Observations of star clusters question the existence of dark matter

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An international team of astrophysicists has made a surprising discovery while analyzing certain star clusters. These star clusters happen to display an unexpected asymmetry in star distribution; however, this verifies the theory of modified Newtonian dynamics (MOND theory) – which imposes itself as an alternative to the concept of dark matter – more than the theory of gravity widely accepted today.

This new study is based on so-called “open” star clusters – clusters of 4 to 5 parsecs on average, from several tens to several thousand stars. These thousands of stars are born in a short time in a huge cloud of gas and are bound together by gravitational forces. These stellar clusters generally survive only a few hundred million years before dissolving: some stars move more slowly, others faster than the cluster as a whole, carrying with them part of the cloud of gas and dust.

We then observe the formation of two “tidal tails”. One of these tails extends behind the cluster as it travels through space, while the other extends forward. Newton’s laws of gravity predict that a star is as likely to be in the front tail as in the back tail; thus, the two tails should in theory contain an almost similar number of stars. However, by observing five known open clusters (the Hyades, Praesepe or the Beehive cluster, Berenike’s Hair, COIN-Gaia 13 and NGC 752), researchers have noticed that this was not the case.

Simulations in perfect agreement with observations

In the clusters we have studied, the front tail always contains many more stars close to the cluster than the back tail. explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute for Radiation Protection and Nuclear Physics. It is not particularly easy to distinguish the stars which belong to the tails of the cluster when the latter is surrounded by millions of stars; this requires evaluating the speed, direction of movement and age of each of the objects.

Dr. Tereza Jerabkova, an astronomer at the European Southern Observatory and co-author of the study, therefore developed a method — the Jerabkova-compact-convergent-point (CCP) method — to accurately count stars located in the tidal tails. Using data collected by the European Space Agency’s Gaia satellite, she and her colleagues were able to map the tails of four nearby open clusters, between 600 and 2000 million years old. This is how they discovered that the leading tails all contained more stars than the trailing tails, at least within a radius of 150 light years from the center of the cluster.

In the Hyades cluster (top), the number of stars (in black) in the forward tidal tail is significantly higher than in the back tail. In the computer simulation based on the MOND theory (below), a similar distribution emerges. © University of Bonn

By running a series of computer simulations, the team found that this asymmetry matched the predictions of modified Newtonian dynamics theory (or MOND theory) very well. ” To put it simply, according to MOND, stars can leave a cluster through two different gates. One leads to the tail tide, the other to the front. However, the first is much narrower than the second, so it is less likely that a star will leave the cluster through this gate. Newton’s theory of gravity, on the other hand, predicts that both doors should be the same width says Professor Pavel Kroupa, from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn.

An explanation for the short lifespan of open clusters

Not only did the simulations coincide perfectly with the observed stellar distribution, but they provided an explanation for another phenomenon: they indeed made it possible to determine that the lifetime of open star clusters was significantly shorter (from 20 to 50 % shorter) than would be expected based on Newton’s laws. ” This explains a long-known mystery. Namely that star clusters in nearby galaxies seem to be disappearing faster than they should Kroupa said.

Many stars and galaxies move too fast relative to their mass; thus, as early as the 1930s, scientists hypothesized that huge amounts of invisible matter — now described as dark matter — could be the cause of this phenomenon. Although no direct evidence has so far confirmed its existence, dark matter is still a widely accepted theory today.

The MOND theory was proposed in the early 1980s as an alternative to the concept of dark matter to explain why galaxies do not seem to obey currently accepted laws of physics. It relies on a modification of Newton’s law of universal gravitation at extremely low accelerations — it predicts that the effects of gravity are stronger at these accelerations than Newton’s laws suggest. If it were to be verified, this would obviously have considerable consequences for other fields of physics; according to Kroupa, it would be necessary to completely “reinvent consumerism”. This is why more evidence will be needed.

The MOND theory is still very controversial, but the team is now exploring new mathematical methods for even more accurate simulations. They could then be applied to other observations and perhaps provide new evidence for this alternative theory.

Source: P. Kroupa et al., Monthly Notices of the Royal Astronomical Society

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