The Standard Model of Cosmology has described how the universe came into being according to the view of most physicists. Now, researchers at the University of Bonn studied the evolution of galaxies within this model and found considerable discrepancies with actual observations.
The University of St. Andrews in Scotland and Charles University in the Czech Republic were also involved in the study. The results were published in the 'Astrophysical Journal'.
Most galaxies visible from Earth resemble 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 rather rarely. This was because, in the model, every galaxy was surrounded by a halo of dark matter. This halo was invisible, but exerted 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," explained Prof. Dr Pavel Kroupa of the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn.
This crash has two effects, the physicist explained, "First, the galaxies penetrate in the process, destroying the disk shape. Second, it reduces the angular momentum of the new galaxy created by the merger." Put simply, this greatly decreased its rotational speed. The rotating motion normally ensured that the centrifugal forces acting during this process cause a new disk to form. However, if the angular momentum is too small, a new disk will not form at all.
In the current study, Kroupa's doctoral student, Moritz Haslbauer, led an international research group to investigate 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 by 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," said Haslbauer.
He added, "There are apparently significantly more flat disk galaxies than can be explained by theory." However, the resolution of the simulations is limited even on today's supercomputers. It may therefore be that the number of disk galaxies that would form in the Standard Model of Cosmology has been underestimated. "However, even if we take this effect into account, there remains a serious difference between theory and observation that cannot be remedied", Haslbauer pointed out.
The situation was different for an alternative to the Standard Model, which dispensed with dark matter. According to the so-called MOND theory (the acronym stands for "MilgrOmiaN Dynamics), galaxies did not grow by merging with each other. Instead, they formed from rotating gas clouds that became more and more condensed. In a MOND universe, galaxies also grew by absorbing gas from their surroundings. However, mergers of full-grown galaxies are rare in MOND. "Our research group in Bonn and Prague has uniquely developed the methods to do calculations in this alternative theory," said Kroupa, who is also a member of the Transdisciplinary Research Units "Modelling" and "Matter" at the University of Bonn. "MOND's predictions are consistent with what we actually see."
However, the exact mechanisms of galaxy growth are not yet fully understood, even with MOND. Additionally, in MOND, Newton's laws of gravity did not apply under certain circumstances, but needed to 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 despite being originally formulated to address galaxies only," said Dr Indranil Banik, who was involved in this research. "Our study proves that young physicists today still have the opportunity to make significant contributions to fundamental physics," Kroupa added.
( With inputs from ANI )
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