Astrophysicists make observations consistent with predictions of an alternative theory of gravity

An international team of astrophysicists has made a puzzling discovery while analyzing certain star clusters. The discovery defies Newton’s laws of gravity, the researchers write in their publication. Instead, the observations are consistent with the predictions of an alternative theory of gravity. However, this is controversial among experts. The results have just been published in the Royal Astronomical Society Monthly Notices.

In their work, the researchers studied open star clusters. These form when thousands of stars are born in a short time in a huge cloud of gas. As they “ignite”, the galactic newcomers carry away the remnants of the gas cloud. In the process, the cluster grows significantly larger. This creates a loose formation of several tens to several thousand stars. The weak gravitational forces acting between them maintain the cohesion of the cluster.

“In most cases, open star clusters only survive a few hundred million years before dissolving,” says Professor Pavel Kroupa of the Helmholtz Institute for Nuclear and Radiation Physics at the University of Good. In the process, they regularly lose stars, which accumulate in two so-called “tidal tails”. One of these tails is pulled behind the cluster as it travels through space. The other, on the contrary, takes the lead like a spearhead.

“According to Newton’s laws of gravity, it’s a matter of luck in which tails a lost star ends up,” says Dr Jan Pflamm-Altenburg of the Helmholtz Institute for Radiation and Nuclear Physics. “Thus, both tails should contain approximately 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 tail front always contains many more stars close to the cluster than the rear tail.”

New method developed to count stars

Until now, it was almost impossible to determine among the millions of stars close to a cluster which belonged to its tails. “To do this, you need to look at the speed, direction of motion, and age of each of these objects,” says Dr. Tereza Jerabkova. The co-author of the article, who did her doctorate in Kroupa’s group, recently left the European Space Agency (ESA) for the European Southern Observatory in Garching. She developed a method that allowed her to accurately count the stars in the tails for the first time.

“So far, five open clusters have been studied near us, four of them by us,” she says. “When we analyzed all the data, we encountered the contradiction with the current theory. The highly accurate survey data from ESA’s Gaia space mission was indispensable for this.”

Observational data, on the other hand, fits much better with a theory that goes by the acronym MOND (“MOdified Newtonian Dynamics”) among experts. “In simple terms, according to MOND, stars can leave a cluster through two different gates,” Kroupa explains. “One leads to the back tidal tail, the other to the front. However, the former is much narrower than the latter – so a star is less likely to leave the cluster through it. Newton’s theory of gravity, on the other hand, predicts that both doors should be the same width.”

Star clusters have shorter lifespans than Newton’s laws predict

The team calculated the expected stellar distribution according to MOND. “The results agree surprisingly well with the observations,” emphasizes Dr. Ingo Thies, who played a key role in the corresponding simulations. “However, we had to resort to relatively simple computational methods to do this. We currently lack the mathematical tools for more detailed analyzes of modified Newtonian dynamics.”

Nevertheless, the simulations also coincided with the observations in another respect: they predicted how long open star clusters should generally survive. And this time frame is significantly shorter than what one would expect according to Newton’s laws. “This explains a long-known mystery,” Kroupa points out. “Namely, star clusters in nearby galaxies appear to be disappearing faster than they should.”

However, the MOND theory is not undisputed among experts. Since Newton’s laws of gravity would not be valid under certain circumstances, but would have to be modified, this would also have far-reaching consequences for other areas of physics. “Again, this solves many of the problems that cosmology faces today,” says Kroupa. The team is now exploring new mathematical methods for even more accurate simulations. They could then be used to find further evidence as to whether the MOND theory is correct or not.