March 3, 2024

Astrophysicist recognizes dark matter

Six years ago, Indranil Panik made a decision. Using binary stars, he wanted to find compelling evidence for an alternative theory of gravity. But then everything turned out completely differently.

Spiral galaxy Messier 74 in a composite image from the James Webb Telescope. As in the Milky Way, there is very little gravity in Messier 74.

NASA/Reuters

Astrophysicist Indranil Panik is still at the beginning of his career. However, he has already made a name for himself in professional circles. Until recently, the 32-year-old researcher was considered a strong proponent of modified Newtonian dynamics, or MOND for short. This is an alternative theory of gravity that attempts to describe the universe without dark matter.

But Panik recently changed his position. “The MOND alternative to dark matter is wrong,” is the headline of one of his articles condition. This transformation is striking. MOND proponents and dark matter advocates have been feuding for years without getting even a step closer. It's rare for someone to completely change their mind.

There is a lack of gravity in the universe

The dispute is over how strong the force holding the universe together is. The traditional answer goes back to Isaac Newton. The law of gravitation, formulated in 1687, states that the force of attraction between two objects decreases with the square of the distance between them. The law has been confirmed several times. It accurately describes how the Moon revolves around the Earth and how the planets revolve around the Sun.

However, on larger scales, Newton's law of gravitation has a deficit: it generates too little gravity. One of the first to notice this was the Swiss astronomer Fritz Zwicky. In the 1930s, he realized that there was a stronger cohesion between the galaxies in the Coma galaxy group than their masses would suggest. Zwicky hypothesized that in addition to the luminous matter of stars, there must be invisible matter in the universe that provides additional gravity.

Astronomers observe a similar gravitational deficit in spiral galaxies like our own Milky Way. There, outer stars orbit the galactic center faster than predicted by Newton's law of gravitation. Here too the contradiction can be explained by dark matter, which strengthens attraction.

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A futile search for dark matter particles

Dark matter is an integral part of the Standard Model of cosmology. However, to this day no one can say what it is made of. It is thought to be caused by unknown elementary particles that do not absorb or emit light and can be observed almost exclusively through the strength of their gravity. Despite decades of research, none of these virtual particles have been discovered yet. Therefore, some researchers consider dark matter to be a theoretical construct that should be avoided.

As an alternative, they promote the theory of gravity proposed by Israeli physicist Mordehai Milgrom in the 1980s. According to Milgrom, it's not the extra invisible matter that makes the outer stars in the galaxy spin faster, but rather the stronger pull of ordinary matter.

According to Milgrom, this modification of Newtonian gravity only occurs when gravity becomes very weak—that is, over large distances. In this MOND system, attraction should not decrease quadratically with the distance between two objects, but only linearly. When the distance doubles, gravity does not become weaker by a factor of four, but only by a factor of two. So, relatively speaking, it's getting stronger.

MOND theory was originally developed by Milgrom to explain the rotation curves of spiral galaxies. However, its proponents are now convinced that it fits astronomical observations better than Newtonian gravity plus dark matter, even for smaller and larger objects. Panek and Hongsheng Zhao of the University of St. Andrews in Scotland said a year ago that agreement with the various observations could only be reached through subsequent modifications. Review article Argue.

A final test with distant binary stars

The reason behind Panic's change in position is the binary stars that orbit each other at a great distance. These objects are particularly suitable for testing MOND theory. On the one hand, they are located in the lunar system when the distance between the stars is several thousand times greater than the distance between the Sun and the Earth. As the calculations show, the two stars should orbit each other 20% faster than Newton's theory of gravitation suggests. On the other hand, this paradox cannot be explained by dark matter. In theory, this number should be too few and far between to have a significant impact on the motion of the two stars.

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Panek says that when he started working on distant binary stars, he was convinced that he would find strong confirmation of the MOND theory. But things went differently.

The next wide binary star, so to speak, is on our doorstep.  Proxima Centauri orbits Alpha Centauri at a distance of 0.2 light-years and takes 590,000 years to complete its full orbit.  The numbers indicate the situation in the next thousands of years.

The next wide binary star, so to speak, is on our doorstep. Proxima Centauri orbits Alpha Centauri at a distance of 0.2 light-years and takes 590,000 years to complete its full orbit. The numbers indicate the situation in the next thousands of years.

ESO/Digital Sky Survey 2

Panek and his colleagues searched through a catalog of large binary stars in the Milky Way, created by astronomers using the European Gaia telescope. Panek explains that the choice of wide binary stars was very restrictive. In the statistical analysis of relative velocities, only binary stars that met strict quality standards were taken into account, Panek says.

the analysis' results It came as a shock to Panek and his co-authors. The MOND theory has been refuted with overwhelming significance. Conversely, Panek says this is not evidence of the existence of dark matter. However, one can say with certainty that MOND theory is not the correct alternative for dark matter. Panek admits, while criticizing himself, that he was too optimistic in the past.

The same data leads to contradictory results

Can we forget MOND theory with this? Not quite yet. Other researchers have also evaluated Gaia data in the past few months. The result is disturbing. Both Chae Kyu-hyun from Sejong University in South Korea and Xavier Hernandez From the National Autonomous University of Mexico independently came to the conclusion that the motion of distant binary stars does not speak against the MOND theory, but in favor of it. This means that the statement is against the statement.

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How this contradiction can be resolved remains unclear. Panek and his colleagues argue that Chai was not constrained enough in selecting binary stars. The uncertainty in velocities was not correctly estimated. If you do this, the supposed MOND signal in your data will disappear. Panek suspects that a similar error may also have had an impact on Hernandez's result.

However, those who criticized are not letting it go. In one Work that has not yet been reviewed They accuse Panek and his co-authors of inconsistent statistical analysis. In addition, the authors neglected to take binary stars into account in their analysis, as they are not far from each other and therefore fall within the Newtonian system. This is necessary for correct calibration of the data.

This dispute also puzzles experts who have worked on MOND theory for years. This is what Stacy McGough of Case Western Reserve University in Cleveland admitted On his blog Triton Station, the situation gives him chills. Everyone expected him to comment on this hot topic. However, he is currently unable to do so.

Pavel Krupa of the University of Bonn, who, like McGough, represents the MOND theory, does not believe that the contradiction can be resolved quickly. In principle, an independent team is now needed to consider the contradictory analyzes impartially. Until then, Krupa sees no reason to move away from MOND theory. There are simply too many observations that could be better described by this theory than by dark matter.

On this point Panek contrasts with his former comrades. For him, MOND theory was no longer a serious alternative to dark matter. This does not prevent him To search for other alternatives. He points to massive structures in the universe such as the El Gordo galaxy group. It is difficult to explain how dark matter can grow so quickly using dark matter alone. Banek is therefore convinced of the necessity of a comprehensive theory of gravity that can be applied on very wide scales. At least on this point, Panek stayed true to himself.

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