Superfluid Universe: Dark Matter as Bose-Einstein Condensate

Original author: Sabine Hossenfelder
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Quantum effects work not only at the subatomic level: they can be spread throughout the galaxy and solve the mystery of dark matter

Most of the matter of the universe is invisible, consists of some substance that leaves no traces in the process of passing through us, and through all the detectors built by scientists to catch it. But this dark matter may not consist of invisible clouds of particles, as most theorists assume. Instead, it may turn out to be something even stranger: a superfluid liquid that condensed into puddles billions of years ago and gave rise to the galaxies we see today.

This is a new assumption.has far reaching implications for cosmology and physics. Superfluid dark matter (STM) solves many theoretical problems associated with clouds of particles. She explains the annoyingly lingering unsuccessful attempts to identify the individual components of these clouds. It also offers a clear scientific path for further exploration and produces certain predictions that can be verified soon.

The STM has important conceptual implications. From this idea it follows that the generally accepted notion of the Universe as a mass of individual particles connected by means of certain forces — as if a child’s designer — misses all the richness of nature. Most of the matter in the Universe may not be at all the same as the matter of which your body consists: it may consist not of atoms or even of such particles as we usually imagine, but be coherent with a whole vast extent.

“For many years, people used the simplest model for TM: particles that do not collide with other particles and do not emit light,” says Justin Khoury, a professor of theoretical physics at the University of Pennsylvania. "But over the past 20 years, observations and computer simulations have noticeably improved, and this model has some problems on galactic scales." TM particles do not collide with themselves, therefore they do not form compact structures equivalent to stars and planets. Since TM, by definition, does not emit light, its gravitational effect is evidence of its existence: an invisible material, judging by all, affects the formation, rotation and movement of galaxies. On the largest scales, TMs without collisions usually correspond well to astronomical observations.

On a smaller scale, this popular and widely used model predicts that in the galactic center should gather more material than can be seen, astronomers - This feature is known as " the problem of inflection " [cusp problem]. Also, this model predicts too many satellite galaxies for the Milky Way, and cannot explain why those satellites that we really have are located almost in the same plane. And finally, TM without collisions says nothing about why the brightness of spiral galaxies corresponds to their rotation speed. This simple model seems too simple.

One possible explanation for these shortcomings may be that physicists have missed one important astrophysical process involved in the formation of the galaxy. But Kouri doesn't think so. From his point of view, this problem speaks of something deeper. The point is not only that the model of cold TM without collisions hardly fits some data, but also that a completely different model fits much better with the very observations with which the standard model has problems. Instead of inventing new, undiscovered particles, another modelsuggests modifying gravity to match TM. The behavior of gravity at distances of thousands and millions of light years cannot be measured directly. Small effects that cannot be detected on Earth can play a fairly large role on the scale of a whole galaxy.

Gravity Modification(MG) is surprisingly successful in some cases and has problems in others. On the one hand, it surprisingly easily corresponds to the rotation of galaxies and explains where the dependence of brightness and speed of rotation come from. MG does not allow such a variety of parameters to appear from galaxy to galaxy, which arises when using clouds of particles - the latter can be completely different. On the other hand, the MG does not cope with the observations of distances much greater or smaller than the size of a typical galaxy. The cold TM model works better on these scales.

The fact that it is extremely difficult to change anything in Einstein's theory of gravity, without completely breaking it, is notoriously renowned. Therefore, most physicists choose a safer alternative in the form of TM, consisting of particles. For them, the emergence of new particles is a beaten way to solve problems, and the associated mathematics is a familiar territory. But Kouri does not want to join any of these parties. He wants to take the best from both, so as to best fit the real universe.

“Usually people tried to solve problems of a galactic scale, modifying gravity; such was the alternative to TM, says Kouri. - And for some reason, perhaps of a social nature, these two approaches were considered mutually exclusive: you are either in the MG camp or in the TM camp consisting of particles. But why not combine them? Of course, Occam's razor would say it would be less convincing. Therefore, our chosen approach is that both phenomena, MG and TM, consisting of particles, may simply be aspects of the same theory. ”

Evidence of the existence of TM accumulate since its discovery by the Swiss astronomer Fritz Zwickymore than 80 years ago. In 1933, Zwicky used the Hooker 254 cm telescope at the Mount Wilson Observatory in California, directing it towards the cluster of Hair of Veronica . This is a swarm of about 1000 galaxies linked together by gravitational attraction. In such a coherent system, the speeds of its components — in this case, galaxies — depend on the total associated mass. Zwicky noted that galaxies move much faster than they would move if only the apparent mass of matter is taken into account, and suggested that the cluster should contain invisible matter. He called her Dunkle Materie, or "dark matter" in German.

Physicists might dismiss this case as a strange deviation. But it turned out that this observation is more the rule than the exception when the American astronomerSince the 1960s, Vera Rubin has studied the rotation of spiral galaxies. The speed of stars in orbits far from the center of the galaxy depends on the total mass (and, therefore, gravitational attraction) of the connected system - in this case, on the mass of the galaxy. Rubin measurements showed that dozens of galaxies rotated faster than one would expect, based only on visible matter. Since the observations of Rubin brought TM under the light of searchlights, she fell into the list of the most popular unsolved problems of physics.

Telescope technology was steadily improving, and evidence in favor of TM, obtained from observations, was gradually accumulated and refined. Now physicists can observe small distortions occurring due to the gravitational curvature of space-time near the galactic clusters. This distortion, known as weak gravitational lensing, slightly warps the appearance of more distant stellar objects; the light coming from them is bent around the cluster, whose attraction acts like a lens. By the strength of this general effect, one can calculate the mass of the cluster and demonstrate the presence of TM. With the help of this method, physicists have even built maps of TM distribution. Comparing them with other methods of proof, they determined that 85% of the matter of the Universe should relate to TM.

Using even more data, physicists were also able to eliminate the idea that TM consists of invisible clumps of ordinary atoms, such as the Earth consists of (technically they are called baryonic matter). This normal matter interacts with itself too much; it would not give the observed distribution of TM. TM also cannot consist of stars collapsed into black holes or other dim astronomical objects. If this were so, these objects would have to greatly exceed the number of stars in our galaxy, which would lead to significant and easily observed gravitational distortions. Also, TM cannot consist of other known particles, such as weakly interacting neutrinos, emitted in large quantities by stars. Neutrinos are not lumpy enough to create observable galactic structures.

It turns out that in order to explain what TM consists of, physicists have to build theories about new, not yet discovered particles. The most commonly used are those that fall into two broad classes: weakly interacting massive particles ( wimps ) and much lighter axions , although there is also no shortage of more complex hypotheses combining different types of particles. But all attempts to detect these particles directly, rather than simply deriving their presence from gravitational attraction, still remain unsuccessful. Instead of solving the puzzle, experiments on their direct detection only deepened it.

"Today it is impossible to be interested in cosmology without being interested in dark matter," says Stefano Liberati, a professor of physics at the International School of Advanced Studies in Italy. Liberati and colleagues independently worked on the explanation of TM , very similar to what Kouri gives. When Liberati first discovered how successful MGs are on a galactic scale, where Cold TM models fail, he immediately tried to think of a way to combine the two models. “It made me think: maybe TM is experiencing some phase transition on a small scale,” he says. - Maybe it turns into some kind of liquid, in particular, into a superfluid one. If it forms condensate on the scale of galaxies, it would actually solve many problems. ”

Superfluids do not exist in everyday life, but physicists are familiar with them. They resemble superconductors — a class of materials in which electricity moves without resistance. When cooled to a temperature close to absolute zero, helium also begins to flow without resistance. It seeps through the smallest pores, and even flows out of the pallets, moving up the walls. Such superfluid behavior is not only characteristic of helium; This is the phase of the state of matter into which other particles can pass at sufficiently low temperatures. This class of ultracold liquids, first predicted in 1924 by Einstein and the Indian physicist Shatiendranath Bose , is today known as the Bose-Einstein condensate.. Liberati realized that TM can also go into a superfluid state.

Bose-Einstein condensates are best studied as a mixture of two components: a superfluid liquid and a normal one. These two components behave differently. Superfluid demonstrates quantum effects at large distances, it does not have viscosity, and unexpected correlations appear on large scales; it behaves as if it consists of much larger particles than it actually is. The other, normal component, behaves like fluids we are used to; sticks to containers and to itself - that is, it has a viscosity. The ratio between the two components depends on the temperature of the condensate: the higher the temperature, the greater the effect of the normal component.

We used to think that quantum physics is dominant only in the microscopic field. But the more physicists learned about quantum theory, the clearer it became that it was not. Bose-Einstein condensates are one of the best-studied substances that allow quantum effects to propagate in the medium. In theory, quantum behavior can propagate over arbitrarily large distances if its perturbations are sufficiently weak.

In a warm and noisy environment like the Earth, fragile quantum effects are quickly destroyed. Therefore, we usually do not face such strange aspects of quantum physics as the possibility of particles to behave like waves. But if you cause quantum behavior in a cold and peaceful place, it will be saved. In such a cold, calm place, like, for example, outer space. There, quantum effects can reach vast distances.

If TM were a Bose-Einstein condensate — one that has a quantum effect across the entire galaxy — this state would naturally explain two different models of TM behavior. Inside the galaxies, most of the TM would be in the superfluid phase. During galactic clusters with a large proportion of intergalactic space, most TMs would be in the normal phase, which would cause a different behavior. According to Kouri and colleagues, it is possible to explain the observed effects of TM using a simple Bose-Einstein condensate model with only a few open parameters (properties that must have the correct values ​​for the model to work).

The idea that TM may be a Bose-Einstein condensate has long been rotated in the astrophysical community, but the new version has its differences. Kouri's new idea is so convincing because he says that superfluid TM can imitate MG: it achieves a goal by combining the best of both models. It turns out that gravity does not need to be modified in order to get the results observed in MG theories. A coherent superfluid liquid can produce the same equations and the same behavior. Thus, the Kouri model combines the advantages of both cold TM and MG, without the disadvantages of both theories.

Superfluid TM can overcome the biggest problem of MG: most astrophysicists dislike it. Many of these researchers came from particle physics, and the MG equations seem unusual to them. For a specialist in particle physics, these equations look unattractive and unnatural. They seem to fit the result. But superfluid TM offers another, perhaps more natural, approach to equations.

According to Kouri, the equations for superfluid TM do not belong to the field of elementary particle physics. They come from condensed matter physics.where they describe not fundamental particles, but long-range behavior appearing on their basis. In the Kouri model, the equations appearing in MG do not describe individual particles. They describe the joint behavior of particles. Such equations are unfamiliar to many specialists in particle physics, so the relationship between superfluidity and MG has gone unnoticed for so long. But, in contrast to the MG equations, the equations describing superfluid liquids already possess a strong theoretical foundation — only in the physics of condensed states.

That Kouri noticed this connection is an unpredictable accident. He came across the literature on condensed matter physics, which used equations very similar to those he saw in MG theories: “And everything else then just fell into place,” he says. “I thought that it all just formed a beautiful picture uniting these two phenomena.”

Returning to the observational evidence of the existence of TM, Kouri's superfluid approach can solve many problems of existing models. To begin with, superfluidity prevents excessive clumping of TM in the centers of galaxies, eliminating the illusory “bend”, since in the superfluidity phase all density fluctuations are aligned. “A superfluid fluid will have a coherent length [distance at which all matter is in one state],” says Liberati. “From this it is already clear that there will be no excesses.”

Superfluidity produces a scheme of attraction identical to the MG equations, so it can be responsible for the observed regularity of the rotation curves of galaxies. However, unlike MG, it behaves only at such temperatures at which the superfluid component prevails. On larger scales, galactic TM clusters turn out to be too excited (that is, too hot) and lose superfluid properties. In this way, superfluid TM could give rise to the formation of visible galaxies, and at the same time, in a phase other than superfluidity, would correspond to the observed cluster structure.

The Kouri approach explains why astronomers do not observe MG evidence inside the solar system. “The sun creates such a strong gravitational field that it locally destroys superfluid coherence,” he says. - Near the solar system you should not think in terms of superfluid coherence. The sun behaves like an admixture. Like a hole in a liquid. ”

Finally, the superfluid fluid model explains why physicists cannot find TM particles. Since the 1980s, dozens of different experiments have been looking for direct evidence of the existence of such particles. These experiments usually use large shielded tanks with different materials, which in rare cases can interact with TM particles and produce the observed signal. Despite the wide variety of techniques and materials, the use of carefully isolated detectors hidden in underground mines to filter out false signals has not been found any conclusive evidence of the existence of TM.

In the absence of a discovery, the idea that TM may be something other than just another type of particles becomes more convincing. “When I was a student, I woke up every thirtieth night after dreaming about modified gravity,” says Nima Arkani-Hamed, a professor of theoretical physics at Princeton. “Then it happened once every 300 nights, and now it happens once every 100. The theme returns.”

If TM is a superfluid liquid, then the particles of which it consists should be light, much lighter than the hypothetical particles TM, which most experiments are looking for. The components of the superfluid fluid are probably too light to be found in current experiments.

An improved and unique prediction of the Kouri model is that superfluid quantum behavior should leave a characteristic trace in collisions of galaxies. When the condensate of TM of one galaxy collides with the condensate of another, as a result there should appear interference patterns - a ripple in the distribution of matter and gravity, which will affect the behavior of galaxies. Superfluid TM also predicts friction between the components of TM in clusters of galaxies; such friction again will give a certain pattern of gravitational attraction. Observations of gravitational lensing can detect these signs of the presence of superfluid TM, if you know exactly what to look for.

For the numerical evaluation of predictions, it is necessary to carry out computer simulations. Kouri is currently working on such a project together with researchers from the University of Oxford. Simulations should also show whether the expected number of satellite galaxies fits better with the theory of superfluid TM, than with the predictions of existing models.

Amanda Weltman, a cosmologist at the University of Cape Town who works with TM, but who did not participate in this study, believes that the new model is “very interesting and creative.” But she says that she will keep her assessments until she sees experimental evidence, some evidence that clearly supports superfluidity: "Such observations will give real weight to their ideas." If the simulations on supercomputers are successful, Cowrie may be able to provide such evidence. And then we have to get used to an even more complex view of the Universe - filled not only with dark matter, but also with frictionless superfluid liquids twisting around bright galaxies.

Arkani-Hamed is more skeptical and not ready to part with cold TM. “But if the wimps are not found in the next set of experiments, they will not be found in the next 20 years,” he says. He believes that the time has come to take a fresh look at models built around unusual particles or modified theories of gravity. Or a model combining the best of two dark worlds.

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