Scientists have made a groundbreaking discovery about dark matter halos that challenges our understanding of the Universe and provides an alternative theory of gravity.
The Standard Model of cosmology suggests that galaxies are predominantly surrounded by a halo of dark matter particles. These dark matter halos are invisible, but their mass exerts a strong gravitational pull on nearby galaxies.
However, a new study led by the University of Bonn and the University of Saint Andrews offers an alternative view to this theory. Their research found that dwarf galaxies in the second closest galaxy cluster to Earth, the Fornax Cluster, have no dark matter halos.
The Kiln Cluster
Dwarf galaxies are tiny, faint galaxies that are usually found in galaxy clusters or near larger galaxies, meaning they can be affected by the gravitational effects of their larger neighbors.
Elena Asencio, PhD student at the University of Bonn and lead author of the research, commented: “We introduce an innovative way to test the standard model based on how much dwarf galaxies are perturbed by the gravitational tides of nearby larger galaxies.” .
Gravitational tides occur when one body’s gravity pulls differently on different parts of another, similar to Earth’s tides caused by the Moon pulling more strongly on the side of the Earth that faces it . The Fornax Cluster is rich in dwarf galaxies; however, recent observations show that some appear distorted as if disturbed by the cluster’s environment.
Pavel Kroupa, a professor at the University of Bonn and Charles University in Prague, said: “These perturbations are not expected in Fornax dwarfs according to the standard model. This is because, according to the standard model, haloes of matter dark of these dwarfs should partially protect them from the tides raised by the cluster.”
New insights into dark matter halos
The researchers looked at the expected level of disturbance of the dwarfs, which depends on their internal properties and the distance to the cluster’s gravitationally powerful center. Large galaxies with low stellar masses and galaxies near the center of the cluster are more easily disturbed or destroyed. The team compared these results with the level of disturbance observed from images obtained by the European Southern Observatory’s VLT Survey Telescope.
Asencio said: “The comparison showed that, if the Standard Model observations are to be explained, the Fornax dwarfs should already be destroyed by gravity from the center of the cluster, even when the tides that lift on a dwarf are sixty-four times weaker than the dwarf’s self-gravity.
“This is not only counterintuitive, but also contradicts previous studies, which found that the external force required to perturb a dwarf galaxy is roughly the same as the dwarf’s self-gravity.”
The researchers claimed that the Standard Model could not explain the morphologies observed in dwarfs in a consistent way. The team then re-ran the analysis using Milgromian dynamics (MOND). Interestingly, instead of assuming halos of dark matter surrounding galaxies, MOND theory suggests a correction to Newtonian dynamics where gravity experiences an increase in the regime of low accelerations.
Dr Indranil Banik from the University of St Andrews explained: “We were not sure that dwarf galaxies could survive in the extreme environment of a galaxy cluster in MOND, due to the absence of protective dark matter halos in this model, but our The results show a remarkable agreement between observations and MOND expectations for the level of perturbation of the Fornax dwarfs.
This is not the first time that a study looking at the effect of dark matter on the dynamics and evolution of galaxies has found that the observations were better explained when they are not surrounded by dark matter.
Kroupa commented: “The number of publications showing incompatibilities between observations and the dark matter paradigm continues to increase every year. It is time to start investing more resources in more promising theories.”
Dr Hongsheng Zhao from the University of St Andrews concluded: “Our results have important implications for fundamental physics. We expect to find more perturbed dwarfs in other clusters, a prediction that other teams should verify.”