Astrophysicists have observed some puzzling behavior in star clusters that seem to defy our current understanding of gravity at cosmic scales. Intriguingly, the observations fit with an alternative theory of gravity that could negate the need for dark matter.
Although it’s since been superseded by Einstein’s theory of general relativity, Newton’s law of universal gravitation still holds pretty well as an explanation for the large-scale structure and movements of the universe. But now, new observations have been made that don’t quite fit these currently accepted models.
An international team of astrophysicists had been investigating open star clusters, which contain thousands of young stars being born from a large cloud of dust and gas. These clusters have a relatively short lifespan before they dissolve, as the stars drift into two “tails” – one in front of the cluster and one behind.
“According to Newton's laws of gravity, it's a matter of chance in which of the tails a lost star ends up,” said Dr. Jan Pflamm-Altenburg, co-author of the study. “So both tails should contain about the same number of stars. However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail.”
In the past it’s been tricky to determine which of a cluster’s stars belong to which tail, but the researchers on the new study developed a method to do so. They call it the Jerabkova-compact-convergent-point (CCP) method, and this was applied to data on four open star clusters gathered by surveys like the Gaia mission. To their surprise they found that in all four clusters, the leading tail had far more stars than the trailing one, in an apparent contradiction of Newton’s laws.
So, the team then simulated the movements of stars in these clusters according to a different hypothesis, known as Modified Newtonian Dynamics (MOND). Essentially, this model suggests that gravity’s effects are stronger at low accelerations than they are in Newton’s laws. And intriguingly, this model’s predictions lined up very well with the observations.
“Put simply, according to MOND, stars can leave a cluster through two different doors,” said Professor Pavel Kroupa, first author of the study. “One leads to the rear tidal tail, the other to the front. However, the first is much narrower than the second – so it’s less likely that a star will leave the cluster through it. Newton's theory of gravity, on the other hand, predicts that both doors should be the same width.”
This isn’t the only way in which the MOND model fits real-world observations better. Star clusters in nearby galaxies have been found to be dissolving faster than Newton’s laws predict – but this would be a natural by-product of MOND.
Another major implication of MOND could shake up astrophysics as we know it – if it was true, then dark matter wouldn’t exist. This mysterious substance was conjured up in the 1930s to explain discrepancies in the observed motion of stars and galaxies, which were seen to be moving much too fast for how much mass they apparently contained. Dark matter fills the gap by adding huge amounts of invisible mass, which scientists have been searching for ever since. Decades of experiments designed to detect dark matter particles have come up empty.
Still, dark matter is the prevailing theory, because it does a very good job of explaining many observed features of the universe and there’s plenty of other evidence that points to its existence. Although there has been other observational evidence supporting MOND, it remains a fringe hypothesis that isn’t widely accepted by the scientific community.
The researchers on the new study are currently exploring other methods to produce more accurate simulations, which could then be applied to other astronomical objects to find more evidence for or against MOND.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
Source: University of Bonn
