The Standard Model of Cosmology describes how the universe came to be in the opinion of most physicists. Researchers from the University of Bonn have now studied the evolution of galaxies within this model, finding considerable discrepancies with actual observations. The University of St. Andrews in Scotland and Charles University in the Czech Republic also participated in the study. The results have just been published in the Journal of Astrophysics.
Most galaxies visible from Earth look like a flat disk with a thickened center. They are therefore similar to the sports equipment of a discus thrower. According to the standard model of cosmology, however, such disks should form quite rarely. Indeed, in the model, each galaxy is surrounded by a halo of dark matter. This halo is invisible, but exerts a strong gravitational pull on nearby galaxies due to its mass. “That’s why we keep seeing galaxies merging with each other in the model universe,” says Professor Dr. Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn.
This crash has two effects, explains the physicist: “First, the galaxies enter the process, destroying the shape of the disk. Second, it reduces the angular momentum of the new galaxy created by the merger. Simply put, this dramatically decreases its rotational speed. The rotational motion normally ensures that the centrifugal forces acting during this process cause a new disc to form. However, if the angular momentum is too small, a new disc will not form at all.
Big gap between prediction and reality
In the current study, Kroupa’s PhD student Moritz Haslbauer led an international research group to study the evolution of the universe using the latest supercomputer simulations. The calculations are based on the standard model of cosmology; they show which galaxies should have formed today if this theory were correct. The researchers then compared their results with what is currently probably the most accurate observational data of the real Universe visible from Earth.
“Here we encountered a significant discrepancy between prediction and reality,” says Haslbauer: “There are apparently many more flat-disc galaxies than can be explained by theory.” However, the resolution of the simulations is limited even on today’s supercomputers. So the number of disk galaxies that would form in the standard model of cosmology may have been underestimated. “However, even if we take this effect into account, there remains a serious difference between theory and observation that cannot be corrected,” Haslbauer points out.
The situation is different for an alternative to the standard model, which does without dark matter. According to the so-called MOND theory (the acronym stands for “MilgrOmiaN Dynamics”), galaxies do not grow by merging with each other. Instead, they are formed by rotating clouds of gas that condense more and more. In a MOND universe, galaxies also grow by absorbing gas from their surroundings. However, adult galaxy mergers are rare in MOND. “Our research group in Bonn and Prague has uniquely developed the methods for performing calculations in this alternative theory,” says Kroupa, who is also a member of the transdisciplinary research units “Modelling” and “Matter” at the University. from Bonn. “MOND’s predictions are consistent with what we are actually seeing.”
Challenge for the standard model
However, the exact mechanisms of galaxy growth are still not fully understood, even with MOND. Moreover, in MOND, Newton’s laws of gravity do not apply in certain circumstances, but must be replaced by the correct ones. This would have far-reaching consequences for other areas of physics. “Nevertheless, the MOND theory solves all known extragalactic cosmological problems, although it was originally formulated to deal only with galaxies,” says Dr. Indranil Banik, who participated in this research. “Our study proves that today’s young physicists still have the opportunity to make significant contributions to fundamental physics,” Kroupa adds.