Why do we need dark matter?

Surprisingly attractive force

Theoretically, gravity should be predictable. We are well acquainted with her, thanks to her we are firmly on the Earth, and our atmosphere does not fly into space. If to take larger scales, this force influenced the evolution of the Universe itself. It’s a shame that sometimes gravity brings us down. In order to explain the spiraling rotation of galaxies and clusters of galaxies by gravity in the form in which we understand it, we need to come up with an entirely new form of matter that no one has ever witnessed – dark matter. To explain the acceleration of the expansion of the universe, we need to invent an equally mysterious entity – dark energy.

But what if we never fully understood gravity? What if somewhere outside our field of view gravity plays out of rules?

To think so is practically heresy, although such ideas are not new. However, recently recent studies of galaxies and unexpected results from the field of quantum informatics are pushing us with new force to rethink our understanding of gravity. There are new radical ideas, in which our ideas about space-time and essence of gravity are thoroughly transformed. In the new picture of the world there is no place for dark matter, and the dark energy, instead of resisting gravity, can partly generate it.

Virtually everything that we know about gravity was given to us by Isaac Newton and Albert Einstein. Newton explained to us that the force of attraction decreases in inverse proportion to the square of the distance, and Einstein – that gravity appears as a result of the curvature of space-time by massive objects.

The law of universal gravitation of Newton says that the stars more distant from the center of the galaxy, the gravitational force acts weaker than the stars located closer to the center of the galaxy, so the speed of the first is lower. However, in the 1970s, astronomers, including Vera Rubin, observed that the speed of stars distant from the center of galaxies did not decrease as predicted. Instead, the speed equalized, which could be explained only by the presence of some invisible matter that surrounded the galaxy and created an additional attraction. Since then we have been unsuccessfully trying to find this matter.

The game is not according to the rules

Far from everyone was involved in the search. In the 1980s, Mordehai Milgrom, then at the University of Princeton, showed that we can explain the oddities in the speed of rotation of galaxies without the participation of dark matter. To do this, we just have to discard the idea that with increasing distances, gravity always behaves the way Newton and Einstein predicted. Milgrom’s theory, known as MOND (modified Newtonian dynamics), suggests that the force of attraction is weakening more smoothly than Newton argued. As soon as the acceleration of an object caused by gravity falls below a certain value, or rather becomes 82 billion times weaker than the acceleration of gravity on the Earth, gravity suddenly switches to a new mode.

Milgrom achieved certain successes, applying his theory to spiral galaxies, but MOND never got spread. For starters, it could not be used to calculate clusters of galaxies that could not form the clusters themselves without the participation of dark matter or without making more radical changes to the theory of gravitation besides those allowed by MOND. Plus, the changes proposed by this theory seemed too random. Why would suddenly the force of attraction change in this seemingly arbitrary point?

And, however, MOND still remains afloat and, to no small extent, due to the fact that dark matter was never found. “There are two possibilities,” says John Moffat of the Perimeter Institute of Theoretical Physics in Waterloo, Canada, “or we’ll find an invisible source of additional attraction and make sure Newton and Einstein were right or we will not find anything.” In this case, we will need to refine the gravity. ”

Last year, perhaps, finally, it was a turning point. Stacy McGaugh, an astronomer from the Case Western Reserve University in Cleveland, Ohio, and his colleagues looked again at more than 150 spiral galaxy galaxies with our Milky Way galaxy. When they compared the calculated force of attraction with the speed of rotation of the disk of galaxies, they found that stars closer to the edge of the disk rotate at abnormally high speeds.

And what from this? After all, we have already observed this behavior many times before, and it can be explained by enveloping the galaxy with a cloud of dark matter. However, with statistical evaluation, McGo used cross-checking. He took all the visible matter in all galaxies and compared the force of attraction of this matter at each point with the speed of rotation of nearby stars. As a result, he received a surprisingly close relationship between the speed of rotation of galaxies and the distribution of visible matter, which they contain.

This Hubble Space Telescope image shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens of the cluster of yellow, elliptical and spiral galaxies – called 0024+1654 – near the photograph’s center.

Lee Smolin, a theorist at the Perimeter Institute in Canada, was impressed. Such a relationship “is tantamount to the law of nature,” he says. This is not expected to see if the galaxy is influenced by something other than visible matter.

Even more surprising is the fact that this close relationship between visible matter and the motion of stars remains in a wide range of different galaxies, even though the dark matter in them is distributed in different ways. Dark matter should not follow resignedly the usual substance. Therefore, either it interacts with ordinary matter or itself more strongly than a simple model predicts, or something is wrong with gravity.

McGo’s work is not the only reason that made us raise this heretical question again. One of the biggest problems for MOND is the behavior of clusters of galaxies. Like the stars on the edge of galaxies, galaxies on the edge of clusters also move too fast – a fact that is explained with the help of dark matter. Observation of the effect of gravitational lensing (a slight curvature of light by the gravitational field of massive objects) suggests that the additional force imparting speed to galaxies is not where the visible matter is. It is simply impossible to explain the behavior of clusters of galaxies without the participation of invisible matter, at least so it is.

The most famous example is the bullet cluster (Bullet CLUS 1E 0657-558, header image), so named for the similarity with the slow-motion image of a bullet tearing a target apart. For many hunters of dark matter this is the best proof that they hunt for this beast knowingly, and it exists. But Pavel Krupa [Pavel Kroupa] from the University of Bonn in Germany claims the opposite – this high-speed intergalactic collision can only be explained by the theory of MOND.

“Comparison with the image of a bullet falling into a target is, of course, a joke for the broad masses,” he says. Krupa argues that in realistic time frames standard gravity is too weak to cause such hot and violent collisions of galaxies, as we observe in the Bullet cluster. Dark matter at the initial stages of the collision is capable of giving it the high speed that we observe, but it will already interfere with all subsequent interactions. “A halo of dark matter resembles a cobweb,” says Krupa. “It captures any galaxy that comes in its way.” Therefore, a pair of colliding galaxies, which continue to move at high speeds even after a collision, is very difficult to explain. “This is a big, big problem for the standard model of cosmology,” says Krupa. “But with a modified gravity … such a problem does not exist.”

The essence of MOND is that at galactic and intergalactic distances, where we can not directly measure the force of gravity, it is stronger than we thought. And it is this, and not some invisible matter, that will be the simplest explanation of why matter on such a scale moves faster and collides more strongly than Newton and Einstein predict.

This does not mean that the MOND theory does not have certain problems when it comes to the interaction within the clusters of galaxies. In the Bullet cluster, using telescopes, we have identified two prominent places where gravitational lensing manifests itself more strongly, and hence there is a higher concentration of mass that does not coincide with the amount of ordinary matter observed by us in these same places.

Milgrom insists that this problem is not such a terrible threat to his model, as many believe. “It’s enough just a small amount of unaccounted for the mass, which can be the most common matter, for example, dead stars or clouds of cold gas, which we have not yet discovered,” he says.

But while this is not confirmed by observations, other scientists are looking for theoretical solutions to this problem. One such solution is a hybrid model in which dark matter behaves like a werewolf – it passes through galaxies unhindered, creating an additional gravitational force consistent with the theory of MOND, but in clusters of galaxies it behaves like ordinary dark matter.

Another option, which unexpectedly again came into fashion – to modify MOND. This is what Moffat is doing. In his understanding, the force of attraction changes after the addition of a repulsive force, which in turn depends on the distance, because of which, at small distances, the attraction force obeys Newton’s inverse square law, but on the outskirts of the galaxy it weakens. In such a picture of the world, gravity is stronger than Newton believed, and behaves as MOND predicts.

Moffat argues that his theory can explain the rotation of galaxies and abnormal velocities in the Bullet cluster. But the main feature of his theory is that near the black holes the force of attraction is stronger than predicted even MOND, which, perhaps, will give us a chance to test this theory.

If we could look at the black hole, we would see a black disk surrounded by a shadow caused by extremely strong gravitational lensing. In 2015, Moffat calculated that according to his theory, the shadow around the supermassive black hole in the center of the Milky Way will be 10 times greater than predicted by GR. And then the scene comes Event Horizon

Telescope (EHT) is a global network of radio telescopes, the launch of which is scheduled for April this year, for the first time able to obtain detailed images of black holes. At least theoretically, we will be able to observe this bloated shadow, if, of course, it is there at all.

However, whatever we choose, the traditional theory of MOND or the modified Moffat gravity, there is a huge problem that can not be closed by the eyes – a glaring absence of a fundamental theory. Why suddenly gravity deviate from the course that Newton and Einstein paved for it, and even, it would seem, at a random point? The answer to this question can be obtained if we radically reconsider our understanding of the essence of gravity.

Last year, Erik Verlinde of the University of Amsterdam in the Netherlands offered a fresh perspective on this issue. Gravitation, he believes, does not arise by itself, but as a result of interactions between entangled bits of quantum information.

Entanglement is a profound and at the same time profoundly paradoxical connection between pairs or groups of particles, when an effect on one particle causes a reaction in others, even if they are separated by large distances. Since the late 1990s, physicists have learned how to obtain Newtonian and Einstein gravity with the help of networks of entangled quantum bits. The problem is that it works only in a theoretical universe known as the Anti-de Sitter Space, which behaves differently than the universe where we live.

The key difference is that in our universe the vacuum is not so calm and immobile. It boils with dark energy, a mysterious substance or force, which is believed to be responsible for accelerating the expansion of space-time.

Instead of trying to solve this problem, Ferlinde looked at how gravity, caused by the interaction between entangled bits of quantum information, behaves in a universe where there is dark energy. As a result, he received a new picture of gravity, in which dark energy gives the entanglement of quantum bits something like additional elasticity.

“It’s as if the dark energy is an elastic medium,” says Ferlinde, “and if you make a lot of it, it deforms this environment.” Additional elasticity, he adds, created by dark energy, feeds the force of attraction at great distances, which eventually leads to the appearance of additional effects at a distance that resemble Milgrom’s theory of MOND.

Ideas Ferlinde made a great impression, but it is still unclear how they are generally connected. “He starts with a dark energy, and says it leads to something that resembles dark matter,” says Sabine Hossenfelder of the Frankfurt Institute for Advanced Studies in Germany. “He is trying hard to reconcile his hypotheses with a great assumption, which in recent years has gained great popularity, that space-time arises from confusion. But I’m not sure that there is a need for this. ”

In a recent study, it was found that if we take the view of Ferrende for gravity, we can explain the anomalies in the gravitational lensing observed near about 30,000 galaxies. But his theory has been criticized for making predictions that actually diverge from MOND. In one scientific paper, in co-authorship with McGo, for example, it is said that the theory of Ferlinde diverges from MOND in the main – the explanation of the anomalous rotation of galaxies. In addition, his theory predicts the motion of planets, which we do not actually observe in our solar system.

Smolin from his side proposed a more modest attempt to deduce MOND-physics from the principles of quantum gravity, and, unlike the theory of Ferlinde, his results do not differ from the theory of MOND. None of them claims that he received a full theory of quantum gravity. But one thing becomes clear – to the question, why gravity behaves so strangely at great distances, theorists began to receive answers.

“We do not know where the final theory will take us, because we have not brought it out yet,” says McGo. “Therefore, before moving forward, we can not escape from the time of confusion and vacillation.”

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