(ORDO NEWS) — It sounds like science fiction to say that there are invisible, undetectable things around us, and that it has a creepy name for dark matter. But there is a lot of evidence that this stuff is very real. So what is dark matter? How do we know it’s there? And how are scientists looking for it?
Everything that we see around us – from plants to planets, from stones to stars, from people to the Perseus cluster of galaxies – is made of matter. But all this makes up only about 15 percent of the total amount of matter in the universe. The vast majority, that is, the remaining 85 percent, are not counted – and we call it dark matter.
This is the name of the complete opposite of ordinary matter. It is made up of particles, and antimatter is made up of antiparticles. A particle and its antiparticle have the same mass, but their individual characteristics will be different. For example, the magnetic moment, charge, spin, the number of leptons and baryons will have opposite values.
The existence of antimatter in the modern sense was predicted back in 1928. Assumptions belonged to Paul Dirac. According to the theory put forward by him, it is possible the existence of particles with an equal mass of an electron, but an opposite charge of equal value. In 1932, Karl D. Anderson was able to prove this hypothesis. He discovered the existence of a positron, it is a twin of an electron or an anti-electron, a part of antimatter. The scientist made this discovery when he was studying cosmic rays.
So the first antiparticle was discovered, and it became a breakthrough for physics. If we start from the standard model, then every particle of ordinary matter has an analogue, and it is represented by an antiparticle. That’s not all, the quark also has its analogue. For example, for a neutron, a proton and an electron, such an analogue will be an antineutron, an antiproton and a positron, respectively.
There are the simplest anti-atoms, the simplest of which is antihydrogen, which contains a positron and an antiproton. Scientists are trying to create anti-cores that will be heavier than antihelium, but they still have not been able to do this. Despite the fact that at the moment it is practically impossible, if we proceed from the laws of physics, it is possible.
In theory, the interaction of antimatter occurs in four forms: electromagnetic, gravitational, strong nuclear and weak. Antimatter is also capable of bending time-space in the same way as ordinary matter does.
About dark matter
In fact, it was not possible to detect dark matter, as it happened with antimatter. However, quite convincing evidence of its existence has been discovered. Long-term observations have allowed scientists to conclude that there must be more dark matter than there is in our universe.
Scientists are using spiral galaxies as ancillary proof that dark matter exists. With what speed such a galaxy will rotate depends on its mass, they increase in direct proportion. Most spiral galaxies, including our Milky Way, are rotating faster than originally thought. It turns out that their mass should be higher than that observed by experts. The difference is represented by absent or unobservable matter, theoretically it can be attributed to dark matter.
In accordance with the theories from which modern science repels, dark matter can interact only through weak and gravitational interactions. The gravitational influence definitely exists, it is noticeable. However, dark matter cannot be seen and is therefore difficult to access and very difficult to detect. Everything would be different if it could produce electromagnetic and strong interactions.
What is the difference?
Antimatter is not only discovered, but can also be synthesized at the will of a person, this happens due to the collision of high-energy charged particles. Moreover, mankind has managed to artificially create antihelium and antihydrogen. Dark matter is not observable. Despite the fact that there is strong evidence of its existence, at the moment it only exists in theory.
If there are other key differences between dark matter and antimatter. As for antimatter, which was formed in the same amount as ordinary matter after the big bang, today we practically do not observe it. No one knows where it went, but there is very little of it in the universe. There is much more dark matter than ordinary matter, but at the moment we only know it by counting.
So what is dark matter?
First, and perhaps most surprisingly, researchers are still unsure of what exactly dark matter is.
Initially, some scientists assumed that the missing mass in the Universe consisted of small faint stars and black holes, although detailed observations did not reveal enough such objects to explain the influence of dark matter, as Don Lincoln wrote about this.
Currently, the main contender for the role of dark matter is a hypothetical particle called a weakly interacting massive particle, or WIMP, which would behave like a neutron, but would be 10-100 times heavier than a proton, as Lincoln wrote. However, this hypothesis only led to more questions.
Who Discovered Dark Matter?
The first hint of its existence is the performances of Giordano Bruno. He intuitively believed that there are “countless and endless” cosmic bodies. It was about objects that escaped from the universal system of Copernicus. Simple stars that are not visible to us. As for the planets, he believed that they were not visible, since they glow very faintly in the reflected rays of their own luminaries.
92 years ago, British physicist James Jeans and Dutch astronomer Jacobus Catein discovered that much of the galactic matter is invisible. It was then that this term was first heard.
And the weight of dark matter was determined by astrophysicists – forty or more gigaelectronvolts (GeV).
Can we detect dark matter?
If dark matter is composed of wimps, they should be all around us, invisible and subtle. So why haven’t they been found yet? While they won’t interact strongly with ordinary matter, there is always a small chance that a dark matter particle could collide with a normal particle such as a proton or electron as it travels through space.
Thus, the researchers conducted experiment after experiment to study the vast numbers of ordinary particles deep underground, where they are shielded from interfering radiation that could mimic the collision of dark matter particles.
What’s the problem? After decades of searching, none of these detectors have made a reliable discovery. Earlier this year, a Chinese experiment, PandaX, reported that they were never able to detect wimps. It is likely that dark matter particles are much smaller than WIMPs, or lack properties that would make them easier to study, said physicist Hi-Bo Yu of the University of California, Riverside.
2. Expanding Universe
There are a number of facts that speak about the properties of the Universe today and in the relatively recent past.
The universe as a whole is homogeneous: all regions in the universe look the same. Of course, this does not apply to small areas: there are areas where many stars are galaxies; there are areas where there are many galaxies – these are clusters of galaxies; there are also regions where there are few galaxies – these are giant voids. But areas 300 million light years and more all look the same. This is unambiguously evidenced by astronomical observations, as a result of which a “map” of the Universe has been drawn up to distances of about 10 billion light years from us 1. It must be said that this “map” serves as a source of the most valuable information about the modern Universe, since it allows one to quantitatively determine exactly how matter is distributed in the Universe.
In fig. 2 shows a fragment of this map 2, covering a relatively small volume of the universe. It can be seen that there are structures of a rather large size in the Universe, but in general the galaxies are “scattered” in it uniformly.
The universe is expanding: galaxies are moving away from each other. Space is stretched in all directions, and the further away from us this or that galaxy is, the faster it moves away from us. Today the rate of this expansion is not high: all distances will double 3 in about 15 billion years, but the rate of expansion was much faster in the past. The density of matter in the Universe decreases over time, and in the future the Universe will be more and more rarefied. On the contrary, the Universe used to be much denser than it is now. The expansion of the Universe is directly evidenced by the “reddening” of light emitted by distant galaxies or bright stars: due to the general stretching of space, the wavelength of light increases during the time it flies towards us. It was this phenomenon that was established by E.
It is remarkable that modern observational data make it possible to measure not only the rate of expansion of the Universe at the present time, but also to trace the rate of its expansion in the past. We will talk about the results of these measurements and the far-reaching conclusions that follow from them. Here we will say about the following: the very fact of the expansion of the Universe, together with the theory of gravity – the general theory of relativity – indicates that in the past the Universe was extremely dense and was expanding extremely rapidly. If we trace the evolution of the Universe back to the past, using the known laws of physics, then we come to the conclusion that this evolution began from the moment of the Big Bang; at that moment, the matter in the Universe was so dense, and the gravitational interaction was so strong that the known laws of physics were inapplicable. 14 billion years have passed since then,
The Universe is “warm”: it contains electromagnetic radiation, characterized by a temperature of T = 2.725 degrees Kelvin (relict photons, which today are radio waves). Of course, this temperature is not high today (below the temperature of liquid helium), but this was far from the case in the past. In the process of expansion, the Universe cools down, so that in the early stages of its evolution, the temperature, like the density of matter, was much higher than today. In the past, the universe was hot, dense, and rapidly expanding.
3 Of course, this does not apply to the distance from the Earth to the Sun or the distance between stars in the Galaxy: the Earth is held near the Sun by gravitational forces, and the distance from it to the Sun does not change due to the expansion of the Universe.
3. The universe in the past
Let us discuss two stages in the evolution of the Universe, about which reliable observational data are available today. One of them, relatively recent, is the stage of transition of matter in the Universe from a plasma state to a gaseous state. This happened at a temperature of 3000 degrees, and the age of the Universe at that time was 300 thousand years (quite a bit compared to the modern 14 billion years). Before that, electrons and protons moved separately from each other, the substance was a plasma. At a temperature of 3000 degrees, electrons and protons merged into hydrogen atoms, and the Universe was filled with this gas. It is important that the plasma is opaque to electromagnetic radiation; photons are all the time emitted, absorbed, scattered by the plasma electrons. Gas, on the other hand, is transparent. This means that the electromagnetic radiation that has come to us with a temperature of 2, 7 degrees freely traveled in the Universe from the moment of the plasma-gas transition, having cooled down (reddened) since then 1100 times due to the expansion of the Universe. This relict electromagnetic radiation has retained information about the state of the Universe at the moment of the plasma-gas transition; with its help we have a photograph (literally!) of the Universe at the age of 300 thousand years, when its temperature was 3000 degrees.
By measuring the temperature of this relict electromagnetic radiation, which came to us from different directions in the sky, we find out which regions were warmer or colder (and therefore denser or thinner) than the average in the Universe, and most importantly – how much warmer or colder they were. The result of these measurements is that the Universe at the age of 300 thousand years was much more homogeneous than it is today: the variations in temperature and density were then less than 10–4 (0.01%) of the average values. Nevertheless, these variations existed: from different directions, electromagnetic radiation comes with slightly different temperatures. This is shown in Fig. 3, which shows the temperature distribution over the celestial sphere (a photograph of the early Universe) minus an average temperature of 2.725 degrees Kelvin; colder areas are shown in blue.
First, he made it possible to establish that our three-dimensional space is Euclidean with a good degree of accuracy: the sum of the angles of a triangle in it is 180 degrees even for triangles with sides, the lengths of which are comparable to the size of the visible part of the Universe, i.e., comparable to 14 billion light years. Generally speaking, general relativity assumes that space may not be Euclidean, but curved; observational data indicate that this is not the case (at least for our region of the Universe). The method for measuring the “sum of the angles of a triangle” on cosmological distance scales is as follows. It is possible to reliably calculate the characteristic spatial size of the regions where the temperature differs from the average: at the time of the plasma-gas transition, this size is determined by the age of the Universe, that is, it is proportional to 300 thousand light years.
In the case of the Euclidean geometry of three-dimensional space, the general theory of relativity unambiguously links the rate of expansion of the Universe with the total density of all forms of energy, just as in the Newtonian theory of gravitation the rate of revolution of the Earth around the Sun is determined by the mass of the Sun. The measured expansion rate corresponds to the total energy density in the modern Universe.
In terms of mass density (since energy is related to mass by E = mc2), this number is.
If the energy in the Universe was entirely determined by the rest energy of ordinary matter, then on average there would be 5 protons in the Universe per cubic meter. We will see, however, that ordinary matter in the universe.
it is possible to establish what was the magnitude (amplitude) of the temperature and density inhomogeneities in the early Universe – it was 10–4–10–5 of the average values. It was from these density irregularities that galaxies and galaxy clusters arose: regions with a higher density attracted the surrounding matter due to gravitational forces, became even denser and eventually formed galaxies.
Since the initial density inhomogeneities are known, the process of galaxy formation can be calculated and the result can be compared with the observed distribution of galaxies in the Universe. This calculation is consistent with observations only if we assume that in addition to ordinary matter in the Universe there is another type of matter – dark matter, whose contribution to the total energy density today is about 25%.
Another stage in the evolution of the Universe corresponds to even earlier times, from 1 to 200 seconds (!) From the moment of the Big Bang, when the temperature of the Universe reached billions of degrees. At this time, thermonuclear reactions were taking place in the Universe, similar to the reactions taking place in the center of the Sun or in a thermonuclear bomb. As a result of these reactions, part of the protons bound with neutrons and formed light nuclei – the nuclei of helium, deuterium and lithium-7. The number of light nuclei formed can be calculated, with the only unknown parameter being the density of the number of protons in the Universe (the latter, of course, decreases due to the expansion of the Universe, but its values at different times are simply related to each other).
A comparison of this calculation with the observed amount of light elements in the Universe is shown in Fig. 4: the lines represent the results of the theoretical calculation depending on the only parameter – the density of ordinary matter (baryons), and the rectangles represent the observational data. Remarkably, there is agreement for all three light nuclei (helium-4, deuterium, and lithium-7); agreement is also with the data on the relic radiation (shown by the vertical stripe in Fig. 4, designated CMB – Cosmic Microwave Background). This agreement indicates that the general theory of relativity and the well-known laws of nuclear physics correctly describe the Universe at the age of 1–200 seconds, when the matter in it had a temperature of a billion degrees or more. It is important for us that all these data lead to the conclusion that is, ordinary matter contributes only 5% to the total energy density in the Universe.
4. The balance of energies in the modern universe
So, the share of ordinary matter (protons, atomic nuclei, electrons) in the total energy in the modern Universe is 5 only 5%. In addition to ordinary matter in the Universe, there are also relic neutrinos – about 300 neutrinos of all types per cubic centimeter. Their contribution to the total energy (mass) in the Universe is small, since the masses of neutrinos are small, and certainly amounts to no more than 3%. The remaining 90–95% of the total energy in the Universe is “no one knows what.”
5 In this case, the matter in the stars is still 10 times less; common matter is found mainly in clouds of gas.
5. Dark matter
Dark matter is akin to ordinary matter in the sense that it is capable of gathering into clumps (the size of, say, a galaxy or a cluster of galaxies) and participates in gravitational interactions in the same way as ordinary matter. Most likely, it consists of new particles not yet discovered in terrestrial conditions.
In addition to cosmological data, measurements of the gravitational field in galaxy clusters and in galaxies are in favor of the existence of dark matter. There are several ways to measure the gravitational field in galaxy clusters, one of which is gravitational lensing.
The gravitational field of the cluster bends the rays of light emitted by the galaxy behind the cluster, that is, the gravitational field acts as a lens. At the same time, several images of this distant galaxy sometimes appear; on the left half of fig. 6 they are blue. The curvature of light depends on the distribution of mass in the cluster, regardless of which particles create this mass. The distribution of mass restored in this way is shown in the right half of Fig. 6 in blue; it can be seen that it is very different from the distribution of the luminous substance. Measured in this way, the masses of galaxy clusters are consistent with the fact that dark matter contributes about 25% of the total energy density in the Universe. Recall that the same number is obtained from comparing the theory of the formation of structures (galaxies, clusters) with observations.
Dark matter is also found in galaxies. This again follows from measurements of the gravitational field, now in galaxies and their surroundings. The stronger the gravitational field, the faster the stars and clouds of gas revolve around the galaxy, so that measurements of the rotation rates depending on the distance to the center of the galaxy make it possible to reconstruct the mass distribution in it. This is illustrated in Fig. 7: with distance from the center of the galaxy, the orbital velocities do not decrease, which indicates that there is non-luminous, dark matter in the galaxy, including far from its luminous part. In our Galaxy, in the vicinity of the Sun, the mass of dark matter is approximately equal to the mass of ordinary matter.
What are dark matter particles? It is clear that these particles should not decay into other, lighter particles, otherwise they would decay during the existence of the Universe. This fact itself testifies to the fact that in nature there is a new, not yet discovered conservation law, which prohibits these particles from decaying. The analogy here is with the law of conservation of electric charge: an electron is the lightest particle with an electric charge, and that is why it does not decay into lighter particles (for example, neutrinos and photons). Further, dark matter particles interact extremely weakly with our matter, otherwise they would have already been detected in terrestrial experiments. Then the area of hypotheses begins. The most plausible (but far from the only!) Hypothesis seems to be that dark matter particles are 100-1000 times heavier than a proton, and that their interaction with ordinary matter is comparable in intensity to that of neutrinos. It is within the framework of this hypothesis that the modern density of dark matter finds a simple explanation: dark matter particles were intensively born and annihilated in the very early Universe at ultra-high temperatures (about 1015 degrees), and some of them have survived to this day. With the specified parameters of these particles, their present number in the Universe is exactly what is needed.
Can we expect the discovery of dark matter particles in the near future under terrestrial conditions? Since today we do not know the nature of these particles, it is impossible to answer this question completely unambiguously. Nevertheless, the outlook appears to be very optimistic.
There are several ways to search for dark matter particles. One of them is associated with experiments at future high-energy accelerators – colliders. If dark matter particles are indeed 100–1000 times heavier than a proton, then they will be produced in collisions of ordinary particles accelerated at colliders to high energies (the energies achieved at existing colliders are not enough for this). The immediate prospects here are associated with the Large Hadron Collider (LHC) under construction at the CERN international center near Geneva, which will receive colliding beams of protons with an energy of 7×7 Teraelectronvolts. It must be said that according to hypotheses popular today, dark matter particles are only one representative of a new family of elementary particles, so that along with the discovery of dark matter particles, one can hope for the discovery of a whole class of new particles and new interactions at accelerators. Cosmology suggests that the world of elementary particles is far from being exhausted by the known today “bricks”!
Another way is to register dark matter particles that fly around us. There are by no means a few of them: with a mass equal to 1000 masses of a proton, here and now there should be 1000 of these particles in a cubic meter. The problem is that they interact extremely weakly with ordinary particles, the substance is transparent to them. Nevertheless, dark matter particles occasionally collide with atomic nuclei, and these collisions can hopefully be registered.
Finally, one more way is associated with the registration of the products of annihilation of dark matter particles among themselves. These particles should accumulate in the center of the Earth and in the center of the Sun (the substance is practically transparent for them, and they are capable of falling into the Earth or the Sun). There they annihilate with each other, and other particles, including neutrinos, are formed. These neutrinos freely pass through the thickness of the Earth or the Sun, and can be registered by special installations – neutrino telescopes. One of these neutrino telescopes is located in the depths of Lake Baikal, the other (AMANDA) – deep in the ice at the South Pole.
to experience interaction in water, as a result of which a charged particle (muon) is formed, the light from which is registered. Since the interaction of neutrinos with matter is very weak, the probability of such an event is small, and very large detectors are required. The construction of a 1 cubic kilometer detector has now begun at the South Pole.
There are other approaches to the search for dark matter particles, for example, the search for the products of their annihilation in the central region of our Galaxy. Time will show which of all these paths will lead to success first, but in any case, the discovery of these new particles and the study of their properties will be a major scientific achievement. These particles will tell us about the properties of the Universe 10-9 s (one billionth of a second!) After the Big Bang, when the temperature of the Universe was 1015 degrees, and dark matter particles intensively interacted with the cosmic plasma.
6. Dark energy
Dark energy is a much stranger substance than dark matter. To begin with, it does not gather in clumps, but is evenly “spilled” in the Universe. There is as much of it in galaxies and galaxy clusters as outside them. The most unusual thing is that dark energy, in a sense, experiences anti-gravity. We have already said that modern astronomical methods can not only measure the current rate of expansion of the Universe, but also determine how it has changed over time. So, astronomical observations 6 indicate that today (and in the recent past) the Universe is expanding with acceleration: the rate of expansion grows with time. In this sense, we can talk about antigravity: the usual gravitational attraction would slow down the scattering of galaxies, but in our Universe, it turns out, the opposite is true.
Such a picture, generally speaking, does not contradict the general theory of relativity, but for this dark energy must have a special property – negative pressure. This sharply distinguishes it from ordinary forms of matter. It would not be an exaggeration to say that the nature of dark energy is the main mystery of fundamental physics in the 21st century.
One of the candidates for the role of dark energy is vacuum. The energy and vacuum density does not change with the expansion of the Universe, and this means the negative pressure of the vacuum 7. Another candidate is a new superweak field that permeates the entire Universe; the term “quintessence” is used for it. There are other candidates, but in any case, dark energy is something completely out of the ordinary.
Another way of explaining the accelerated expansion of the Universe is to assume that the laws of gravity themselves change over cosmological distances and cosmological times. This hypothesis is far from harmless: attempts to generalize the general theory of relativity in this direction face serious difficulties.
Apparently, if such a generalization is at all possible, then it will be associated with the idea of the existence of additional dimensions of space, in addition to the three dimensions that we perceive in everyday experience.
Unfortunately, at present there are no visible ways of direct experimental investigation of dark energy under terrestrial conditions. This, of course, does not mean that new brilliant ideas in this direction cannot appear in the future, but today hopes for clarification of the nature of dark energy (or, more broadly, the reasons for the accelerated expansion of the Universe) are associated exclusively with astronomical observations and with obtaining new, more accurate cosmological data. We have to find out in detail how exactly the Universe expanded at a relatively late stage of its evolution, and this, hopefully, will allow us to make a choice between different hypotheses.
6 We are talking about observations of type 1a supernovae.
7 The change in energy with a change in volume is determined by pressure, ΔE = —pΔV. With the expansion of the Universe, the energy of the vacuum grows with the volume (the energy density is constant), which is possible only if the pressure of the vacuum is negative. Note that the opposite signs of pressure and vacuum energy directly follow from Lorentz invariance.
Is dark matter composed of more than one particle?
Ordinary matter is made up of particles like protons and electrons, as well as more more unusual particles like neutrinos, muons and pions. As a result, some researchers are asking the question: “Could dark matter, which makes up 85 percent of the matter in the universe, be as complex?”
“There is no compelling reason to believe that all dark matter in the universe is built from one type of particle. Dark protons can combine with dark electrons to form dark atoms and create configurations as diverse and interesting as those found in the visible world, ”said physicist Andrei Katz from Harvard University.
While such assumptions were increasingly presented in physics laboratories, figuring out how to confirm or refute them has still eluded scientists.
Do dark photons exist?
Along with additional dark matter particles, there is the possibility that dark matter experiences forces similar to those experienced by ordinary matter.
Some researchers were looking for “dark photons” that would be like the photons that normal particles exchange and cause electromagnetic force.
If dark photons do exist, electron-positron pairs could annihilate and produce one of the unknown particles, potentially opening up a whole new part of the universe.
Could dark matter be composed of axions?
As physicists lose their love for WIMPs more and more, other dark matter particles begin to gain their interest. One of the main replacements is a hypothetical particle known as an axion (currently being searched for in several experiments), which would be extremely light, possibly much smaller than a proton.
Recent computer simulations have raised the likelihood that these axions could form star-like objects capable of producing detectable radiation that closely resembles fast radio bursts.
What are the properties of dark matter?
Astronomers discovered dark matter through its gravitational interactions with ordinary matter, suggesting that this is its primary way of claiming its existence in the universe. But when trying to understand the true nature of dark matter, researchers are stumped.
According to some theories, dark matter particles must be their own antiparticles, which means that two dark matter particles will annihilate each other when they meet.
The experiment with a magnetic alpha spectrometer on the International Space Station has been searching for characteristic signs of this annihilation since 2011 and has already recorded hundreds of thousands of events. Scientists are still not sure if they come from dark matter. Unfortunately, the signal has yet to help them pinpoint what dark matter is.
Does dark matter exist in every galaxy?
Because dark matter is vastly superior to ordinary matter, it is often referred to as the governing force that forms large structures such as galaxies and galaxy clusters.
So it came as a surprise when astronomers announced earlier this year that they had discovered a galaxy called NGC 1052-DF2 that seemed to contain no dark matter at all.
“Dark matter does not appear to be a prerequisite for galaxy formation,” said Peter van Dokkum of Yale University.
However, in the summer, a separate team published an analysis suggesting that van Dokkum’s team had incorrectly measured the distance to the galaxy, meaning that its visible matter was much dimmer and lighter than the first finds, and that most of its mass was in dark matter than previously thought.
What is known about anti-nuclei?
All antiatomic achievements of mankind refer only to antihydrogen. Antiatoms of other elements have not yet been synthesized in the laboratory and have not been observed in nature. The reason is simple: antinuclei are even more difficult to create than antiprotons.
The only way we know to create anti-nuclei is to collide heavy nuclei of high energies and see what happens there. If the collision energy is high, thousands of particles, including antiprotons and antineutrons, will be born and scatter in all directions. Antiprotons and antineutrons, accidentally flying out in the same direction, can combine with each other – you get an anti-nucleus.
The ALICE detector is able to distinguish between different nuclei and antinuclei in terms of energy release and the direction of swirling in a magnetic field.
The method is simple, but not too ineffective: the probability of synthesizing a nucleus in this way drops sharply with an increase in the number of nucleons. The lightest anti-nuclei, antideuterons, were first observed exactly half a century ago. Antihelium-3 was seen in 1971. Antitriton and antihelium-4 are also known, the latter being discovered quite recently, in 2011. Heavier antinuclei have not yet been observed.
Two parameters describing nucleon-nucleon interactions (scattering length f0 and effective radius d0) for different pairs of particles. Red asterisk – the result for a pair of antiprotons obtained by the STAR collaboration
Unfortunately, you can’t make antiatoms this way. Anti-nuclei are not only rarely born, but also have too much energy and fly out in all directions. It is unrealistic to try to catch them at the collider and then take them out through a special channel and cool them down.
However, sometimes it is enough to carefully track the anti-nuclei on the fly to get some interesting information about the anti-nuclear forces acting between the antinucleons. The simplest thing is to accurately measure the mass of anti-nuclei, compare it with the sum of the masses of antiprotons and antineutrons, and calculate the mass defect, i.e. the binding energy of the nucleus. This was done recently by the ALICE experiment at the Large Hadron Collider; the binding energy for antideuteron and antihelium-3 coincided within the error with ordinary nuclei.
Another, more subtle effect was studied by the STAR experiment at the American heavy ion collider RHIC. He measured the angular distribution of the antiprotons produced and found out how it changes when two antiprotons are emitted in a very close direction. Correlations between antiprotons made it possible for the first time to measure the properties of the “anti-nuclear” forces acting between them (scattering length and effective interaction radius); they coincided with what is known about the interaction of protons.
Is there antimatter in space?
When Paul Dirac deduced the existence of positrons from his theory, he fully admitted that real antiworlds could exist somewhere in space. Now we know that there are no stars, planets, galaxies from antimatter in the visible part of the Universe. The point is not even that the annihilation explosions are not visible; it is simply completely inconceivable how they could have formed at all and survive to the present time in a constantly evolving universe.
But the question “how did it happen” is another tremendous mystery of modern physics; in scientific parlance, it is called the problem of baryogenesis. According to the cosmological picture of the world, in the earliest universe, particles and antiparticles were equally divided. Then, due to violation of CP-symmetry and baryon number, a small, at the level of one billionth, excess of matter over antimatter should have appeared in the dynamically developing universe. When the universe cooled down, all antiparticles were annealed with particles, only this excess of matter survived, which gave rise to the universe that we are observing. It is because of him that at least something interesting remained in her, it is thanks to him that we generally exist. How exactly this asymmetry arose is unknown. There are many theories, but which one is correct is unknown. It is only clear that it must definitely be some kind of New physics.
Although there are no planets and stars from antimatter, there is still antimatter in space. Fluxes of positrons and antiprotons of different energies are recorded by satellite observatories of cosmic rays, such as PAMELA, Fermi, AMS-02. The fact that positrons and antiprotons come to us from space means that they are born somewhere there. The high-energy processes that can generate them are known in principle: these are highly magnetized neighborhoods of neutron stars, various explosions, acceleration of cosmic rays at the fronts of shock waves in the interstellar medium, etc. The question is whether they can explain all the observed properties of the flux of cosmic antiparticles. If it turns out that not, this will be evidence in favor of the fact that some of their fraction arises from the decay or annihilation of dark matter particles.
Here, too, there is a mystery. In 2008, the PAMELA observatory detected a suspiciously large number of high-energy positrons compared to what theoretical simulations predicted. These results were long ago confirmed by the AMS-02 installation – one of the modules of the International Space Station and, in general, the largest particle detector launched into space (and assembled, guess where? – right, at CERN). This excess of positrons excites the mind of theorists – after all, it is not “boring” astrophysical objects that may be responsible for it, but heavy particles of dark matter, which decay or annihilate into electrons and positrons. There is no clarity here yet, but the AMS-02 setup, as well as many critical physicists, are studying this phenomenon very carefully.
The ratio of antiprotons to protons in cosmic rays of different energies. Points are experimental data, multicolored curves are astrophysical expectations with various errors.
The situation with antiprotons is also unclear. In April this year, AMS-02 presented preliminary results of a new research cycle at a special scientific conference. The main highlight of the report was the statement that AMS-02 sees too many high-energy antiprotons – and this may also be a hint of decay of dark matter particles. However, other physicists disagree with such a cheerful conclusion. It is now believed that the antiproton data of AMS-02, with some stretch, can be explained by ordinary astrophysical sources. One way or another, everyone is eagerly awaiting the new positron and antiproton data from AMS-02.
AMS-02 has already registered millions of positrons and a quarter of a million antiprotons. But the creators of this setup have a bright dream – to catch at least one anti-nucleus. This will be a real sensation – it is absolutely incredible that anti-nuclei were born somewhere in space and would fly to us. So far, no such case has been found, but the dataset continues, and who knows what surprises nature has in store for us.
Antimatter – antigravity? How does she even feel gravity?
If you rely only on experimentally tested physics and do not go into exotic, not yet confirmed theories, then gravity should act on antimatter in the same way as on matter. No antigravity is expected for antimatter. If we allow ourselves to look a little further, beyond the limits of the known, then purely theoretically possible variants are possible when there is something additional to the load to the usual universal gravitational force, which acts differently on matter and antimatter. No matter how ghostly this possibility may seem, it needs to be tested experimentally, and for this it is necessary to carry out experiments to test how antimatter feels gravity.
For a long time, it really was not possible to do this for the simple reason that for this it is necessary to create individual atoms of antimatter, trap them, and conduct experiments with them. Now they have learned how to do this, so the long-awaited test is just around the corner.
The main supplier of the results is the same CERN with its extensive program for the study of antimatter. Some of these experiments have already indirectly verified that gravity is fine with antimatter. For example, a recent BASE experiment found that the (inert) mass of an antiproton matches the mass of a proton with very high precision. If gravity acted on antiprotons somehow differently, physicists would notice the difference – after all, the comparison was made in the same setup and under the same conditions. The result of this experiment: the effect of gravity on antiprotons coincides with the effect on protons with an accuracy of better than one millionth.
However, this measurement is indirect. To make it more convincing, I would like to conduct a direct experiment: take several atoms of antimatter, drop them and see how they will fall in the field of gravity. Such experiments are also carried out or are being prepared at CERN. The first attempt was not very impressive. In 2013, the ALPHA experiment – which by then had already learned to keep a cloud of antihydrogen in its trap – tried to determine where the antiatoms would fall if the trap was turned off. Alas, due to the low sensitivity of the experiment, it was not possible to get an unambiguous answer: too little time had passed, antiatoms were rushing back and forth in a trap, and outbreaks of annihilation occurred here and there.
The situation is promised to radically improve two other CERN experiments: GBAR and AEGIS. Both of these experiments will test in different ways how a cloud of supercold antihydrogen falls in the gravitational field. Their expected accuracy in measuring the acceleration of gravity for antimatter is about 1%. Both facilities are currently under assembly and debugging, and the main research will begin in 2017, when the antiproton moderator AD will be supplemented with a new storage ring ELENA.
Variants of the behavior of a positron in a solid.
What happens if a positron gets into a substance?
Formation of molecular positronium on a quartz surface.
If you have read this far, you already know very well that as soon as a particle of antimatter enters ordinary matter, annihilation occurs: the particles and antiparticle disappear and turn into radiation. But how fast is this happening? Imagine a positron that flew in from a vacuum and entered a solid. Does it annihilate upon contact with the first atom? Not at all necessary! The anniligation of an electron and a positron is not an instantaneous process; it requires a long atomic time scale. Therefore, the positron manages to live a bright and full of non-trivial events life in matter. First, the positron can pick up an unattended electron and form a bound state – positronium (Ps). With a suitable spin orientation, positronium can live tens of nanoseconds before annihilation. Being in solid matter,
Secondly, while drifting in matter, positronium can come to the surface and stick there – this is a positron (or rather, positronium) analogue of the adsorption of atoms. At room temperature, he does not sit in one place, but actively travels along the surface. And if this is not an external surface, but a nanometer-sized pore, then positronium is trapped in it for a long time.
Further more. In the standard material for such experiments, porous quartz, the pores are not isolated, but are united by nanochannels into a common network. Lukewarm positronium, crawling along the surface, will have time to examine hundreds of pores. And since a lot of positroniums are formed in such experiments and almost all of them crawl out into the pores, sooner or later they bump into each other and, interacting, sometimes form the most real molecules – molecular positronium, Ps2. Further, it is already possible to study how the positronium gas behaves, what excited states positronium has, etc. And don’t think that this is purely theoretical reasoning; all the listed effects have already been verified and studied experimentally.
Does antimatter have practical uses?
Of course. In general, any physical process, if it opens before us a certain new facet of our world and does not require any super-cost, will certainly find practical applications. Moreover, such applications, which we ourselves would not have guessed, if not for the discovery and study of the scientific side of this phenomenon.
The most famous applied application of antiparticles is PET, positron emission tomography. In general, nuclear physics has an impressive track record of medical applications, and antiparticles have not been left idle here either. With PET, a small dose of a drug containing an unstable isotope with a short lifetime (minutes and hours) and degrading due to positive beta decay is injected into the patient’s body. The drug accumulates in the necessary tissues, the nuclei disintegrate and emit positrons, which annihilate nearby and give out two gamma quanta of a certain energy. The detector registers them, determines the direction and time of their arrival, and restores the place where the decay occurred. Thus, it is possible to construct a three-dimensional map of the distribution of matter with a high spatial resolution and with a minimum radiation dose.
Positrons can also be used in materials science, for example, to measure the porosity of a substance. If the substance is solid, then the positrons, stuck in the substance at a sufficient depth, rather quickly annihilate and emit gamma quanta. If there are nanopores inside the substance, annihilation is delayed, since positronium sticks to the surface of the pore. By measuring this delay, it is possible to find out the degree of nanoporosity of a substance by a non-contact and non-destructive method. As an illustration of this technique, there is a recent work on how nanopores appear and tighten in the thinnest layer of ice during vapor deposition on the surface. A similar approach also works when studying structural defects in semiconductor crystals, for example, vacancies and dislocations, and makes it possible to measure the structural fatigue of a material.
Medical applications can also be found for antiprotons. Now at the same CERN, the ACE experiment is being carried out, which studies the effect of an antiproton beam on living cells. Its goal is to study the prospects for the use of antiprotons for the treatment of cancerous tumors.
Energy release of an ion beam and X-ray when passing through a substance.
This idea may horrify the reader out of habit: how so, with an antiproton beam – and for a living person ?! Yes, and it is much safer than X-ray irradiation of a deep tumor! An antiproton beam of specially selected energy becomes an effective tool in the hands of the surgeon, with which it is possible to burn out tumors deep inside the body and minimize the impact on the surrounding tissues. Unlike X-rays, which burns everything that falls under the beam, heavy charged particles on their way through matter release the bulk of their energy in the last centimeters before stopping. By adjusting the energy of the particles, you can vary the depth at which the particles stop; it is on this area with a size of millimeters that the main radiation effect will have to be.
Such proton beam radiotherapy has long been used in many well-equipped clinics around the world. Recently, some of them are switching to ion therapy, which uses a beam of carbon ions rather than protons. For them, the energy release profile is even more contrasting, which means that the effectiveness of the “therapeutic against side effects” pair increases. But it has long been proposed to try antiprotons for this purpose. After all, when they get into the substance, they not only give up their kinetic energy, but also annihilate after stopping – and this increases the energy release several times. Where this additional energy deposition is deposited is a complex issue and must be carefully examined before starting clinical trials.
This is what the ACE experiment does. During it, researchers pass a beam of antiprotons through a cuvette with a bacterial culture and measure their survival depending on the location, on the parameters of the beam, and on the physical characteristics of the environment. This methodical and, perhaps, boring collection of technical data is an important starting point for any new technology.
What do astronomers know about dark matter?
Dark matter is a substance that does not interact with other matter using electromagnetic (EM) or strong nuclear forces. The absence of electromagnetic interactions means that it cannot emit, absorb, reflect, refract or scatter light. This naturally makes it a rather difficult subject to observe. However, about 85% of all matter in the universe is dark matter.
So far, scientists do not have a single practical evidence that dark matter really exists, but there are theoretical ones. Here are the three main ones.
Galactic rotation curves
As one object revolves around the other, the object in orbit must constantly accelerate towards the center (or more accurately, they both accelerate towards their combined center of mass). Without this acceleration, the orbital body will simply fly away.
The faster the orbiting body moves, the more acceleration is required to keep it in orbit. Since in this case the acceleration is due to gravity, this means that the central mass must be larger.
This knowledge allows scientists to “weigh” different parts of the galaxy, as well as measure rotation speeds by comparing redshifts on the approaching and receding sides of the galaxy. When weighing in, astronomers see a discrepancy between the mass of all objects in the galaxy and its total mass.
Red shift – shift of spectral lines of chemical elements to the red (long-wave) side. This phenomenon can be an expression of weak diffuse scattering, Doppler effect or gravitational redshift, or a combination of both. For the first time, the shift of spectral lines in the spectra of celestial bodies was described by the French physicist Hippolyte Fizeau in 1848 and proposed to explain the shift the Doppler effect caused by the radial velocity of the star.
According to general relativity, whenever light passes through a gravitational field, it is slightly distorted. This acts like a gravitational lens and can produce, for example, “Einstein rings” like in the image below.
Einstein’s general theory of relativity states that the gravity of such large cosmic objects as galaxies bends the space around it and deflects rays of light. This creates a distorted image of another galaxy – a light source.
The “Einstein Ring” in the image above is a distorted image of one galaxy (highlighted in blue) behind another (red) galaxy in the center. Light from blue travels in all directions, but is bent by the gravity of the red galaxy. This means that light, which, for example, was originally directed directly at the Earth, will never reach our planet – unlike light that had a different direction, but was distorted by the lens and seems to come from all directions at once. This process explains the appearance of the ring.
In weak gravitational lenses, statistical analysis of the distortion in the light we receive allows us to “notice” the gravitational field between the Earth and distant galaxies. Often there is more mass in this field – and therefore more matter – than scientists can explain.
An example of gravitational lensing, which, from the point of view of the existing theory, proves the presence of dark matter, is a photograph of the Bullet galaxy cluster located in the constellation Carina.
The picture shows the aftermath of a collision of two galaxies. Red on the image shows areas of visible matter, blue – dark matter, the presence of which is determined by gravitational lensing.
Such a clear separation is explained by the fact that most of the luminous matter in a galaxy cluster is located in an intra-cluster environment – in a hot, dense plasma. When parts of the plasma collide with each other, a significant amount of matter slows down and remains in the center. But dark matter weakly interacts with matter, so its components from two clusters can freely pass through each other – this leads to the separation shown in the photograph.
For the first few hundred thousand years after the Big Bang, the universe was hot enough to ionize strongly. This temporarily made it almost opaque to light – the photons rotated like any other particle. However, when everything cooled down enough, significant amounts of protons and electrons combined to form neutral hydrogen, which became transparent enough to most of the light around it. This process happened quite quickly (in terms of cosmological time) – as a result, all the light contained in the Universe, relatively speaking, was suddenly released outside, taking a picture at that stage of its evolution. This is how you can describe the relic radiation in a simplified way.
To record this light, scientists can point radio telescopes in any direction – and depending on the area of observation, the temperature will change slightly. The difference in temperature is due to the presence or absence of dark matter in this area.
What is unusual about the first galaxy?
DF2 is a galaxy that is part of a large group led by the massive elliptical galaxy NGC 1052. The galaxy attracted the attention of scientists because it looked different in photographs taken by the Dragonfly and Sloan Digital Sky Survey (SDSS) satellites. On the first, the galaxy was a blur of weak light, while on the second, a group of point objects.
Based on these observations, scientists led by Peter van Dokkum identified ten globular clusters (large groups of old stars) within the galaxy and found that they move three times slower than when there is a lot of dark matter. The fact is that if the mass of the galaxy was greater than the mass of visible objects, the clusters would rotate faster.
The scientific community evaluated the publication critically – it was called the researchers’ mistake that they observed only ten clusters and only for two nights. Skeptics believed that scientists may have missed key details of the movement of star clusters, and this as a result skewed their estimate of the mass of the galaxy and its visible matter.
And in the second?
The only way to prove the correctness of their observations was to search for a second galaxy that would contain the minimum amount of dark matter – and such a galaxy was discovered in March 2019.
The researchers published two scientific articles – in the first, they re-measured the mass of DF2 using an advanced Hubble camera and a ten-meter telescope at the Keck Observatory in Hawaii. This time, astronomers observed not only the speed of movement of the clusters, but also the speed of rotation of the stars inside them. As a result, scientists have established that DF2 is a transparent ultradiffuse galaxy, the size of which is roughly the size of the Milky Way. Only the stars in it turned out to be about 200 times less.
The second article was devoted to the discovery of a similar DF2 galaxy – DF4, which is located in the same cluster next to the galaxy NGC 1052. The researchers believe that, firstly, galaxies with a minimum amount of dark matter are not uncommon, and, secondly, that large the galaxy could steal dark matter from its smaller neighbors.
How can the absence of dark matter be evidence of its presence?
To understand the statement that the absence of dark matter in two galaxies confirms its presence in the Universe in accordance with the General Theory of Relativity, it is worth considering the criticism of the idea of the presence of dark matter.
Some scientists do not agree that dark matter exists in the Universe, and theoretical evidence of its presence is attributed to the so-called modified Newtonian dynamics (MOND). This alternative theory states that gravity on a cosmic scale does not work as predicted by Isaac Newton or Albert Einstein. This means that General Relativity, on which theories about the existence of dark matter are based, does not work in the case of galaxies.
For example, theoretical physicist Eric Verlinde of the University of Amsterdam published a research paper in 2016 that looked at gravity as a byproduct of quantum interactions and suggested that the extra gravity attributed to dark matter is the effect of dark energy – background energy woven into the fabric of space – time of the universe.
In other words, Verlinde believes that dark matter is not matter, but only the interaction between ordinary matter and dark energy.
The discovery by scientists from Yale University demonstrates that dark matter can be separated from ordinary matter – provided that both detected galaxies behave in accordance with the standard theory of gravity. That is, the processes occurring in them can be explained using the equations discovered by Newton and Kepler.
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