Why do we need dark matter and can we do without it?
This is a translation of an article by Mark Anderson from the magazine New Scientist (March 18-24, 2017).
In theory, gravity should be a predictable power. We are well acquainted with her, thanks to her we are firmly standing on the Earth, and our atmosphere does not fly off into space. If we take a larger scale, then this force influenced the evolution of the Universe itself. What a shame that sometimes gravity brings us. 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 a completely new form of matter that no one has ever witnessed with our own eyes - 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 does not play by the rules?
To think so is practically a heresy, although such ideas are not new. Recently, however, recent studies of galaxies and unexpected results from the field of quantum informatics are pushing us to rethink our understanding of gravity. New radical ideas appear in which our ideas about space-time and the essence of gravity are thoroughly transformed. In the new picture of the world there is no place for dark matter, and dark energy, instead of resisting gravity, can partly generate it.
')
Virtually everything we know about gravity was given to us by Isaac Newton and Albert Einstein. Newton explained to us that the force of attraction decreases inversely proportional 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 the world Newton says that the stars, more distant from the center of the galaxy, the force acts weaker than the stars located closer to the center of the galaxy, so the speed of the first movement is lower. However, in the 1970s, astronomers, including Vera Rubin [Vera Rubin], noticed that the speed of stars away from the center of galaxies did not decrease as predicted. Instead, the speed leveled off, which could only be explained by the presence of some invisible matter that surrounded the galaxy and created additional attraction. Since then, we have been trying unsuccessfully to find this matter.
In search, however, not all participated. In the 1980s, Mordehai Milgrom, then working at Princeton University, showed that we can explain the strangeness in the speed of rotation of galaxies without the participation of dark matter. To do this, you just need to drop the idea that with increasing distances, gravity always behaves as Newton and Einstein predicted. Milgrom’s theory, known as MOND (Modified Newtonian Dynamics), suggests that gravity weakens more smoothly than Newton claimed. 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 free fall on Earth, gravity suddenly switches to a new mode.
Milgrom has achieved some success by applying his theory to spiral galaxies, but MOND has not spread. To begin with, with its help it was impossible to calculate clusters of galaxies that could not form clusters without the participation of dark matter or without making more radical changes to the theory of gravity besides those that were allowed by MOND. Plus, the changes proposed by this theory seemed too random. Why would the force of attraction change at this seemingly arbitrary point?
And yet, however, MOND still remains afloat, and to a lesser extent due to the fact that dark matter has never been detected. “There are two possibilities,” says John Moffat from the Perimeter Institute for Theoretical Physics in Waterloo, Canada, “either we find an invisible source of additional attraction and see that Newton and Einstein were right, or we will not find anything. In this case, we will need to refine gravity. ”
Last year, perhaps finally, the turning point came. Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland, Ohio, and his colleagues have re-looked at more than 150 spiral galaxies of skosha 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 closer to the edge of the disk, the stars rotate at abnormally high speeds.
And what from this? After all, this behavior we have repeatedly observed before, and it can be explained by enveloping the galaxy with a cloud of dark matter. However, in a statistical evaluation, McGaw used cross-check. 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 that they contain.
Lee Smolin, a theorist at the Perimeter Institute in Canada, was amazed. Such a relationship is “equivalent to the law of nature,” he says. You don’t expect to see this if galaxies are affected by something other than visible matter.
Even more surprising is the fact that this close relationship between visible matter and the movement of stars persists in a wide range of different galaxies, even though the dark matter in them is distributed differently. Dark matter must not follow the usual substance without reproach. Therefore, it either interacts with ordinary matter or is stronger itself than the simple model predicts, or something is wrong with gravity.
McGaw'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, the galaxies on the edge of clusters also move too fast - a fact that is explained by dark matter. Observing the effect of gravitational lensing (a slight curvature of light by the gravitational field of massive objects) suggests that the additional force that gives speed to galaxies is not where 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 considered.
The most famous example is the Bullet Cluster (Bullet CLuster 1E 0657-558, title image), so named for its similarity with the slow-motion image of a bullet tearing a target apart. For many dark matter hunters, this is the best proof that they hunt this beast for a reason, 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 that hits the target is, of course, a joke for the masses,” he says. Krupa argues that in a realistic time frame, standard gravity is too weak a force to cause such hot and frantic collisions of galaxies, as we observe in the Bullet cluster. Dark matter at the initial stages of a collision can give it the high speed that we observe, but it will interfere with all subsequent interactions. "The dark matter halo resembles a spider web," says Krupa. "It captures any galaxy in its path." Therefore, a pair of colliding galaxies that continue to move at high speeds even after a collision is very difficult to explain. “This is a big, big problem for the standard cosmology model,” says Krupa. "But with modified gravity ... there is no such problem."
The essence of MOND is that at galactic and intergalactic distances, where we cannot 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 scales moves faster and collides more strongly than Newton and Einstein predict.
This does not mean that the MOND theory has no definite problems when it comes to interaction within clusters of galaxies. With the help of telescopes in the Bullet cluster, we identified two pronounced places where gravitational lensing is more pronounced, which means there is a higher mass concentration, which does not coincide with the amount of ordinary matter observed by us in the 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 unrecorded mass, which may be the most ordinary matter, for example, dead stars or clouds of cold gas that we haven’t yet discovered,” he says.
But while observations are not confirmed, 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 without difficulty, creating an additional force of attraction consistent with the MOND theory, but in clusters of galaxies it behaves like ordinary dark matter.
Another option that suddenly came into vogue again was to modify MOND. This is what Moffat does. In his understanding, the force of attraction changes after the addition of the repulsive force, which in turn depends on the distance, because of which at small distances the force of attraction obeys the inverse Newton squares law, but on the outskirts of the galaxy it weakens. In such a picture of the world, gravity is stronger than Newton thought, and it behaves as MOND predicts.
Moffat argues that his theory can explain the rotation of galaxies and the anomalous velocities in the Bullet cluster. But the main feature of his theory is that, near black holes, the forces of attraction are stronger than even MOND predicts, which may 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 an extremely strong gravitational lensing. In 2015, Moffat calculated that according to his theory, the shadow around a supermassive black hole in the center of the Milky Way would be 10 times more than the GRT predicts. And here comes Event Horizon.
Telescope (EHT) is a global network of radio telescopes, the launch of which is scheduled for April of this year, for the first time capable of obtaining detailed images of black holes. At least theoretically, we can 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 cannot be ignored - the glaring lack of underlying theory. Why would gravity suddenly deviate from the course that Newton and Einstein had laid for it, and, 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, Eric Ferlinde [Erik Verlinde] from the University of Amsterdam in the Netherlands offered a fresh perspective on this issue. Gravity, he believes, does not arise by itself, but as a result of the interactions between the entangled bits of quantum information.
Entanglement is a deep and at the same time deeply paradoxical connection between pairs or groups of particles, when an impact 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 using networks of entangled quantum bits. The problem is that it only works in the theoretical universe known as the Anti-de Sitter Space, which behaves differently from the universe where we live.
The key difference is that in our universe the vacuum is not so calm and motionless. He rages 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 the tangled 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 extra elasticity.
“It’s as if the dark energy is an elastic medium,” says Ferlinde, “and if mass is introduced there, it will deform this medium.” Additional elasticity, he adds, created by dark energy, feeds the force of attraction at large distances, which ultimately leads to additional effects at a distance that resemble Milgrom's MOND theory.
Ferlinde's ideas made a great impression, but it is not yet clear how coherent they are at all. “It starts with dark energy, and says it leads to something that resembles dark matter,” says Sabine Hossenfelder of the Frankfurt Institute for Advanced Research in Germany. “He is trying with all his might to link his hypotheses with a big assumption, which has gained great popularity in recent years, that space-time arises from entanglement. But I'm not sure that there is a need. ”
In a recent study, it was found that if we take Ferlind’s point of view on gravity, then we can explain the anomalies in 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 MacGaw, for example, it is said that Ferlind's theory disagrees with MOND in the main thing - the explanation of the anomalous rotation of galaxies. In addition, his theory predicts the movement of the planets, which we actually do not observe in our solar system.
For his part, Smolin proposed a more modest attempt to derive MOND physics from the principles of quantum gravity, and, unlike Ferlind’s theory, his results do not disagree with the MOND theory. None of them declares that he obtained the complete theory of quantum gravity. But one thing becomes clear - to the question why gravity behaves so strangely at large distances, theorists began to receive answers.
“We don’t know where the final theory will lead us, because we haven’t brought it out yet,” says McGo. "Therefore, before moving forward, we can not escape from the pores of confusion and vacillation."
Surprisingly appealing force
In theory, gravity should be a predictable power. We are well acquainted with her, thanks to her we are firmly standing on the Earth, and our atmosphere does not fly off into space. If we take a larger scale, then this force influenced the evolution of the Universe itself. What a shame that sometimes gravity brings us. 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 a completely new form of matter that no one has ever witnessed with our own eyes - 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 does not play by the rules?
To think so is practically a heresy, although such ideas are not new. Recently, however, recent studies of galaxies and unexpected results from the field of quantum informatics are pushing us to rethink our understanding of gravity. New radical ideas appear in which our ideas about space-time and the essence of gravity are thoroughly transformed. In the new picture of the world there is no place for dark matter, and dark energy, instead of resisting gravity, can partly generate it.
')
Virtually everything we know about gravity was given to us by Isaac Newton and Albert Einstein. Newton explained to us that the force of attraction decreases inversely proportional 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 the world Newton says that the stars, more distant from the center of the galaxy, the force acts weaker than the stars located closer to the center of the galaxy, so the speed of the first movement is lower. However, in the 1970s, astronomers, including Vera Rubin [Vera Rubin], noticed that the speed of stars away from the center of galaxies did not decrease as predicted. Instead, the speed leveled off, which could only be explained by the presence of some invisible matter that surrounded the galaxy and created additional attraction. Since then, we have been trying unsuccessfully to find this matter.
Game not by the rules
In search, however, not all participated. In the 1980s, Mordehai Milgrom, then working at Princeton University, showed that we can explain the strangeness in the speed of rotation of galaxies without the participation of dark matter. To do this, you just need to drop the idea that with increasing distances, gravity always behaves as Newton and Einstein predicted. Milgrom’s theory, known as MOND (Modified Newtonian Dynamics), suggests that gravity weakens more smoothly than Newton claimed. 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 free fall on Earth, gravity suddenly switches to a new mode.
Milgrom has achieved some success by applying his theory to spiral galaxies, but MOND has not spread. To begin with, with its help it was impossible to calculate clusters of galaxies that could not form clusters without the participation of dark matter or without making more radical changes to the theory of gravity besides those that were allowed by MOND. Plus, the changes proposed by this theory seemed too random. Why would the force of attraction change at this seemingly arbitrary point?
And yet, however, MOND still remains afloat, and to a lesser extent due to the fact that dark matter has never been detected. “There are two possibilities,” says John Moffat from the Perimeter Institute for Theoretical Physics in Waterloo, Canada, “either we find an invisible source of additional attraction and see that Newton and Einstein were right, or we will not find anything. In this case, we will need to refine gravity. ”
Last year, perhaps finally, the turning point came. Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland, Ohio, and his colleagues have re-looked at more than 150 spiral galaxies of skosha 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 closer to the edge of the disk, the stars rotate at abnormally high speeds.
And what from this? After all, this behavior we have repeatedly observed before, and it can be explained by enveloping the galaxy with a cloud of dark matter. However, in a statistical evaluation, McGaw used cross-check. 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 that they contain.
Lee Smolin, a theorist at the Perimeter Institute in Canada, was amazed. Such a relationship is “equivalent to the law of nature,” he says. You don’t expect to see this if galaxies are affected by something other than visible matter.
Even more surprising is the fact that this close relationship between visible matter and the movement of stars persists in a wide range of different galaxies, even though the dark matter in them is distributed differently. Dark matter must not follow the usual substance without reproach. Therefore, it either interacts with ordinary matter or is stronger itself than the simple model predicts, or something is wrong with gravity.
McGaw'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, the galaxies on the edge of clusters also move too fast - a fact that is explained by dark matter. Observing the effect of gravitational lensing (a slight curvature of light by the gravitational field of massive objects) suggests that the additional force that gives speed to galaxies is not where 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 considered.
The most famous example is the Bullet Cluster (Bullet CLuster 1E 0657-558, title image), so named for its similarity with the slow-motion image of a bullet tearing a target apart. For many dark matter hunters, this is the best proof that they hunt this beast for a reason, 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 that hits the target is, of course, a joke for the masses,” he says. Krupa argues that in a realistic time frame, standard gravity is too weak a force to cause such hot and frantic collisions of galaxies, as we observe in the Bullet cluster. Dark matter at the initial stages of a collision can give it the high speed that we observe, but it will interfere with all subsequent interactions. "The dark matter halo resembles a spider web," says Krupa. "It captures any galaxy in its path." Therefore, a pair of colliding galaxies that continue to move at high speeds even after a collision is very difficult to explain. “This is a big, big problem for the standard cosmology model,” says Krupa. "But with modified gravity ... there is no such problem."
The essence of MOND is that at galactic and intergalactic distances, where we cannot 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 scales moves faster and collides more strongly than Newton and Einstein predict.
This does not mean that the MOND theory has no definite problems when it comes to interaction within clusters of galaxies. With the help of telescopes in the Bullet cluster, we identified two pronounced places where gravitational lensing is more pronounced, which means there is a higher mass concentration, which does not coincide with the amount of ordinary matter observed by us in the 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 unrecorded mass, which may be the most ordinary matter, for example, dead stars or clouds of cold gas that we haven’t yet discovered,” he says.
But while observations are not confirmed, 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 without difficulty, creating an additional force of attraction consistent with the MOND theory, but in clusters of galaxies it behaves like ordinary dark matter.
Another option that suddenly came into vogue again was to modify MOND. This is what Moffat does. In his understanding, the force of attraction changes after the addition of the repulsive force, which in turn depends on the distance, because of which at small distances the force of attraction obeys the inverse Newton squares law, but on the outskirts of the galaxy it weakens. In such a picture of the world, gravity is stronger than Newton thought, and it behaves as MOND predicts.
Moffat argues that his theory can explain the rotation of galaxies and the anomalous velocities in the Bullet cluster. But the main feature of his theory is that, near black holes, the forces of attraction are stronger than even MOND predicts, which may 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 an extremely strong gravitational lensing. In 2015, Moffat calculated that according to his theory, the shadow around a supermassive black hole in the center of the Milky Way would be 10 times more than the GRT predicts. And here comes Event Horizon.
Telescope (EHT) is a global network of radio telescopes, the launch of which is scheduled for April of this year, for the first time capable of obtaining detailed images of black holes. At least theoretically, we can 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 cannot be ignored - the glaring lack of underlying theory. Why would gravity suddenly deviate from the course that Newton and Einstein had laid for it, and, 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, Eric Ferlinde [Erik Verlinde] from the University of Amsterdam in the Netherlands offered a fresh perspective on this issue. Gravity, he believes, does not arise by itself, but as a result of the interactions between the entangled bits of quantum information.
Entanglement is a deep and at the same time deeply paradoxical connection between pairs or groups of particles, when an impact 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 using networks of entangled quantum bits. The problem is that it only works in the theoretical universe known as the Anti-de Sitter Space, which behaves differently from the universe where we live.
The key difference is that in our universe the vacuum is not so calm and motionless. He rages 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 the tangled 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 extra elasticity.
“It’s as if the dark energy is an elastic medium,” says Ferlinde, “and if mass is introduced there, it will deform this medium.” Additional elasticity, he adds, created by dark energy, feeds the force of attraction at large distances, which ultimately leads to additional effects at a distance that resemble Milgrom's MOND theory.
Ferlinde's ideas made a great impression, but it is not yet clear how coherent they are at all. “It starts with dark energy, and says it leads to something that resembles dark matter,” says Sabine Hossenfelder of the Frankfurt Institute for Advanced Research in Germany. “He is trying with all his might to link his hypotheses with a big assumption, which has gained great popularity in recent years, that space-time arises from entanglement. But I'm not sure that there is a need. ”
In a recent study, it was found that if we take Ferlind’s point of view on gravity, then we can explain the anomalies in 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 MacGaw, for example, it is said that Ferlind's theory disagrees with MOND in the main thing - the explanation of the anomalous rotation of galaxies. In addition, his theory predicts the movement of the planets, which we actually do not observe in our solar system.
For his part, Smolin proposed a more modest attempt to derive MOND physics from the principles of quantum gravity, and, unlike Ferlind’s theory, his results do not disagree with the MOND theory. None of them declares that he obtained the complete theory of quantum gravity. But one thing becomes clear - to the question why gravity behaves so strangely at large distances, theorists began to receive answers.
“We don’t know where the final theory will lead us, because we haven’t brought it out yet,” says McGo. "Therefore, before moving forward, we can not escape from the pores of confusion and vacillation."
Source: https://habr.com/ru/post/402745/
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