universe.
In response to this article, 33 scientists, including the founders of the theory of inflation (Alan Gut, Alexey Starobinsky, Andrei Linde) and other famous scientists, such as Stephen Hawking, published a response letter in the same journal in which they categorically disagree with IS&L's claims .
the site asked cosmologists and astrophysicists to comment on the validity of these claims, the difficulties in interpreting the predictions of inflationary theories, and the need to reconsider the approach to the theory of the early Universe.
One of the founders of the theory of inflationary expansion, Stanford University physics professor Andrei Linde, considers the claims to be far-fetched, and the critics’ approach itself to be unconscionable: “If you answer in detail, you will end up with a long scientific article, but in short it will look like propaganda. This is what people use. In short, the leader of the critics is Steinhardt, who has been trying for 16 years to create an alternative to the theory of inflation, and his articles are error upon error. Well, when you can’t do it yourself, you have a desire to criticize more popular theories, using methods well known from history textbooks. Most theorists have stopped reading them, but journalists love them. Physics has almost nothing to do with it.”
Candidate of Physical and Mathematical Sciences, employee of the Institute of Nuclear Research of the Russian Academy of Sciences Sergei Mironov reminds that scientific truth cannot be born in polemics at a non-professional level. The critical article, in his opinion, is written scientifically and argumentatively; it brings together various problems of inflationary theory. Reviews like these are necessary and help prevent science from becoming ossified.
However, the situation changes when such a discussion moves onto the pages of a popular publication, because whether it is right to promote one’s scientific idea in this way is a moot point. In this regard, Mironov notes that the response to criticism looks ugly, since some of its authors are not experts at all in the field in question, and the other writes popular texts about the inflation model. Mironov points out that the response article was written as if the authors had not even read the IS&L work, and they did not provide any counterarguments to it. Statements about the provocative manner in which the critical note was written mean that “the authors of the response simply fell for trolling.”
"Share of Truth"
However, scientists, including supporters of the inflation model, recognize its shortcomings. Physicist Alexander Vilenkin, professor and director of the Institute of Cosmology at Tufts University in Medford (USA), who made important contributions to the development of modern inflation theory, notes: “There is some truth in the statements of Steinhardt and colleagues, but I think that their claims are extremely exaggerated. Inflation predicts the existence of many regions like ours, with initial conditions determined by quantum fluctuations. Theoretically, any initial conditions are possible with some probability. The problem is that we don't know how to calculate these probabilities. The number of regions of each type is infinite, so we have to compare infinite numbers - this situation is called the measure problem. Of course, the absence of a single measure derived from fundamental theory is a worrying sign.”
Sergei Mironov considers the mentioned multitude of models to be a shortcoming of the theory, since this allows it to be adjusted to any experimental observations. This means that the theory does not satisfy Popper's criterion (according to this criterion, a theory is considered scientific if it can be refuted by experiment - approx. website), at least for the foreseeable future. Also among the problems of Mironov’s theory is the fact that within the framework of inflation, the initial conditions require fine adjustment of the parameters, which makes it, in a sense, not natural. A specialist in the early Universe, candidate of physical and mathematical sciences, employee of the Gran Sasso Scientific Institute of the National Institute of Nuclear Physics (Italy) Sabir Ramazanov also recognizes the reality of these problems, but notes that their existence does not necessarily mean that the inflationary theory is incorrect, but a number of it aspects really deserve deeper thought.
The creator of one of the first inflation models, Academician of the Russian Academy of Sciences, chief researcher at the Institute of Theoretical Physics of the Russian Academy of Sciences, Alexey Starobinsky, explains that one of the simplest models, which Andrei Linde proposed in 1983, was indeed refuted. It predicted too many gravitational waves, so Linde recently pointed out that inflation models need to be reconsidered.
Critical experiment
Astronomers pay special attention to the fact that an important prediction made possible by the theory of inflation was the prediction of relict gravitational waves. Oleg Verkhodanov, a specialist in the analysis of cosmic microwave background radiation and observational cosmology, Doctor of Physical and Mathematical Sciences, leading researcher at the Special Astrophysical Observatory of the Russian Academy of Sciences, considers this forecast a significant observational test for the simplest variants of inflationary expansion, while for the critically advocated “Big Rebound” theory such a decisive there is no experiment.
Illustration of the Big Bounce theory
Wikimedia Commons
Therefore, it will be possible to talk about another theory only if serious restrictions are placed on relict waves. Sergei Mironov also calls the potential discovery of such waves a serious argument in favor of inflation, but notes that so far their amplitude is only limited, which has already made it possible to discard some options, which are being replaced by others that do not predict too strong primary gravitational disturbances. Sabir Ramazanov agrees with the importance of this test and, moreover, believes that the inflation theory cannot be considered proven until this phenomenon is discovered in observations. Therefore, while the key prediction of the inflation model about the existence of primary gravitational waves with a flat spectrum has not been confirmed, it is too early to talk about inflation as a physical reality.
“The correct answer, from which they are diligently trying to lead the reader away”
Alexey Starobinsky examined IS&L's claims in detail. He identified three main claims.
Statement 1: Inflation predicts anything. Or nothing.
“The correct answer, which IS&L tries to steer the reader away from, is that words like “inflation,” “quantum field theory,” “particle model” are very general: they combine many different models, varying in degree of complexity ( for example, the number of types of neutrinos),” explains Starobinsky.
After scientists fix the free parameters included in each specific model from experiments or observations, the model’s predictions are considered unambiguous. The modern Standard Model of elementary particles contains about 20 such parameters (mainly the masses of quarks, the masses of neutrinos and their mixing angle). The simplest viable inflation model contains only one such parameter, the value of which is fixed by the measured amplitude of the initial spectrum of matter inhomogeneities. After this, all other predictions are clear.
The academician clarifies: “Of course, it can be complicated by adding new terms of different physical nature, each of which will be included with a new free numerical parameter. But, firstly, in this case the predictions will not be “anything”, but definite. And secondly, and this is the most important thing, today’s observations show that these terms are not needed; at the current level of accuracy of about 10% they are not there!”
Statement 2. It is unlikely that in the models under consideration an inflationary stage will arise at all, since in them the potential energy of the inflaton has a long, flat “plateau”.
“The statement is false,” Starobinsky is categorical. “In my work in 1983 and 1987, it was proven that the inflationary regime in models of this type is general, that is, it arises in a set of initial conditions with a non-zero measure.” This was subsequently proven using more stringent mathematical criteria, with numerical simulations, etc.
The results of the Planck experiment, according to Starobinsky, questioned the point of view repeatedly expressed by Andrei Linde. According to it, inflation must necessarily begin at the Planck density of matter, and, already starting from this limiting parameter for the classical description of space-time, matter was distributed uniformly. However, the evidence discussed above did not suggest this. That is, in models of this type, before the stage of inflationary expansion there is an anisotropic and inhomogeneous stage of the evolution of the Universe with a greater curvature of space-time than during inflation.
“To make it clearer, let’s use the following analogy,” explains the cosmologist. - In the general theory of relativity, one of the general solutions is rotating black holes, described by the Kerr metric. Just because black holes are general solutions doesn't mean they are everywhere. For example, they are not in the Solar System and its surroundings (luckily for us). This means that if we search, we will definitely find them. That's how it happened." In the case of inflation, the same thing happens - this intermediate stage is not present in all solutions, but in a fairly wide class of them, so that it may well arise in a single implementation, that is, for our Universe, which exists in one copy. But how likely this one-time event is is completely determined by our hypotheses about what preceded inflation.
Statement 3. The quantum phenomenon of “eternal inflation”, which occurs in almost all inflation models and entails the emergence of a multiverse, leads to complete uncertainty in the predictions of the inflationary scenario: “Everything that can happen, happens.”
“The statement is partly false, partly has no relation to the observed effects in our Universe,” the academician is adamant. - Although the words in quotation marks were borrowed by IS&L from the reviews of Vilenkin and Gut, their meaning is distorted. There they stood in a different context and meant no more than the banal remark even for a schoolchild that the equations of physics (for example, mechanics) can be solved for any initial conditions: somewhere and someday these conditions will be realized.”
Why does “eternal inflation” and the formation of a “multiverse” not affect all processes in our Universe after the end of the inflationary stage? The fact is that they occur outside our light cone of the past (by the way, of the future too),” explains Starobinsky. Therefore, it is impossible to say for sure whether they occur in our past, present or future. “Strictly speaking, this is true up to exponentially small quantum gravitational effects, but in all existing consistent calculations such effects have always been neglected,” the academician emphasizes.
“I don’t want to say that it’s not interesting to explore what lies outside our light cone of the past,” continues Starobinsky, “but this is not yet directly connected with observational data. However, here too, IS&L confuses the reader: if “eternal inflation” is described correctly, then under given conditions at the beginning of the inflationary stage, no arbitrariness in predictions arises (although not all my colleagues agree with this). Moreover, many predictions, in particular the spectrum of matter inhomogeneities and gravitational waves arising at the end of inflation, do not depend on these initial conditions at all,” the cosmologist adds.
“There is no urgent need to revise the fundamentals of the physics of the early Universe”
Oleg Verkhodanov notes that there is no reason yet to abandon the current paradigm: “Of course, inflation has room for interpretation - a family of models. But even among them, you can choose the ones that most correspond to the distribution of spots on the CMB map. So far, most of the Planck mission results are in favor of inflation.” Alexey Starobinsky notes that the very first model with the de Sitter stage preceding the hot Big Bang, which he proposed back in 1980, is in good agreement with the data of the Planck experiment, to which IS&L appeals. (during the de-Sitter stage, which lasted about 10–35 seconds, the Universe rapidly expanded, the vacuum filling it seemed to stretch without changing its properties - website note).
Sabir Ramazanov generally agrees with him: “A number of predictions - the Gaussian nature of the spectrum of primary disturbances, the absence of constant curvature modes, the slope of the spectrum - were confirmed in the WMAP and Planck data. Inflation deservedly plays a dominant role as a theory of the early Universe. At the moment, there is no urgent need to revise the fundamentals of the physics of the early Universe.” Cosmologist Sergei Mironov also recognizes the positive qualities of this theory: “The very idea of inflation is extremely elegant, it allows us to solve all the fundamental problems of the hot Big Bang theory in one fell swoop.”
“In general, the result of the IS&L article is empty chatter from beginning to end,” sums up Starobinsky. “It has nothing to do with the real problems that cosmologists are working on now.” And at the same time, the academician adds: “Another thing is that any model - like Einstein’s general theory of relativity, like the modern model of elementary particles, and the inflation model - is not the last word in science. It is always only approximate, and at some level of accuracy, small corrections to it will certainly appear, from which we will learn a lot, since new physics will stand behind them. It’s precisely these small corrections that astronomers are looking for now.”
Since the mid-1970s, physicists began working on theoretical models of the Grand Unification of the three fundamental forces - strong, weak and electromagnetic. Many of these models concluded that very massive particles carrying a single magnetic charge must have been produced in abundance shortly after the Big Bang. When the age of the Universe reached 10 -36 seconds (according to some estimates, even a little earlier), the strong interaction separated from the electroweak interaction and became independent. In this case, point topological defects with a mass 10 15 - 10 16 greater than the mass of the then non-existent proton were formed in vacuum. When, in turn, the electroweak interaction was divided into weak and electromagnetic and true electromagnetism appeared, these defects acquired magnetic charges and began a new life - in the form of magnetic monopoles.
 |
The separation of fundamental interactions in our early Universe had the character of a phase transition. At very high temperatures, the fundamental interactions were combined, but when cooled below the critical temperature, separation did not occur [this can be compared to the supercooling of water]. At this moment, the energy of the scalar field associated with the unification exceeded the temperature of the Universe, which endowed the field with negative pressure and caused cosmological inflation. The Universe began to expand very quickly, and at the moment of symmetry breaking (at a temperature of about 10 28 K) its size increased 10 50 times. The scalar field associated with the unification of interactions disappeared, and its energy was transformed into the further expansion of the Universe. |
HOT BIRTH |
This beautiful model presented cosmology with an unpleasant problem. “Northern” magnetic monopoles annihilate when they collide with “southern” ones, but otherwise these particles are stable. Due to their huge nanogram-scale mass by the standards of the microcosm, soon after birth they were obliged to slow down to non-relativistic speeds, disperse throughout space and survive until our times. According to the standard Big Bang model, their current density should be approximately the same as that of protons. But in this case, the total density of cosmic energy would be at least a quadrillion times higher than the real one.
All attempts to discover monopoles have so far failed. As the search for monopoles in iron ores and sea water has shown, the ratio of their number to the number of protons does not exceed 10 -30. Either these particles are not present at all in our region of space, or there are so few of them that instruments are unable to register them, despite a clear magnetic signature. This is also confirmed by astronomical observations: the presence of monopoles should affect the magnetic fields of our Galaxy, but this has not been detected.
Of course, we can assume that monopoles never existed at all. Some models of the unification of fundamental interactions do not actually prescribe their appearance. But the problems of the horizon and a flat Universe remain. It so happened that in the late 1970s, cosmology faced serious obstacles, which clearly required new ideas to overcome.
NEGATIVE PRESSURE
And these ideas were not slow to appear. The main one was the hypothesis according to which in outer space, in addition to matter and radiation, there is a scalar field (or fields) that creates negative pressure. This situation seems paradoxical, but it occurs in everyday life. A positive pressure system, such as compressed gas, loses energy and cools as it expands. An elastic band, on the contrary, is in a state of negative pressure, because, unlike gas, it tends not to expand, but to contract. If such a tape is quickly stretched, it will heat up and its thermal energy will increase. As the Universe expands, a field with negative pressure accumulates energy, which, when released, can generate particles and quanta of light.
FLAT PROBLEM |
ASTRONOMERS HAVE ALREADY LONG BEEN SURE THAT IF THE CURRENT OUTER SPACE IS DEFORMED, IT IS PRETTY MODERATELY. |
FLAT PROBLEM |
Negative pressure can have different values. But there is a special case when it is equal to the density of cosmic energy with the opposite sign. In this situation, this density remains constant as space expands, since negative pressure compensates for the growing “rarefaction” of particles and light quanta. From the Friedmann-Lemaitre equations it follows that the Universe in this case expands exponentially.
The exponential expansion hypothesis solves all three problems above. Suppose that the Universe arose from a tiny “bubble” of highly curved space, which underwent a transformation that endowed space with negative pressure and thereby caused it to expand according to an exponential law. Naturally, after this pressure disappears, the Universe will return to its previous “normal” expansion.
PROBLEM SOLVING
Let us assume that the radius of the Universe before entering the exponential phase was only several orders of magnitude greater than the Planck length, 10 -35 m. If in the exponential phase it grows, say, 10 50 times, then by its end it will reach thousands of light years. Whatever the difference in the space curvature parameter from unity before the expansion begins, by the end of the expansion it will decrease by 10 -100 times, that is, the space will become perfectly flat!
The problem of monopoles is solved in a similar way. If the topological defects that became their predecessors arose before or even during the process of exponential expansion, then by the end of it they should move away from each other at gigantic distances. Since then, the Universe has expanded considerably, and the density of monopoles has dropped to almost zero. Calculations show that even if you examine a cosmic cube with an edge of a billion light years, then with the highest degree of probability there will not be a single monopole.
The exponential expansion hypothesis also suggests a simple way out of the horizon problem. Let us assume that the size of the embryonic “bubble” that laid the foundation for our Universe did not exceed the path that light managed to travel after the Big Bang. In this case, thermal equilibrium could be established in it, ensuring equality of temperatures throughout the entire volume, which was preserved during exponential expansion. A similar explanation is present in many cosmology textbooks, but you can do without it.
FROM ONE BUBBLE
At the turn of the 1970s and 1980s, several theorists, the first of whom was the Soviet physicist Alexei Starobinsky, considered models of the early evolution of the Universe with a short stage of exponential expansion. In 1981, American Alan Guth published a paper that brought this idea to widespread attention. He was the first to understand that such an expansion (most likely completed at the age mark of 10 -34 s) eliminates the problem of monopoles, which he initially dealt with, and points the way to resolving problems with flat geometry and the horizon. Guth beautifully called this expansion cosmological inflation, and the term became generally accepted.
THERE, BEHIND THE HORIZON |
THE HORIZON PROBLEM IS CONNECTED WITH THE CMB RADIATION, FROM WHATEVER POINT ON THE HORIZON IT CAME, ITS TEMPERATURE IS CONSTANT WITH AN ACCURACY OF UP TO 0.001%. |
FLAT PROBLEM |
But Guth's model still had a serious drawback. It allowed for the emergence of many inflationary areas colliding with each other. This led to the formation of a highly disordered cosmos with an inhomogeneous density of matter and radiation, which is completely different from real outer space. However, soon Andrei Linde from the Physical Institute of the Academy of Sciences (FIAN), and a little later Andreas Albrecht and Paul Steinhardt from the University of Pennsylvania showed that if you change the equation of the scalar field, then everything falls into place. This led to a scenario in which our entire observable Universe arose from a single vacuum bubble, separated from other inflationary regions by unimaginably large distances.
CHAOTIC INFLATION
In 1983, Andrei Linde made another breakthrough by developing the theory of chaotic inflation, which made it possible to explain both the composition of the Universe and the homogeneity of the cosmic microwave background radiation. During inflation, any previous inhomogeneities in the scalar field are stretched so much that they practically disappear. At the final stage of inflation, this field begins to rapidly oscillate near the minimum of its potential energy. At the same time, particles and photons are born in abundance, which intensively interact with each other and reach an equilibrium temperature. So at the end of inflation, we have a flat, hot Universe, which then expands according to the Big Bang scenario. This mechanism explains why today we observe cosmic microwave background radiation with tiny temperature fluctuations, which can be attributed to quantum fluctuations in the first phase of the existence of the Universe. Thus, the theory of chaotic inflation resolved the horizon problem without the assumption that before the onset of exponential expansion, the embryonic Universe was in a state of thermal equilibrium.
According to Linde's model, the distribution of matter and radiation in space after inflation simply must be almost perfectly homogeneous, with the exception of traces of primary quantum fluctuations. These fluctuations gave rise to local fluctuations in density, which eventually gave rise to galaxy clusters and the cosmic voids separating them. It is very important that without inflationary “stretching” the fluctuations would be too weak and would not be able to become the embryos of galaxies. In general, the inflationary mechanism has an extremely powerful and universal cosmological creativity - if you like, it appears as a universal demiurge. So the title of this article is by no means an exaggeration.
On scales of the order of hundredths of the size of the Universe (now hundreds of megaparsecs), its composition was and remains homogeneous and isotropic. However, on the scale of the entire cosmos, homogeneity disappears. Inflation stops in one area and begins in another, and so on ad infinitum. This is a self-reproducing endless process that generates a branching set of worlds - the Multiverse. The same fundamental physical laws can be realized there in different guises - for example, intranuclear forces and the charge of an electron in other universes may turn out to be different from ours. This fantastic picture is currently being discussed in all seriousness by both physicists and cosmologists.
FIGHT OF IDEAS
“The main ideas of the inflationary scenario were formulated three decades ago,” explains Andrei Linde, one of the authors of inflationary cosmology, professor at Stanford University. - After this, the main task was to develop realistic theories based on these ideas, but only the criteria for realism changed more than once. In the 1980s, the dominant view was that inflation could be understood using Grand Unified models. Then hopes faded, and inflation began to be interpreted in the context of the theory of supergravity, and later - the theory of superstrings. However, this path turned out to be very difficult. Firstly, both of these theories use extremely complex mathematics, and secondly, they are designed in such a way that it is very, very difficult to implement an inflationary scenario with their help. Therefore, progress here has been rather slow. In 2000, three Japanese scientists, with considerable difficulty, obtained, within the framework of the theory of supergravity, a model of chaotic inflation, which I had come up with almost 20 years earlier. Three years later, we at Stanford did work that showed the fundamental possibility of constructing inflationary models using superstring theory and, on its basis, explaining the four-dimensionality of our world. Specifically, we found that this way we can obtain a vacuum state with a positive cosmological constant, which is necessary to trigger inflation. Our approach was successfully developed by other scientists, and this greatly contributed to the progress of cosmology. It is now clear that superstring theory allows for the existence of a gigantic number of vacuum states, giving rise to the exponential expansion of the Universe.
Now we should take one more step and understand the structure of our Universe. This work is underway, but is encountering enormous technical difficulties, and what the result will be is not yet clear. My colleagues and I have been working for the past two years on a family of hybrid models that rely on both superstrings and supergravity. There is progress, we are already able to describe many really existing things. For example, we are close to understanding why the vacuum energy density is now so low, which is only three times higher than the density of particles and radiation. But we need to move on. We are eagerly awaiting observations from the Planck space observatory, which measures the spectral characteristics of the CMB at very high resolution. It is possible that the readings from its instruments will put entire classes of inflation models under the knife and give impetus to the development of alternative theories.”
Inflationary cosmology boasts many remarkable achievements. She predicted the flat geometry of our Universe long before astronomers and astrophysicists confirmed this fact. Until the end of the 1990s, it was believed that with full consideration of all matter in the Universe, the numerical value of the parameter Ω does not exceed 1/3. It took the discovery of dark energy to make sure that this value is practically equal to unity, as follows from the inflationary scenario. Fluctuations in the temperature of the cosmic microwave background radiation were predicted and their spectrum was calculated in advance. There are many similar examples. Attempts to refute the inflation theory have been made repeatedly, but no one has succeeded. In addition, according to Andrei Linde, in recent years the concept of a plurality of universes has emerged, the formation of which can well be called a scientific revolution: “Despite its incompleteness, it is becoming part of the culture of a new generation of physicists and cosmologists.”
AS WELL AS EVOLUTION
“The inflationary paradigm is now implemented in many variants, among which there is no recognized leader,” says Alexander Vilenkin, director of the Institute of Cosmology at Tufts University. - There are many models, but no one knows which one is correct. Therefore, I would not talk about any dramatic progress achieved in recent years. Yes, and there are still enough difficulties. For example, it is not entirely clear how to compare the probabilities of events predicted by a particular model. In an eternal universe, any event must occur countless times. So to calculate probabilities you need to compare infinities, and this is very difficult. There is also the unresolved problem of the onset of inflation. Most likely, you cannot do without it, but it is not yet clear how to get to it. And yet the inflationary picture of the world has no serious competitors. I would compare it with Darwin's theory, which at first also had many inconsistencies. However, she never had an alternative, and in the end she won the recognition of scientists. It seems to me that the concept of cosmological inflation will cope perfectly with all the difficulties.”
One of the fragments of the first microsecond of the life of the universe played a huge role in its further evolution.
Loss of Communication The cosmic microwave background radiation we now see from Earth comes from a distance of 46 billion light years (according to the companion scale), having traveled just under 14 billion years. However, when this radiation began its journey, the age of the Universe was only 300,000 years. During this time, the light could travel only 300,000 light years (small circles), and the two points in the illustration simply could not communicate with each other - their cosmological horizons do not intersect.
Alexey Levin
The conceptual breakthrough became possible thanks to a very beautiful hypothesis, born in attempts to find a way out of three serious inconsistencies of the Big Bang theory - the problem of a flat Universe, the problem of the horizon and the problem of magnetic monopoles.
Rare particle
Since the mid-1970s, physicists have begun working on theoretical models of the Grand Unification of the three fundamental forces—strong, weak, and electromagnetic. Many of these models concluded that very massive particles carrying a single magnetic charge must have been produced in abundance shortly after the Big Bang. When the age of the Universe reached 10 -36 seconds (according to some estimates, even a little earlier), the strong interaction separated from the electroweak interaction and became independent. In this case, point topological defects with a mass 10 15 -10 16 greater than the mass of the then non-existent proton were formed in vacuum. When, in turn, the electroweak interaction was divided into weak and electromagnetic and true electromagnetism appeared, these defects acquired magnetic charges and began a new life - in the form of magnetic monopoles.
The cosmic microwave background radiation that we now see from Earth comes from a distance of 46 billion light years (on the accompanying scale), having traveled just under 14 billion years. However, when this radiation began its journey, the age of the Universe was only 300,000 years. During this time, the light could travel only 300,000 light years (small circles), and the two points in the illustration simply could not communicate with each other - their cosmological horizons do not intersect.
This beautiful model presented cosmology with an unpleasant problem. “Northern” magnetic monopoles annihilate when they collide with “southern” ones, but otherwise these particles are stable. Due to their huge nanogram-scale mass by the standards of the microcosm, soon after birth they were obliged to slow down to non-relativistic speeds, disperse throughout space and survive until our times. According to the standard Big Bang model, their current density should be approximately the same as that of protons. But in this case, the total density of cosmic energy would be at least a quadrillion times higher than the real one.
All attempts to discover monopoles have so far failed. As the search for monopoles in iron ores and sea water has shown, the ratio of their number to the number of protons does not exceed 10 -30. Either these particles are not present at all in our region of space, or there are so few of them that instruments are unable to register them, despite a clear magnetic signature. This is also confirmed by astronomical observations: the presence of monopoles should affect the magnetic fields of our Galaxy, but this has not been detected.
Flat problem
Astronomers have long been convinced that if the current outer space is deformed, it is quite moderate. Friedmann and Lemaitre's models allow us to calculate what this curvature was shortly after the Big Bang to be consistent with modern measurements. The curvature of space is estimated using the dimensionless parameter Ω, equal to the ratio of the average density of cosmic energy to its value at which this curvature becomes zero, and the geometry of the Universe, accordingly, becomes flat. About forty years ago there was no longer any doubt that if this parameter differs from unity, it would be no more than ten times in one direction or another. It follows that one second after the Big Bang it differed from unity up or down by only 10 -14! Is such a fantastically precise “tuning” accidental or is it due to physical reasons? This is exactly how the problem was formulated by American physicists Robert Dicke and James Peebles in 1979.
Of course, we can assume that monopoles never existed at all. Some models of the unification of fundamental interactions do not actually prescribe their appearance. But the problems of the horizon and a flat Universe remain. It so happened that in the late 1970s, cosmology faced serious obstacles, which clearly required new ideas to overcome.
Negative pressure
And these ideas were not slow to appear. The main one was the hypothesis according to which in outer space, in addition to matter and radiation, there is a scalar field (or fields) that creates negative pressure. This situation seems paradoxical, but it occurs in everyday life. A positive pressure system, such as compressed gas, loses energy and cools as it expands. An elastic band, on the contrary, is in a state of negative pressure, because, unlike gas, it tends not to expand, but to contract. If such a tape is quickly stretched, it will heat up and its thermal energy will increase. As the Universe expands, a field with negative pressure accumulates energy, which, when released, can generate particles and quanta of light.
The local geometry of the universe is determined by the dimensionless parameter Ω: if it is less than one, the universe will be hyperbolic (open), if more - spherical (closed), and if exactly equal to one - flat. Even very small deviations from unity can lead to a significant change in this parameter over time. The illustration in blue shows a graph of the parameter for our Universe.
Negative pressure can have different values. But there is a special case when it is equal to the density of cosmic energy with the opposite sign. In this situation, this density remains constant as space expands, since negative pressure compensates for the growing “rarefaction” of particles and light quanta. From the Friedmann-Lemaitre equations it follows that the Universe in this case expands exponentially.
The exponential expansion hypothesis solves all three problems above. Suppose that the Universe arose from a tiny “bubble” of highly curved space, which underwent a transformation that endowed space with negative pressure and thereby caused it to expand according to an exponential law. Naturally, after this pressure disappears, the Universe will return to its previous “normal” expansion.
Problem solving
Let us assume that the radius of the Universe before entering the exponential phase was only several orders of magnitude greater than the Planck length, 10 -35 m. If in the exponential phase it grows, say, 10 50 times, then by its end it will reach thousands of light years. Whatever the difference in the space curvature parameter from unity before the expansion begins, by the end of the expansion it will decrease by 10 -100 times, that is, the space will become perfectly flat!
The problem of monopoles is solved in a similar way. If the topological defects that became their predecessors arose before or even during the process of exponential expansion, then by its end they should move away from each other at gigantic distances. Since then, the Universe has expanded considerably, and the density of monopoles has dropped to almost zero. Calculations show that even if you examine a cosmic cube with an edge of a billion light years, then with the highest degree of probability there will not be a single monopole.
The exponential expansion hypothesis also suggests a simple way out of the horizon problem. Let us assume that the size of the embryonic “bubble” that laid the foundation for our Universe did not exceed the path that light managed to travel after the Big Bang. In this case, thermal equilibrium could be established in it, ensuring equality of temperatures throughout the entire volume, which was preserved during exponential expansion. A similar explanation is present in many cosmology textbooks, but you can do without it.
From one bubble
At the turn of the 1970s and 1980s, several theorists, the first of whom was the Soviet physicist Alexei Starobinsky, considered models of the early evolution of the Universe with a short stage of exponential expansion. In 1981, American Alan Guth published a paper that brought this idea to widespread attention. He was the first to understand that such an expansion (most likely completed at the age mark of 10 -34 s) eliminates the problem of monopoles, which he initially dealt with, and points the way to resolving problems with flat geometry and the horizon. Guth beautifully called this expansion cosmological inflation, and the term became generally accepted.
Normal expansion at speeds lower than the speed of light leads to the fact that the entire Universe will sooner or later be inside our event horizon. Inflationary expansion at speeds significantly exceeding the speed of light has led to the fact that only a small part of the Universe formed during the Big Bang is accessible to our observation. This allows us to solve the horizon problem and explain the same temperature of the relict radiation coming from different points in the sky.
But Guth's model still had a serious drawback. It allowed for the emergence of many inflationary areas colliding with each other. This led to the formation of a highly disordered cosmos with an inhomogeneous density of matter and radiation, which is completely different from real outer space. However, soon Andrei Linde from the Physical Institute of the Academy of Sciences (FIAN), and a little later Andreas Albrecht and Paul Steinhardt from the University of Pennsylvania showed that if you change the equation of the scalar field, then everything falls into place. This led to a scenario in which our entire observable Universe arose from a single vacuum bubble, separated from other inflationary regions by unimaginably large distances.
Chaotic inflation
In 1983, Andrei Linde made another breakthrough by developing the theory of chaotic inflation, which made it possible to explain both the composition of the Universe and the homogeneity of the cosmic microwave background radiation. During inflation, any previous inhomogeneities in the scalar field are stretched so much that they practically disappear. At the final stage of inflation, this field begins to rapidly oscillate near the minimum of its potential energy. At the same time, particles and photons are born in abundance, which intensively interact with each other and reach an equilibrium temperature. So at the end of inflation, we have a flat, hot Universe, which then expands according to the Big Bang scenario. This mechanism explains why today we observe cosmic microwave background radiation with tiny temperature fluctuations, which can be attributed to quantum fluctuations in the first phase of the existence of the Universe. Thus, the theory of chaotic inflation resolved the horizon problem without the assumption that before the onset of exponential expansion, the embryonic Universe was in a state of thermal equilibrium.
According to Linde's model, the distribution of matter and radiation in space after inflation simply must be almost perfectly homogeneous, with the exception of traces of primary quantum fluctuations. These fluctuations gave rise to local fluctuations in density, which eventually gave rise to galaxy clusters and the cosmic voids separating them. It is very important that without inflationary “stretching” the fluctuations would be too weak and would not be able to become the embryos of galaxies. In general, the inflationary mechanism has an extremely powerful and universal cosmological creativity - if you like, it appears as a universal demiurge. So the title of this article is by no means an exaggeration.
On scales of the order of hundredths of the size of the Universe (now hundreds of megaparsecs), its composition was and remains homogeneous and isotropic. However, on the scale of the entire cosmos, homogeneity disappears. Inflation stops in one region and begins in another, and so on ad infinitum. This is a self-reproducing endless process that generates a branching set of worlds - the Multiverse. The same fundamental physical laws can be realized there in different guises - for example, intranuclear forces and the charge of an electron in other universes may turn out to be different from ours. This fantastic picture is currently being discussed in all seriousness by both physicists and cosmologists.
The expanding sphere demonstrates a solution to the problem of a flat Universe within the framework of inflationary cosmology. As the radius of the sphere increases, the selected area of its surface becomes more and more flat. In exactly the same way, the exponential expansion of space-time during inflation has led to the fact that our Universe is now almost flat.
Struggle of ideas
“The main ideas of the inflationary scenario were formulated three decades ago,” explains Andrei Linde, one of the authors of inflationary cosmology, Stanford University professor, to PM. - After this, the main task was to develop realistic theories based on these ideas, but only the criteria for realism changed more than once. In the 1980s, the dominant view was that inflation could be understood using Grand Unified models. Then hopes faded, and inflation began to be interpreted in the context of the theory of supergravity, and later - the theory of superstrings. However, this path turned out to be very difficult. Firstly, both of these theories use extremely complex mathematics, and secondly, they are designed in such a way that it is very, very difficult to implement an inflationary scenario with their help. Therefore, progress here has been rather slow. In 2000, three Japanese scientists, with considerable difficulty, obtained, within the framework of the theory of supergravity, a model of chaotic inflation, which I had come up with almost 20 years earlier. Three years later, we at Stanford did work that showed the fundamental possibility of constructing inflationary models using superstring theory and, on its basis, explaining the four-dimensionality of our world. Specifically, we found that this way we can obtain a vacuum state with a positive cosmological constant, which is necessary to trigger inflation. Our approach was successfully developed by other scientists, and this greatly contributed to the progress of cosmology. It is now clear that superstring theory allows for the existence of a gigantic number of vacuum states, giving rise to the exponential expansion of the Universe.
Now we should take one more step and understand the structure of our Universe. This work is underway, but is encountering enormous technical difficulties, and what the result will be is not yet clear. My colleagues and I have been working for the past two years on a family of hybrid models that rely on both superstrings and supergravity. There is progress, we are already able to describe many really existing things. For example, we are close to understanding why the vacuum energy density is now so low, which is only three times higher than the density of particles and radiation. But we need to move on. We are eagerly awaiting observations from the Planck space observatory, which measures the spectral characteristics of the CMB at very high resolution. It is possible that the readings from its instruments will put entire classes of inflation models under the knife and give impetus to the development of alternative theories.”
The cosmological inflation model, which solves many of the problems with the Big Bang theory, states that in a very short time the size of the bubble from which our Universe was formed increased by 10 50 times. After this, the Universe continued to expand, but much more slowly.
Inflationary cosmology boasts many remarkable achievements. She predicted the flat geometry of our Universe long before astronomers and astrophysicists confirmed this fact. Until the end of the 1990s, it was believed that with full consideration of all matter in the Universe, the numerical value of the parameter does not exceed 1/3. It took the discovery of dark energy to make sure that this value is practically equal to unity, as follows from the inflationary scenario. Fluctuations in the temperature of the cosmic microwave background radiation were predicted and their spectrum was calculated in advance. There are many similar examples. Attempts to refute the inflation theory have been made repeatedly, but no one has succeeded. In addition, according to Andrei Linde, in recent years the concept of a plurality of universes has emerged, the formation of which can well be called a scientific revolution: “Despite its incompleteness, it is becoming part of the culture of a new generation of physicists and cosmologists.”
On par with evolution
“The inflationary paradigm is now implemented in many variants, among which there is no recognized leader,” says Alexander Vilenkin, director of the Institute of Cosmology at Tufts University. — There are many models, but no one knows which one is correct. Therefore, I would not talk about any dramatic progress achieved in recent years. Yes, and there are still enough difficulties. For example, it is not entirely clear how to compare the probabilities of events predicted by a particular model. In an eternal universe, any event must occur countless times. So to calculate probabilities you need to compare infinities, and this is very difficult. There is also the unresolved problem of the onset of inflation. Most likely, you cannot do without it, but it is not yet clear how to get to it. And yet the inflationary picture of the world has no serious competitors. I would compare it with Darwin's theory, which at first also had many inconsistencies. However, she never had an alternative, and in the end she won the recognition of scientists. It seems to me that the concept of cosmological inflation will cope perfectly with all the difficulties.”
