Dark energy. Dark energy in the universe

Everything we see around us (stars and galaxies) is no more than 4-5% of the total mass in the Universe!

According to modern cosmological theories, our Universe consists of only 5% of ordinary, so-called baryonic matter, which forms all observable objects; 25% dark matter detected due to gravity; and dark energy, making up as much as 70% of the total.

The terms dark energy and dark matter are not entirely successful and represent a literal, but not semantic, translation from English.

In a physical sense, these terms only imply that these substances do not interact with photons, and they could just as easily be called invisible or transparent matter and energy.

Many modern scientists are convinced that research aimed at studying dark energy and matter will likely help answer the global question: what awaits our Universe in the future?

Clumps the size of a galaxy

Dark matter is a substance that most likely consists of new particles that are still unknown in terrestrial conditions and has properties inherent in ordinary matter itself. For example, it is also capable, like ordinary substances, of gathering into clumps and participating in gravitational interactions. But the size of these so-called clumps can exceed an entire galaxy or even a cluster of galaxies.

Approaches and methods for studying dark matter particles

At the moment, scientists around the world are trying in every possible way to discover or artificially obtain particles of dark matter under terrestrial conditions, using specially developed ultra-technological equipment and many different research methods, but so far all their efforts have not been crowned with success.

One method involves conducting experiments at high-energy accelerators, commonly known as colliders. Scientists, believing that dark matter particles are 100-1000 times heavier than a proton, assume that they should be generated in the collision of ordinary particles accelerated to high energies through a collider. The essence of another method is to register dark matter particles found all around us. The main difficulty in registering these particles is that they exhibit very weak interaction with ordinary particles, which are inherently transparent to them. And yet, dark matter particles very rarely collide with atomic nuclei, and there is some hope of registering this phenomenon sooner or later.

There are other approaches and methods for studying dark matter particles, and only time will tell which of them will be the first to succeed, but in any case, the discovery of these new particles will be the most important scientific achievement.

Substance with anti-gravity

Dark energy is an even more unusual substance than dark matter. It does not have the ability to gather into clumps, as a result of which it is evenly distributed throughout the entire Universe. But its most unusual property at the moment is antigravity.

The nature of dark matter and black holes

Thanks to modern astronomical methods, it is possible to determine the rate of expansion of the Universe at the present time and simulate the process of its change earlier in time. As a result of this, information was obtained that at the moment, as well as in the recent past, our Universe is expanding, and the pace of this process is constantly increasing. This is why the hypothesis about the antigravity of dark energy arose, since ordinary gravitational attraction would have a slowing effect on the process of “galaxy recession”, restraining the rate of expansion of the Universe. This phenomenon does not contradict the general theory of relativity, but dark energy must have negative pressure - a property that no currently known substance has.

Candidates for the role of "Dark Energy"

The mass of the galaxies in the Abel 2744 cluster is less than 5 percent of its total mass. This gas is so hot that it only glows in X-rays (red in this image). The distribution of invisible dark matter (which makes up about 75 percent of the cluster's mass) is colored blue.

One of the putative candidates for the role of dark energy is vacuum, the energy density of which remains unchanged during the expansion of the Universe and thereby confirms the negative pressure of the vacuum. Another putative candidate is the “quintessence” - a previously unknown ultra-weak field that supposedly passes through the entire Universe. There are also other possible candidates, but not one of them has so far contributed to obtaining an exact answer to the question: what is dark energy? But it is already clear that dark energy is something completely supernatural, remaining the main mystery of fundamental physics of the 21st century.

What's happened dark matter and dark energy The Universe: structure of space with photos, volume in percentage, influence on objects, research, expansion of the Universe.

About 80% of the space is represented by material that is hidden from direct observation. This is about dark matter– a substance that does not produce energy or light. How did the researchers realize that it was dominant?

In the 1950s, scientists began to actively study other galaxies. During the analyses, they noticed that the Universe is filled with more material than can be captured on “ visible eye" Proponents of dark matter emerged every day. Although there was no direct evidence of its existence, theories grew, as did workarounds for observation.

The material we see is called baryonic matter. It is represented by protons, neutrons and electrons. It is believed that dark matter is capable of combining baryonic and non-baryonic matter. For the Universe to remain in its usual integrity, dark matter must be present in an amount of 80%.

The elusive substance can be incredibly difficult to find if it contains baryonic matter. Among the candidates are brown and white dwarfs, as well as neutron stars. Supermassive black holes can also add to the difference. But they must have contributed more influence than what scientists saw. There are also those who think that dark matter must consist of something more unusual and rare.

Hubble telescope composite image showing a ghostly ring of dark matter in the galaxy cluster Cl 0024+17

Most of the scientific world believes that the unknown substance is represented mainly by non-baryonic matter. The most popular candidate is WIMPS (weakly interacting massive particles), whose mass is 10-100 times greater than that of a proton. But their interaction with ordinary matter is too weak, making it more difficult to find.

Neutrinos, massive hypothetical particles that are larger in mass than neutrinos, but are characterized by their slowness, are now being examined very carefully. They haven't been found yet. As possible options the smaller neutral axiom and intact photons are also taken into account.

Another possibility is that knowledge about gravity is outdated and needs to be updated.

Invisible dark matter and dark energy

But if we don’t see something, how can we prove that it exists? And why did we decide that dark matter and dark energy are something real?

The mass of large objects is calculated from their spatial movement. In the 1950s, researchers looking at spiral galaxies assumed that material close to the center would move much faster than material farther away. But it turned out that the stars were moving at the same speed, which meant there was much more mass than previously thought. The gas studied in elliptical types showed the same results. The same conclusion suggested itself: if we were guided only by visible mass, then galaxy clusters would have collapsed long ago.

Albert Einstein was able to prove that large universal objects are capable of bending and distorting light rays. This allowed them to be used as a natural magnifying lens. By studying this process, scientists were able to create a map of dark matter.

It turns out that most of our world is represented by a still elusive substance. You will learn more interesting things about dark matter if you watch the video.

Dark matter

Physicist Dmitry Kazakov about the overall energy balance of the Universe, the theory of hidden mass and dark matter particles:

If we talk about matter, then dark matter certainly leads in percentage terms. But overall it takes up only a quarter of everything. The universe abounds dark energy.

Since the Big Bang, space has begun a process of expansion that continues today. The researchers believed that eventually the initial energy would run out and it would slow down. But distant supernovae demonstrate that space does not stop, but picks up speed. All this is only possible if the amount of energy is so huge that it overcomes the gravitational influence.

Dark matter and dark energy: a mystery explained

We know that the Universe is mostly dark energy. This is a mysterious force that causes space to increase the rate of expansion of the Universe. Another mysterious component is dark matter, which maintains contact with objects only through gravity.

Scientists can't see dark matter through direct observation, but the effects can be studied. They manage to capture light that is bent by the gravitational force of invisible objects (gravitational lensing). They also notice moments when the star rotates around the galaxy much faster than it should.

All this is explained by the presence of a huge amount of elusive substance that affects mass and speed. In fact, this substance is shrouded in mystery. It turns out that researchers can rather say not what is in front of them, but what “it” is not.

This collage shows images of six different galaxy clusters taken by NASA's Hubble Space Telescope. The clusters were discovered during attempts to study the behavior of dark matter in galaxy clusters during their collision

Dark matter... dark. It does not produce light and is not observable in direct view. Therefore, we exclude stars and planets.

It does not act as a cloud of ordinary matter (such particles are called baryons). If baryons were present in dark matter, it would show up in direct observation.

We also exclude black holes, because they act as gravitational lenses that emit light. Scientists do not observe enough lensing events to calculate the amount of dark matter that must be present.

Although the Universe is a huge place, it all began with the smallest structures. It is believed that dark matter began to condense to create "building blocks" with normal matter, producing the first galaxies and clusters.

To find dark matter, scientists use various methods:

• The Large Hadron Collider.
• instruments like WNAP and the Planck space observatory.
• direct view experiments: ArDM, CDMS, Zeplin, XENON, WARP and ArDM.
• indirect detection: gamma ray detectors (Fermi), neutrino telescopes (IceCube), antimatter detectors (PAMELA), X-ray and radio sensors.

Methods for searching for dark matter

Physicist Anton Baushev on weak interactions between particles, radioactivity and the search for traces of annihilation:

Delving deeper into the mystery of dark matter and dark energy

Scientists have never been able to literally see dark matter, because it does not contact baryonic matter, which means it remains elusive to light and other types of electromagnetic radiation. But researchers are confident in its presence, as they observe the impact on galaxies and clusters.

Standard physics says that stars located at the edges of a spiral galaxy should slow down. But it turns out that stars appear whose speed does not obey the principle of location in relation to the center. This can only be explained by the fact that the stars feel the influence of invisible dark matter in the halo around the galaxy.

The presence of dark matter can also decipher some of the illusions observed in the depths of the universe. For example, the presence of strange rings and arcs of light in galaxies. That is, light from distant galaxies passes through the distortion and is amplified by an invisible layer of dark matter (gravitational lensing).

So far we have a few ideas about what dark matter is. The main idea is exotic particles that are not in contact with ordinary matter and light, but have power in the gravitational sense. Now several groups (some using the Large Hadron Collider) are working on creating dark matter particles to study them in the laboratory.

Others think the influence can be explained by a fundamental modification of gravitational theory. Then we get several forms of gravity, which differs significantly from the usual picture and the laws established by physics.

The Expanding Universe and Dark Energy

The situation with dark energy is even more confusing and the discovery itself became unpredictable in the 1990s. Physicists have always thought that the force of gravity works to slow down and one day may stop the process of universal expansion. Two teams took on the task of measuring the speed and both, to their surprise, detected acceleration. It's like you throw an apple into the air and know that it is bound to fall down, but it moves further and further away from you.

It became clear that acceleration was influenced by a certain force. Moreover, it seems that the wider the Universe, the more “power” this force gains. Scientists decided to call it dark energy.

Recently, in cosmology - the science that studies the structure and evolution of the Universe - the term “dark energy” has become widely used, causing at least slight bewilderment among people far from these studies. It is often paired with another “dark” term - “dark matter”, and it is also mentioned that, according to observational data, these two substances provide 95% of the total density of the Universe. Let us shed a ray of light on this “kingdom of darkness.”

IN scientific literature The term “dark energy” appeared at the end of the last century to refer to the physical environment that fills the entire Universe. Unlike various types substances and radiation, from which it is possible (at least theoretically) to completely clear or shield a certain volume, dark energy in the modern Universe is inextricably linked with every cubic centimeter of space. With some stretch, we can say that space itself has mass and participates in gravitational interaction. (Recall that according to the well-known formula E = mc 2, energy is equivalent to mass.)

The first word in the term "dark energy" indicates that this form of matter does not emit or absorb any electromagnetic radiation, particularly light. It interacts with ordinary matter only through gravity. The word “energy” contrasts this medium with structured matter, that is, consisting of particles, emphasizing that it does not participate in the process of gravitational crowding leading to the formation of galaxies and their clusters. In other words, the density of dark energy, unlike ordinary and dark matter, is the same at all points in space.

To avoid confusion, let us immediately note that we proceed from a materialistic idea of ​​the world around us, which means that everything that fills the Universe is matter. If matter is structured, it is called substance, and if not, such as a field, then it is called energy. The substance, in turn, is divided into ordinary and dark, focusing on whether it interacts with electromagnetic radiation. True, according to the established tradition in cosmology, dark matter is usually called “dark matter”. Energy is also divided into two types. One of them is just radiation, another substance filling the Universe. Once upon a time, it was radiation that determined the evolution of our world, but now its role has dropped to almost absolute zero, more precisely to 3 degrees Kelvin - the temperature of the so-called cosmic microwave radiation coming from all directions in space. This is a remnant (relic) of the hot youth of our Universe. But we might never have known about another type of energy, which does not interact with matter or radiation and manifests itself exclusively gravitationally, if not for research in the field of cosmology.

With radiation and ordinary matter consisting of atoms, we constantly deal with Everyday life. We know much less about dark matter. Nevertheless, it has been fairly reliably established that its physical carrier is certain weakly interacting particles. Even some properties of these particles are known, for example, that they have mass, and they move much slower than light. However, they have never been recorded by artificial detectors.

Einstein's biggest mistake

The question of the nature of dark energy is even murkier. Therefore, as often happens in science, it is better to answer it by describing the background of the question. It begins in 1917, a memorable year for our country, when the creator of the general theory of relativity, Albert Einstein, publishing a solution to the problem of the evolution of the Universe, introduced the concept of a cosmological constant into scientific circulation. In his equations describing the properties of gravity, he designated it by the Greek letter “lambda” (Λ). This is how it got its second name - lambda member. The purpose of the cosmological constant was to make the Universe stationary, that is, unchanging and eternal. Without the lambda term, the equations of general relativity predicted that the universe should be unstable, like a balloon that has suddenly lost all its air. Einstein did not seriously study such an unstable Universe, but limited himself to restoring balance by introducing a cosmological constant.

However, later, in 1922-1924, our outstanding compatriot Alexander Friedman showed that in the fate of the Universe the cosmological constant cannot play the role of a “stabilizer”, and ventured to consider unstable models of the Universe. As a result, he managed to find non-stationary solutions of Einstein’s equations, not yet known at that time, in which the Universe as a whole contracted or expanded.

In those years, cosmology was a purely speculative science, attempting to purely theoretically apply physical equations to the Universe as a whole. Therefore, Friedman's solutions were initially perceived - including by Einstein himself - as a mathematical exercise. They remembered it after the discovery of the recession of galaxies in 1929. Friedmann's solutions were excellent for describing observations and became the most important and widely used cosmological model. And Einstein later called the cosmological constant his “biggest scientific mistake.”

Distant supernovae

Gradually, the observational base of cosmology became more and more powerful, and researchers learned not only to ask questions of nature, but also to get answers to them. And along with the new results, the number of arguments in favor of the real existence of Einstein’s “biggest scientific mistake” grew. They started talking about this loudly in 1998 after the observation of distant supernovae, which indicated that the expansion of the Universe was accelerating. This meant that there was a certain repulsive force operating in the Universe, and therefore a corresponding energy, similar in its manifestations to the effect of the lambda term in Einstein’s equations. Essentially, the lambda term is a mathematical description of the simplest special case of dark energy.

Let us recall that according to observations, cosmological expansion obeys Hubble's law: the greater the distance between two galaxies, the faster they move away from each other, and the speed, determined by the red shift in the spectra of galaxies, is directly proportional to the distance. But until recently, Hubble's law was directly tested only at relatively small distances - those that could be measured more or less accurately. How the Universe expanded in the distant past, that is, over large distances, could only be judged from indirect observational data. It was possible to directly test Hubble's law at large distances only at the end of the 20th century, when a way appeared to determine the distances to distant galaxies from the supernovae that flare up in them.

A supernova is a moment in the life of a massive star when it experiences a catastrophic explosion. Supernovae come in different types depending on the specific circumstances preceding the cataclysm. During observations, the type of flare is determined by the spectrum and shape of the light curve. Supernovae, designated Ia, occur in the thermonuclear explosion of a white dwarf whose mass has exceeded a threshold of ~1.4 solar masses, called the Chandrasekhar limit. As long as the white dwarf's mass is below a threshold value, the star's gravitational force is balanced by the pressure of the degenerate electron gas. But if in a close binary system matter flows onto it from a neighboring star, then at a certain moment the electron pressure turns out to be insufficient and the star explodes, and astronomers record another type Ia supernova explosion. Since the threshold mass and the reason why a white dwarf explodes are always the same, such supernovae at maximum brightness should have the same, and very high, luminosity and can serve as a “standard candle” for determining intergalactic distances. If we collect data on many such supernovae and compare the distances to them with the redshifts of the galaxies in which the outbursts occurred, we can determine how the expansion rate of the Universe has changed in the past and select the appropriate cosmological model, in particular the appropriate value of the lambda term (dark density). energy).

However, despite the simplicity and clarity of this method, it faces a number of serious difficulties. First of all, the lack of a detailed theory of the explosion of Type Ia supernovae makes their status as a standard candle precarious. The nature of the explosion, and therefore the luminosity of the supernova, can be affected by the rotation speed of the white dwarf, chemical composition its core, the amount of hydrogen and helium that flowed onto it from a neighboring star. How all this affects the light curves is not yet known with certainty. Finally, supernovae do not flare in empty space, but in galaxies, and the light of the flare may, for example, be weakened by a random cloud of gas and dust encountered on the way to Earth. All this casts doubt on the possibility of using supernovae as standard candles. And if this were the only argument in favor of the existence of dark energy, this article would hardly have been written. So while the supernova argument has sparked widespread debate about dark energy (and even the term itself), cosmologists' confidence in its existence rests on other, more compelling arguments. Unfortunately, they are not so simple, and therefore they can only be described in the most general terms.

A Brief History of Times

By modern ideas, the birth of the Universe must be described in terms of the not yet created quantum theory of gravity. The concept of “age of the Universe” makes sense for moments of time no earlier than 10-43 seconds. On a smaller scale, it is no longer possible to talk about the linear flow of time that we are accustomed to. The topological properties of space also become unstable. Apparently, on small scales, space-time is filled with microscopic “wormholes” - a kind of tunnels connecting separated regions of the Universe. However, it is also impossible to talk about distances or the order of events. In the scientific literature, such a state of space-time with a fluctuating topology is called quantum foam. For reasons still unknown, perhaps due to quantum fluctuations, a physical field appears in the space of the Universe, which at about 10-35 seconds causes the Universe to expand with colossal acceleration. This process is called inflation, and the field that causes it is called inflaton. Unlike economics, where inflation is a necessary evil that must be fought, in cosmology inflation, that is, the exponentially rapid expansion of the Universe, is a good thing. It is to her that we owe that the Universe has gained big size and flat geometry. At the end of this short epoch of accelerated expansion, the energy stored in the inflaton gives rise to the matter we know: a mixture of radiation and massive particles heated to enormous temperatures, as well as dark energy, barely noticeable against their background. We can say that this is the Big Bang. Cosmologists speak of this moment as the beginning of a radiation-dominated era in the evolution of the Universe, since most of the energy at this time comes from radiation. However, the expansion of the Universe continues (although now without acceleration) and it affects the main types of matter in different ways. The tiny density of dark energy does not change over time, the density of matter falls in inverse proportion to the volume of the Universe, and the density of radiation decreases even faster. As a result, after 300 thousand years, the dominant form of matter in the Universe becomes matter, most of which is dark matter. From this moment on, the growth of disturbances in the density of matter, barely smoldering at the stage of radiation dominance, becomes fast enough to lead to the formation of galaxies, stars and planets so necessary for humanity. The driving force of this process is gravitational instability, leading to the crowding of matter. Barely noticeable inhomogeneities remained from the moment of inflaton decay, but as long as radiation dominated the Universe, it prevented the development of instability.
Now dark matter begins to play a major role. Under the influence of their own gravity, regions of increased density stop in their expansion and begin to contract, as a result of which gravitationally bound systems called halos are formed from dark matter. In the gravitational field of the Universe, “holes” are formed into which ordinary matter rushes. Accumulating inside the halo, it forms galaxies and their clusters. This process of formation of structures began more than 10 billion years ago and continued to grow until the last turning point in the evolution of the Universe occurred. After 7 billion years (about half the current age of the Universe), the density of matter, which continued to decline due to cosmological expansion, became less than the density of dark energy. Thus, the era of dominance of matter ended, and now dark energy controls the evolution of the Universe. Whatever its physical nature, it manifests itself in the fact that the cosmological expansion again, as in the era of inflation, begins to accelerate, only this time very slowly. But even this is enough to slow down the formation of structures, and in the future it should stop altogether: any insufficiently dense formations will be dissipated by the accelerating expansion of the Universe. The time “window” in which gravitational instability operates and galaxies arise will close in tens of billions of years. The further evolution of the Universe depends on the nature of dark energy. If this is a cosmological constant, then the accelerated expansion of the Universe will continue forever. If dark energy is an ultra-weak scalar field, then after it reaches a state of equilibrium, the expansion of the Universe will begin to slow down, and possibly be replaced by compression. While the physical nature of dark energy is unknown, all these are nothing more than speculative hypotheses. Thus, only one thing can be said with certainty: the accelerated expansion of the Universe will continue for several tens of billions of years. During this time, our cosmic home - the Milky Way galaxy - will merge with its neighbor - the Andromeda Nebula (and most of the smaller satellite galaxies that are part of the Local Group). All other galaxies will fly away to great distances, so that many of them will not be visible even with the most powerful telescope. As for the cosmic microwave background radiation, which brings us so much vital information about the structure of the Universe, then its temperature will drop almost to zero, and this source of information will be lost. Humanity will remain Robinson on the island with the ephemeral prospect of acquiring at least Friday.

Large-scale structure of the Universe

Cosmologists have two main sources of knowledge about the large-scale structure of the Universe. First of all, this is the distribution of luminous matter, that is, galaxies, in the space surrounding us. The three-dimensional map shows into what structures - groups, clusters, superclusters - galaxies are united and what are the characteristic sizes, shapes and numbers of these formations. This makes it clear how matter is distributed in the modern Universe.

Another source of information is the distribution of the intensity of the cosmic microwave background radiation over the celestial sphere. A map of the sky in the microwave range carries information about the distribution of density inhomogeneities in the early Universe, when its age was about 300 thousand years - it was then that matter became transparent to radiation. The angular distances between the spots on the microwave map indicate the size of the irregularities at that time, and the differences in brightness (by the way, they are very small, on the order of a hundredth of a percent) indicate the degree of compaction of the embryos of future galaxy clusters. Thus, we have, as it were, two time slices: the structure of the Universe at moments 300 thousand and 14 billion years after the Big Bang.

The theory says that the characteristics of the observed structures strongly depend on how much of the matter in the Universe is matter (regular and dark). Calculations based on observational data show that its share today is about 30% (of which only 5% is ordinary matter consisting of atoms). This means that the remaining 70% is matter that is not included in any structures, that is, dark energy. This argument is not so transparent, since behind it there are complex calculations that describe the formation of structures in the Universe. However, it is indeed more powerful. This can be illustrated with this analogy. Imagine that an extraterrestrial civilization is seeking to discover intelligent life on Earth. One group of researchers noticed powerful radio emission coming from our planet, which periodically changes frequency and intensity, and attributes this to the work of electronic equipment. Another group sent a probe to Earth and photographed squares of fields, road lines, and city nodes. The first argument is, of course, simpler, but the second is more convincing.

Continuing this analogy, we can say that even more clear evidence of intelligent life would be the observation of the formation of the listed structures. Of course, it is not yet possible for humans to observe in real time how clusters of galaxies are formed. Nevertheless, it is possible to determine how their number changed during the evolution of the Universe. The fact is that, due to the finite speed of light, observing objects at large distances is equivalent to looking into the past.

The rate of formation of galaxies and their clusters is determined by the growth rate of density disturbances, which, in turn, depends on the parameters of the cosmological model, in particular on the ratio of matter and dark energy. In a Universe with a large proportion of dark energy, disturbances grow slowly, which means that today there should be only slightly more galaxy clusters than in the past, and their number will decrease slowly with distance. In contrast, in a Universe without dark energy, the number of clusters decreases quite quickly as we go deeper into the past. By determining the rate at which new galaxy clusters are appearing from observations, we can obtain an independent estimate of the dark energy density.

There are other independent observational arguments confirming the existence of a homogeneous medium, which has a decisive influence on the structure and evolution of the Universe. We can say that the statement about the existence of dark energy was the result of the development of the entire observational cosmology of the twentieth century.

Vacuum and other models

While most cosmologists no longer doubt the existence of dark energy, there is still no clarity regarding its nature. However, this is not the first time that physicists find themselves in such a situation. Many new theories begin with phenomenology, that is, a formal mathematical description of a particular effect, and intuitive explanations appear much later. For today, describing physical properties dark energy, cosmologists pronounce words that for the uninitiated are more like a spell: this is a medium whose pressure is equal to the energy density in magnitude, but opposite in sign. If this strange relationship is substituted into Einstein’s equation from the general theory of relativity, it turns out that such a medium is gravitationally repelled from itself and, as a result, expands rapidly and will never gather into any clumps.

This is not to say that we often deal with such matter. However, this is exactly how physicists have been describing vacuum for many years. According to modern concepts, elementary particles do not exist in empty space, but in a special environment - a physical vacuum, which precisely determines their properties. This medium can be in different states, differing in the density of stored energy, and in different types of vacuum, elementary particles behave differently.

Our ordinary vacuum has the least energy. The existence of an unstable, more energetic vacuum, which corresponds to the so-called electroweak interaction, has been experimentally discovered. It begins to appear at particle energies above 100 gigaelectronvolts - this is only an order of magnitude below the limit of the capabilities of modern accelerators. Even more energetic types of vacuum are predicted theoretically. It can be assumed that our ordinary vacuum does not have zero energy density, but just one that gives the desired value of the lambda term in Einstein’s equation.

However, this nice idea The idea of ​​attributing dark energy to the vacuum does not excite researchers working at the intersection of particle physics and cosmology. The fact is that this type of vacuum should correspond to a particle energy of only about a thousandth of an electronvolt. But this energy range, which lies on the border between infrared and radio radiation, has long been studied far and wide by physicists, and nothing anomalous has been found there.

Therefore, researchers are inclined to believe that dark energy is a manifestation of a new ultra-weak field that has not yet been discovered in laboratory conditions. This idea is similar to that which underlies modern inflationary cosmology. There, too, the ultra-fast expansion of the young Universe occurs under the influence of the so-called scalar field, only its energy density is much higher than that which is responsible for the current slow acceleration in the expansion of the Universe. It can be assumed that the field, which is the carrier of dark energy, remained as a relic of the Big Bang and was in a state of “hibernation” for a long time while the dominance of first radiation and then dark matter lasted.

Negative pressure and gravitational repulsion

When describing dark energy, cosmologists believe that its main property is negative pressure. It leads to the appearance of repulsive gravitational forces, which non-specialists sometimes refer to as antigravity. This statement contains two paradoxes at once. Let's look at them sequentially.

How can pressure be negative? The pressure of an ordinary substance is known to be associated with the movement of molecules. Hitting the wall of the vessel, the gas molecules transfer their impulse to it, push it away, and put pressure on it. Free particles cannot create negative pressure, they cannot “pull the blanket over themselves,” but in a solid body this is quite possible. A good analogy for the negative pressure of dark energy is the shell balloon. Every square centimeter of it is stretched and tends to shrink. If a gap appeared somewhere in the shell, it would immediately shrink into a small rubber rag. But while there is no rupture, the negative tension is evenly distributed over the entire surface. Moreover, if the balloon is inflated, the rubber will become thinner, and the energy stored in its tension will increase. The density of matter and dark energy behaves in a similar way as the Universe expands.

Why does negative pressure speed up expansion? It would seem that, under the influence of the negative pressure of dark energy, the Universe should contract or, at least, slow down its expansion, which began at the moment of the Big Bang. But the opposite is true, because the negative pressure of dark energy is too... great.

The fact is that, according to the general theory of relativity, gravity depends not only on mass (more precisely, energy density), but also on pressure. The greater the pressure, the stronger the gravity. And the greater the negative pressure, the weaker it is! True, the pressures achievable in laboratories and even in the center of the Earth and the Sun are too low for their effect on gravity to be noticeable. But the negative pressure of dark energy, on the contrary, is so great that it overpowers the attraction of both its own mass and the mass of all other matter. It turns out that a massive substance with very strong negative pressure paradoxically does not compress, but, on the contrary, swells under the influence of its own gravity. Imagine a totalitarian state that, in an effort to ensure its security, clamps down on freedom to such an extent that citizens flee the country en masse, rebel and ultimately destroy the state itself. Why do excessive efforts to strengthen the state result in its destruction? This is the nature of people - they resist suppression. Why does extreme negative pressure lead to expansion instead of compression? These are the properties of gravity expressed by Einstein's equation. Of course, an analogy is not an explanation, but it helps to “get your head around” the paradoxes of dark energy.

How to weigh the structure?

Dark energy is the most important evidence of the existence of phenomena that are not described by modern physics. Therefore, a detailed study of its properties is the most important task of observational cosmology. To find out the physical nature of dark energy, it is necessary first of all to study as accurately as possible how the expansion mode of the Universe changed in the past. One can try to directly measure the dependence of the rate of expansion on distance. However, due to the lack of reliable methods in astronomy for determining extragalactic distances, it is almost impossible to achieve the required accuracy along this path. But there are other, more promising ways to measure dark energy, which are a logical extension of the structural argument for its existence.

As already noted, the rate of formation of structures very much depends on the density of dark energy. It itself cannot cluster and create structures and prevents the gravitational clustering of dark and ordinary matter. By the way, this is why in our era those lumps of matter that have not yet begun to shrink gradually “dissolve” in the sea of ​​dark energy, ceasing to “feel” mutual attraction. Humanity, thus, is witnessing the maximum rate of formation of structures in the history of the Universe. In the future it will only decrease.

To determine how the density of dark energy has changed over time, you need to learn how to “weigh” the structure of the Universe - galaxies and their clusters - at different redshifts. There are many ways to do this, because the objects of measurement - galaxies - are well studied and visible even at great distances. The most straightforward approach is to carefully count the galaxies and their structures using the aforementioned three-dimensional map of the spatial distribution of galaxies. In another method, the mass of a structure is estimated from the inhomogeneous gravitational field it creates. As light passes through the structure, it is deflected by its gravity, causing the images of distant galaxies we see to be distorted. This effect is called gravitational lensing. By measuring the resulting distortions, it is possible to determine (weigh) the structure along the path of light. The first successful observations have already been made using this method, and space experiments are planned for the future - after all, it is necessary to achieve maximum measurement accuracy.

So, we live in a world whose expansion dynamics are controlled by a form of matter unknown to us. And the only reliable knowledge about it, besides the fact of its existence, is the equation of state of a vacuum-like type, that same peculiar connection between energy density and pressure. We do not yet know whether and how the nature of this relationship changes over time. This means that all discussions about the future of the Universe are essentially speculative, based largely on the aesthetic views of their authors. But we have entered an era of precise cosmology, based on high-tech observational instruments and advanced statistical methods for data processing. If astronomy continues to develop at the same pace as today, the mystery of dark energy will be solved by the current generation of researchers.

Physicists love catchphrases. For some time now, it has been customary among them to give “unscientific” names to newly discovered entities. Take strange and charm quarks, for example. So dark energy is not a synonym for dark forces, but a term coined to designate some unusual properties of our Universe.

The discovery of dark energy was made using astronomical methods and came as a complete surprise to most physicists. Dark energy is perhaps the main mystery modern natural science. It is likely that its solution will become the most important event in physics of the 21st century, comparable in scale to the largest discoveries of the recent past, such as the discovery of the phenomenon of the expansion of the Universe.

It is even possible that such a radical development of the theory will occur that it will be on a par with the creation of the general theory of relativity, the discovery of the curvature of space-time and the connection of this curvature with gravitational forces. We are now at the beginning of the journey, and talking about dark energy is an opportunity to look into the “laboratory” of physicists at a time when their work is in full swing.

A little history

The fact that “something is wrong” in our Universe became clear to cosmologists by the early 1990s. For clarification, it is useful to recall the law of expansion of the Universe. Galaxies that are distant from each other scatter, and the further away the galaxy, the faster it moves away from us. Quantitatively, the expansion rate is characterized by the Hubble parameter. By the early 1990s, the value of the Hubble parameter in the modern Universe was quite well measured: the rate of expansion of the Universe today is such that galaxies located at a distance of 1 billion light years from Earth are escaping from us at a speed of 24 thousand km/s.

Note that the Hubble parameter depends on time: in the distant past, the Universe expanded much faster than it does now, and, accordingly, the Hubble parameter was much larger.

In the modern theory of gravity - the general theory of relativity - the Hubble parameter is uniquely related to two other characteristics of the Universe: firstly, with the total energy density of all forms of matter, vacuum, etc., and secondly, with the curvature of three-dimensional space. Our three-dimensional space, generally speaking, does not have to be Euclidean; its geometry may, for example, be similar to the geometry of a sphere; The sum of the angles of a triangle may not equal 180°. In this case, the “elasticity” of space from the point of view of the expansion of the Universe plays the same role as the energy density.

By the early 1990s, the energy density of “normal” matter in the modern Universe was also estimated with good accuracy. It is “normal” in the sense that it experiences the same gravitational interactions as ordinary matter. The matter, however, is complicated by the fact that most of the “normal” matter is so-called dark matter. Dark matter apparently consists of new, not yet discovered in terrestrial experiments, elementary particles that interact extremely weakly with matter (weaker than neutrinos!), but equally experience gravitational interaction. It was precisely by the effect of gravitational attraction that it was discovered. Moreover, measurements of gravitational forces in galaxy clusters made it possible to determine the mass of dark matter in them, and ultimately in the Universe as a whole. Thus, the total energy density of “normal” matter was found (the famous formula is valid for it E = mс 2).

And what happened? It turned out that “normal” matter is clearly not enough to explain the measured rate of expansion of the Universe. Moreover, there is a severe shortage: the “shortage” was about 2/3 (according to modern estimates - about 70%). There were two possible explanations for this fact: either three-dimensional space is curved, and the missing contribution to the Hubble parameter is associated with its “elasticity,” or there is new form energy, which later became known as “dark energy”.

From a theoretical point of view, both of these possibilities - the non-Euclidean nature of space and dark energy - looked extremely implausible.

Let's start with the curvature of three-dimensional space. As the Universe expands, space smoothes out and its curvature decreases. If the curvature is different from zero now, then it was greater in the past than it is today. However, the energy (mass) density of matter decreases even faster as the Universe expands. This means that in the past the relative contribution of curvature to the Hubble parameter was very small, and the main contribution - by a large margin - was the contribution of matter. In order for the expansion of the Universe to be 70% ensured by curvature today, it is necessary to “adjust” the value of the radius of curvature of space in the past with fantastic accuracy - a second after the Big Bang it should have been equal to a billion radii of the part of the Universe observed at that time, no more and no less! Without such a fit, the curvature today would be either many orders of magnitude greater or many orders of magnitude less than needed to explain the observations.

This problem was one of the main considerations that led to the idea of ​​the inflationary stage of the evolution of the Universe. According to the inflation theory, proposed by Alexei Starobinsky and independently by Alan Guth and shaped by the work of Andrei Linde, Andreas Albrecht and Paul Steinhardt, the Universe, at a very early stage of its evolution, went through a stage of extremely rapid, exponential expansion (inflation, inflation). At the end of this stage, the Universe warmed up to a very high temperature, and the era of the hot Big Bang began.

Although the inflationary stage most likely lasted only a small fraction of a second, during this time the Universe expanded by tens or hundreds of orders of magnitude (or much more) and the curvature of space dropped to almost zero. Thus, inflationary theory leads to the prediction that the space of the modern Universe with the highest degree Euclidean precision. This, of course, goes against the hypothesis that the Universe is expanding today by 70% due to curvature.

The action of dark energy is similar to the cosmological inflation of the first moments of the Universe, only on a completely different scale - an insignificant energy density, slow acceleration. This small scale big mystery, it is completely unclear how dark energy can be related to the physics of particles and fields known to us. We will return to this riddle later.

In the dilemma of whether dark energy or curvature is responsible for the missing 70% of the density of the Universe, the latter has long been more popular. The breakthrough came in 1998–1999, when two US teams, one led by Adam Reiss and Brian Schmidt and the other by Saul Perlmutter, reported observations of distant Type Ia supernovae. From these observations it followed that our Universe is expanding at an accelerating rate. This property is completely consistent with the idea of ​​dark energy, while the curvature of space does not lead to accelerated expansion.

A few words about type Ia supernovae. These are white dwarfs that, fueled by matter from a companion star, reached the so-called Chandrasekhar limit, after which they lost stability, exploded and collapsed into neutron stars. The Chandrasekhar limit is the same for all white dwarfs; the white dwarfs themselves are similar to each other, therefore the explosions are in a certain sense the same. In other words, type Ia supernovae are “standard candles”: knowing the absolute luminosity and measuring the apparent brightness (the flow of energy coming to Earth), you can determine the distance to each of them. At the same time, it is possible to establish the speed at which each of the supernovae is moving away from us (using the Doppler effect).

Supernovae are very bright objects and can be seen at great distances. In other words, the distant supernovae that we observe now exploded a long time ago, and therefore their escape speed was determined by the rate of expansion of the Universe then, in the distant past. Thus, observations of type Ia supernovae make it possible to determine the expansion rate at a relatively early stages evolution of the Universe (8 billion years ago and even a little earlier) and trace the dependence of this rate on time. This is what made it possible to establish that the Universe is expanding at an accelerating rate.

The final proof that the curvature of the three-dimensional space of the Universe is small was obtained by studying the map of the cosmic microwave background radiation.

During the era of relict photon emission, the Universe was not exactly homogeneous. The inhomogeneities that existed then were the embryos of structures - the first stars, galaxies, clusters of galaxies. At that time, plasma inhomogeneities were sound waves. It is important that at that time the Universe had a characteristic distance scale. Sound waves with a long length and, accordingly, a long period, had not yet had time to develop by the era of radiation of relict photons, and waves with the “correct” length had just managed to reach the phase of maximum amplitude. This “correct” wavelength represents the “standard ruler” of the era of CMB photon emission; its size is reliably calculated in the hot Big Bang theory and appears in the CMB map.

At the turn of the 20th–21st centuries, in the BOOMERanG and MAXIMA experiments, the angle at which the “standard ruler” just discussed is visible was measured for the first time. It is clear that this angle depends on the geometry of space: if the sum of the angles of a triangle exceeds 180°, then this angle is larger. As a result, it was found that our three-dimensional space is Euclidean with a good degree of accuracy. Subsequent measurements confirmed this conclusion. From the point of view of the expansion of the Universe, the existing results mean that the curvature of space makes a negligible contribution (less than 1%) to the Hubble parameter. The expansion rate of the Universe is now 70% due to dark energy.

They don't know anything about her anymore

What properties of dark energy are currently known? There are few such properties, only three. But what is known can rightly cause astonishment.

The first is the fact that, unlike “normal” matter, dark energy does not cluster, does not gather into objects such as galaxies or their clusters - it is “spread” evenly throughout the Universe. This statement, like any statement based on observations or experiments, is true with a certain accuracy. However, from observations it follows that deviations from homogeneity, if they exist, should be very small in magnitude.

We have already talked about the second property: dark energy causes the Universe to expand with acceleration. In this way, dark energy is also strikingly different from normal matter, which slows down expansion. The two described properties indicate that dark energy, in a certain sense, experiences antigravity; for it there is gravitational repulsion instead of gravitational attraction. High Density Areas normal matter, due to gravitational attraction, collects matter from the surrounding space, these areas themselves are compressed and form dense clumps. For an anti-gravitating substance, the opposite is true: areas with increased density (if any) are stretched due to gravitational repulsion, inhomogeneities are smoothed out and no clumps are formed.

The third property of dark energy is that its density does not depend on time. Also surprising: the Universe is expanding, the volume is growing, but the energy density remains constant. There seems to be a contradiction here with the law of conservation of energy. Over the past 8 billion years, the Universe has doubled in size. An area of ​​space that then had, say, a size of 1 m, today has a size of 2 m, its volume has increased 8 times, and the energy in this volume has increased by the same amount. The non-conservation of energy is obvious.

In fact, the increase in energy as the Universe expands does not contradict the laws of physics. Dark energy is designed in such a way that expanding space does work on it, which leads to an increase in the energy of this substance in the expanding volume of space. True, the expansion of space is itself caused by dark energy, so the situation is reminiscent of Baron Munchausen pulling himself out of the swamp by his hair. And yet there is no contradiction: in a cosmological context it is impossible to introduce the concept full energy, which includes the energy of the gravitational field itself. So there is also no law of conservation of energy, which prohibits the increase or decrease in the energy of any form of matter.

The statement about the constancy of the density of dark energy is also based on astronomical observations, and therefore is also true with a certain accuracy. To characterize this accuracy, we point out that over the past 8 billion years, the density of dark energy has changed by no more than 1.1 times. Today we can say this with confidence.

Note that the second and third properties of dark energy - the ability to lead to an accelerated expansion of the Universe and its constancy in time (or, more generally, a very slow dependence on time) - are in fact closely related. This connection follows from the equations of general relativity. Within the framework of this theory, the accelerated expansion of the Universe occurs precisely when the energy density in it either does not change at all or changes very slowly. Thus, the antigravity of dark energy and its difficult relationships with the law of conservation of energy - two sides of the same coin.

This essentially exhausts reliable information about dark energy. Then the area of ​​hypotheses begins. Before talking about them, let's briefly discuss one general issue.

Why now?

If in the modern Universe dark energy makes the largest contribution to the total energy density, then in the past this was far from the case. Let's say 8 billion years ago, normal matter was 8 times denser, and the density of dark energy was the same (or almost the same) as now. From this it is easy to conclude that at that time the ratio between the rest energy of normal matter and dark energy was in favor of the former: dark energy was about 13%, and not 70% as it is today. Due to the fact that normal matter played the main role at that time, the expansion of the Universe occurred with slowdown . Even earlier, the influence of dark energy on expansion was very weak.

So, the influence of dark energy and the acceleration of the expansion of the Universe caused by it are very recent phenomena by cosmological standards: the acceleration began “only” 6.5 billion years ago. On the other hand, since the density of normal matter decreases with time, but the density of dark energy does not, dark energy will soon (again by cosmological standards) completely dominate. This means that the current stage of cosmological evolution is a transitional period when dark energy already plays a noticeable role, but the expansion of the Universe is determined not only by it, but also by normal matter. Is this particularity of our time a coincidence or is there some deep property of our Universe behind it? This question is “why now?” - remains open for now.

Candidates

If there were no gravity, the absolute value of energy would have no physical meaning. In all theories describing nature, with the exception of the theory of gravitational interactions, only difference energies of certain states. Thus, when speaking about the binding energy of a hydrogen atom, we mean the difference between two quantities: the total rest energy of a free proton and electron, on the one hand, and the rest energy of the atom, on the other. It is this energy difference that is released (transferred to the born photon) when an electron and proton combine to form an atom. If it were not for gravitational interaction, talking about vacuum energy would be pointless , her there would simply be nothing to compare it to.

The fact is that vacuum energy, like any other energy, “weighs” gravitates . Vacuum is a state with the lowest energy (therefore, by the way, energy cannot be taken away from it), but this energy does not have to be equal to zero; from a theoretical point of view, it can be both positive and negative. Whether it can be calculated “from first principles” is a big question. But in any case, vacuum energy, if it is positive, has exactly the properties that dark energy should have: homogeneity in space and constancy in time.

As we said above, in the general theory of relativity, the last property automatically means that vacuum energy leads to the accelerated expansion of the Universe.

We emphasize that homogeneity in space and constancy in time are exact, not approximate properties of the vacuum. The vacuum energy density is a universal constant (at least in the part of the Universe that we observe). It must be said that this constant - the cosmological constant, the Λ-term - was introduced into his equations by Einstein. True, he did not identify it with vacuum energy, but this is a question of terminology, at least with the modern understanding of the essence of the matter. Einstein later abandoned his idea - perhaps in vain.

Why does the idea of ​​dark energy as vacuum energy not satisfy many physicists? First of all, this is due to the absurdly small value of the vacuum energy density, which is necessary for agreement between theory and observations.

In a vacuum, virtual particles are born and die all the time, there are field condensates in it - the vacuum is more like a complex medium than an absolute void. These are not just speculations: the features of the vacuum are reflected in the properties of elementary particles and their interactions and are ultimately determined, albeit indirectly, from numerous experiments. The energy of the vacuum, in principle, should “know” how it is structured, what its structure is and what the values ​​of the parameters characterizing it are (for example, field condensates).

Now let's imagine a theoretical angel who has studied elementary particle physics, but has not heard anything about our Universe. Let's ask this theorist to predict the vacuum energy density. Based on the energy scales characteristic of fundamental interactions and the corresponding length scales, he will make his estimate - and be mistaken by an unimaginable number of times - by tens of orders of magnitude. Our theorist would predict such more energy vacuum and such a rate of expansion of the Universe caused by it that the houses on the next street should fly away from us at speeds close to the speed of light!

The problem of vacuum energy puzzled theoretical physicists long before the discovery of dark energy. Thus, in the 1920s and 1930s, this problem worried Wolfgang Pauli, who wrote in 1933: “This energy [of vacuum; then they used the term “zero point energy,” Nullpunktsenergie] should be unobservable in principle, since it is not emitted, absorbed, or scattered... and since, as is obvious from experience, it does not create a gravitational field.” Why is this happening? One possibility is that the energy of empty space somehow still changes over time and eventually becomes close to zero. Specific theoretical models illustrating this possibility are extremely difficult to construct, but not impossible; it is even more difficult to fit them into a cosmological context.

If dark energy is vacuum energy, then trying to understand why it has such a small value can be done using a completely different logic. Let us imagine that the Universe is extremely large, that it is many times larger than the part we observe. Let us further assume that in different very vast parts of the Universe, a variety of vacuum states with very different energy densities can be realized. This possibility, by the way, is not theoretically excluded; Moreover, this is exactly what seems to be the case in superstring theory, especially if the Universe was going through an inflationary stage. Regions of the Universe where the vacuum energy density is too high in absolute value look completely different from our region: where the vacuum energy is large and positive, space expands so quickly that stars and galaxies simply do not have time to form; in regions with large negative vacuum energy, the expansion of space quickly gives way to compression, and these regions collapse long before the formation of stars. In both cases, cosmological evolution is incompatible with the existence of observers like us. And, conversely, we could only appear where the vacuum energy density is very close to zero - and that’s where we appeared.

This, as they say, anthropic view of the problem of vacuum energy was expressed more than 20 years ago in the works of Andrei Linde and Steven Weinberg. Now it is popular among a significant part of theoretical physicists. The other part perceives it as a way to get away from the problem. The most balanced approach is probably not to rule out the anthropic explanation as a possible final answer, but still try to find alternative solution problems of vacuum energy and dark energy.

An alternative to vacuum as a carrier of dark energy may serve as some new field, “spilled” in the Universe. In this version, the energy of the new field is dark energy. This field should be new because the presence of known fields (for example, electromagnetic fields) everywhere in the Universe would influence the behavior of matter too much and lead to effects that would have been discovered long ago. In addition, the known fields are such that their energy does not have the properties of dark energy listed above.

The hypothetical new field should be characterized by an energy scale of the order of 0.002 eV. Although this is a very small scale in terms of known interactions, it does not seem completely implausible. Indeed, we already know that the scale of different interactions varies greatly. Thus, the mentioned scale of strong interactions (200 MeV) is 10 19 times smaller than the scale of gravitational forces. Such a gigantic difference, of course, requires an explanation in itself, but this is a separate issue. In any case, the existence of different energy scales in nature is a fact, and the introduction of a new small scale does not seem to be an insurmountable obstacle.

The new field, generally speaking, changes during the evolution of the Universe. Its energy density also changes. In order for this change not to be too rapid, the quanta of the new field - new particles - must have an extremely small mass; they say this field should be easy.

Finally, a new field is a new force (just as the gravitational field corresponds to gravitational, and the electromagnetic field corresponds to electric and magnetic forces). A light field with extremely low mass is a long-range force similar to gravity. In order not to conflict with experiments testing the general theory of relativity, the interaction of this field with ordinary matter should be very weak, weaker than gravitational one.

All these properties do not look attractive to the theorist, but they can be accepted. It is important that the hypothesis of a new field, at least in principle, allows for experimental verification - with the help of observations, it is possible to identify changes in the field energy density over time. This will definitely reject the hypothesis about the vacuum nature of dark energy and, on the contrary, will serve as an indication of the existence of a new light field in the Universe. In addition, in the future we can hope to discover the heterogeneity of the distribution of dark energy in space. This would be definitive proof that dark energy is new field energy and not anything else.

On the other hand, today there are no visible ways to register a new light field in laboratory experiments, accelerators, etc. The reason is the extremely weak interaction of this field with matter. However, we still know too little, and, as they say, never say “never.”

Physicists discuss different types hypothetical light fields, the energy of which could act as dark energy. In the simplest version from a theoretical point of view, the energy density of the new field decreases with time. For a field of this type, the term “quintessence” is used. However, the opposite possibility cannot be excluded, when the energy density growing with time; a field of this type is called a “phantom”. Phantom would be a very exotic field; Nothing like this has ever been found in nature. The distinction between quintessence and phantom, as we will discuss below, is important from the point of view of remote future Universe.

Finally, another possible explanation for dark energy is that there really is no dark energy. If general relativity does not apply on modern cosmological length and time scales, then there is no need for dark energy.

Of course, this view of dark energy cannot ignore the fact that general relativity has been well tested on smaller distance scales. Therefore you need to create new theory gravity, which would transform into the general theory of relativity at these distances, but would otherwise describe the evolution of the Universe at relatively late stages, close to ours. This is a difficult task, especially if we take into account the requirement of self-consistency, internal consistency of the theory. Nevertheless, such attempts are being made, and some of them look quite promising.

One possibility is to allow Newton's constant of gravity to vary in space and time according to certain equations. Unfortunately, the most beautiful versions of the theory that realize this possibility were rejected by experiments testing general relativity. If you don’t pursue beauty, then models that explain the accelerated expansion of the Universe and are consistent with everything that is known about gravity can be built along this path. Such models, as a rule, predict deviations from the general theory of relativity, which, although small, are experimentally detectable in the future.

Let us also note the idea that our space can have more than three dimensions. At the same time, additional dimensions at ordinary distances do not manifest themselves, but at cosmological distances of billions of light years, the force lines of the gravitational field can “spread” into additional dimensions, which is why gravity will no longer be described by the usual Newton’s law. A completely satisfactory theory that explains the accelerated expansion of the Universe in this way has not yet been constructed; In the models proposed to date, this idea is only partially implemented. It is remarkable, however, that these models lead to their predictions for experiment. Among them is the possibility of changing Newton's gravitational law to small distances; small but detectable corrections to the general theory of relativity in the solar system, etc.

So, the recently discovered features of the expansion of the Universe have raised a new question: are they caused by vacuum energy, the energy of a new light field, or new gravity at extremely large distances? Theoretical study of these possibilities is in full swing, and the answer, as usual in physics, must ultimately be provided by new experiments.

Dark energy and the future of the Universe

With the discovery of dark energy, ideas about what the distant future of our Universe might be like have changed dramatically. Before this discovery, the question of the future was clearly associated with the question of the curvature of three-dimensional space. If, as many previously believed, the curvature of space determined 70% of the current rate of expansion of the Universe, and there was no dark energy, then the Universe would expand without limit, gradually slowing down. Now it is clear that the future is determined by the properties of dark energy.

Since we know these properties poorly now, we cannot yet predict the future. One can only consider different variants. It is difficult to say what is happening in theories with new gravity, but other scenarios can be discussed now.

If dark energy is constant over time, as is the case with vacuum energy, then the Universe will always experience accelerated expansion. Most galaxies will eventually move away from ours to an enormous distance, and our Galaxy, along with its few neighbors, will turn out to be an island in the void. If dark energy is quintessential, then in the distant future the accelerated expansion may stop and even be replaced by compression. In the latter case, the Universe will return to a state with hot and dense matter, a “Big Bang in reverse” will occur, back in time.

An even more dramatic fate awaits the Universe if dark energy settles - a phantom, such that its energy density increases without limit. The expansion of the Universe will become more and more rapid, it will accelerate so much that galaxies will be torn out of clusters, stars from galaxies, planets from the solar system. It will come to the point that electrons will break away from atoms, and atomic nuclei will split into protons and neutrons. There will be, as they say, a Big Rip.

Such a scenario, however, does not seem very likely. Most likely, the phantom's energy density will remain limited. But even then, the Universe may face an unusual future. The fact is that in many theories, phantom behavior - an increase in energy density over time - is accompanied by instabilities of the phantom field. In this case, the phantom field in the Universe will become highly inhomogeneous, its energy density in different parts of the Universe will be different, some parts will rapidly expand, and some may experience collapse. The fate of our Galaxy will depend on which region it falls into.

All this, however, relates to the future, distant even by cosmological standards. In the next 20 billion years, the Universe will remain almost the same as it is now. We have time to understand the properties of dark energy and thereby more definitely predict the future - and perhaps influence it.

• Cosmological models
  • Big Bang
  • Friedman's Universe
• Timeline of cosmology

There are two options for explaining the essence of dark energy:

To date (2012), all known reliable observational data do not contradict the first hypothesis, so it is accepted in cosmology as standard. The final choice between the two options requires high-precision measurements of the rate of expansion of the Universe to understand how this rate changes over time. The expansion rate of the Universe is described by the cosmological equation of state. Resolving the equation of state for dark energy is one of the most pressing problems in modern observational cosmology.

Dark energy should also make up a significant portion of the so-called hidden mass of the Universe.

Discovery of dark energy

Based on observations of Type Ia supernovae carried out in the late 1990s, it was concluded that the expansion of the Universe is accelerating over time. These observations were then supported by other sources: measurements of the CMB, gravitational lensing, Big Bang nucleosynthesis. All obtained data fits well into the lambda-CDM model.

Supernovae and the Accelerating Universe

The cosmological constant has a negative pressure equal to its energy density. The reasons why the cosmological constant has a negative pressure follow from classical thermodynamics. The amount of energy contained in a “vacuum box” of volume V, equals ρV, Where ρ - energy density of the cosmological constant. Increasing the volume of the “box” ( dV positive) leads to its increase internal energy, and this means doing negative work. Since the work done by a change in volume dV, equals pdV, Where p- pressure, then p negative and, in fact, p = −ρ(coefficient c², connecting mass and energy, is equal to 1).

The most important unsolved problem of modern physics is that most quantum field theories, based on the energy of the quantum vacuum, predict a huge value of the cosmological constant - many orders of magnitude greater than what is admissible according to cosmological concepts. The usual formula of quantum field theory for the summation of vacuum zero-point oscillations of the field (with a cutoff at the wave number of vibrational modes corresponding to the Planck length) gives a huge vacuum energy density. This value, therefore, must be compensated by some action that is almost equal (but not exactly equal) in magnitude, but has the opposite sign. Some theories of supersymmetry (SATHISH) require that the cosmological constant be exactly zero, which also does not help resolve the problem. This is the essence of the “cosmological constant problem”, the most difficult “fine-tuning” problem in modern physics: not a single way has been found to derive from particle physics the extremely small value of the cosmological constant defined in cosmology. Some physicists, including Steven Weinberg, believe the so-called. The “anthropic principle” is the best explanation for the observed fine balance of energy in the quantum vacuum.

Despite these problems, the cosmological constant is in many ways the most parsimonious solution to the problem of an accelerating Universe. A single numerical value explains many observations. Therefore, the current generally accepted cosmological model (lambda-CDM model) includes the cosmological constant as an essential element.

Quintessence

An alternative approach was proposed in 1987 by German theoretical physicist Christoph Wetterich. Wetterich proceeded from the assumption that dark energy is a kind of particle-like excitation of a certain dynamic scalar field called quintessence. The difference from the cosmological constant is that the density of quintessence can vary in space and time. To prevent quintessence from “assembling” and forming large-scale structures following the example of ordinary matter (stars, etc.), it must be very light, that is, have a large Compton wavelength.

No evidence of the existence of quintessence has yet been discovered, but such existence cannot be ruled out. The quintessence hypothesis predicts a slightly slower acceleration of the Universe compared to the cosmological constant hypothesis. Some scientists believe that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variations of fundamental constants in space or time. The existence of scalar fields is predicted by the standard model and string theory, but it poses a problem similar to the cosmological constant variant: renormalization theory predicts that scalar fields should acquire significant mass.

The problem of cosmic coincidence raises the question of why the acceleration of the Universe began at a particular point in time. If the acceleration in the Universe began before this moment, stars and galaxies simply would not have time to form, and life would have no chance of arising, at least in the form we know. Proponents of the “anthropic principle” consider this fact to be the best argument in favor of their constructions. However, many quintessence models include so-called “tracking behavior”, which solves this problem. In these models, the quintessence field has a density that adjusts to the radiation density (without reaching it) until the moment of development of the Big Bang, when an equilibrium of matter and radiation is achieved. After this point, the quintessence begins to behave like the sought-after “dark energy” and eventually dominates the Universe. This development naturally sets dark energy levels low.

On the other hand, dark energy may dissipate over time or even change its repulsive effect to an attractive one. In this case, gravity will prevail and lead the Universe to the “Big Crunch”. Some scenarios assume a "cyclical model" of the Universe. Although these hypotheses have not yet been confirmed by observations, they are not completely rejected. Accurate measurements of the rate of acceleration must play a decisive role in establishing the ultimate fate of the Universe (developing according to the Big Bang theory).

The accelerated expansion of the Universe was discovered in 1998 from observations of Type Ia supernovae. For this discovery, Saul Perlmutter, Brian P. Schmidt and Adam Riess received the 2006 Shao Prize in Astronomy and the 2011 Nobel Prize in Physics.

see also

Notes

Links

• Dark energy near us - a popular brochure, A. D. Chernina, SAI MSU.
• A.D. Chernin: Physical vacuum and cosmic anti-gravity
• Documentary - Dark Matter, Dark Energy (2008)
• A.D. Chernin. Dark energy and universal antigravity. // UFN, 178 , 267 (2008).
• V. N. Lukash, V. A. Rubakov. Dark energy: myths and reality. // UFN, 178 , 301 (2008). (Commentary to the article by A. D. Chernin)
• Robert R. Caldwell, Marc Kamionkowski, Nevin N. Weinberg, Phantom Energy and Cosmic Doomsday (astro-ph:0302506)
• Mark Trodden, Jonathan Fan. Dark Worlds

Wikimedia Foundation.

2010.

See what “Dark Energy” is in other dictionaries: DARK ENERGY - (TE) strange energy of the non-baryonic world (see), present in our Universe and manifested in the form of antigravity, the ability to “push off” from ordinary matter. As a result of numerous (500,000 from 1995 to 2005) observations from ...