Doing a simulation of galaxy formation in MOND gravity is quite difficult. But, the first attempt, with admittedly simplified assumptions, has performed well.
In cooperation with Dr. Benoit Famaey in Strasbourg, we have now simulated for the first time whether galaxies would form in a MOND universe and if so, which ones," says Kroupa's doctoral student Nils Wittenburg. To do this he used a computer program for complex gravitational calculations which was developed in Kroupa's group. Because with MOND, the attraction of a body depends not only on its own mass, but also on whether other objects are in its vicinity.
The scientists then used this software to simulate the formation of stars and galaxies, starting from a gas cloud several hundred thousand years after the Big Bang. "In many aspects, our results are remarkably close to what we actually observe with telescopes," explains Kroupa. For instance, the distribution and velocity of the stars in the computer-generated galaxies follow the same pattern that can be seen in the night sky. "Furthermore, our simulation resulted mostly in the formation of rotating disk galaxies like the Milky Way and almost all other large galaxies we know," says the scientist. "Dark matter simulations, on the other hand, predominantly create galaxies without distinct matter disks - a discrepancy to the observations that is difficult to explain."
Calculations based on the existence of dark matter are also very sensitive to changes in certain parameters, such as the frequency of supernovae and their effect on the distribution of matter in galaxies. In the MOND simulation, however, these factors hardly played a role.
From here. The paper is:
The formation and evolution of galaxies is highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies takes place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses (POR) code. These are the first-ever galaxy formation simulations done in MOND with detailed hydrodynamics, including star formation, stellar feedback, radiative transfer and supernovae. These models start from simplified initial conditions, in the form of isolated, rotating gas spheres in the early Universe. These collapse and form late-type galaxies obeying several scaling relations, which was not a priori expected. The formed galaxies have a compact bulge and a disk with exponentially decreasing surface mass density profiles and scale lengths consistent with observed galaxies, and vertical stellar mass distributions with distinct exponential profiles (thin and thick disk). This work thus shows for the first time that disk galaxies with exponential profiles in both gas and stars are a generic outcome of collapsing gas clouds in MOND. These models have a slight lack of stellar angular momentum because of their somewhat compact stellar bulge, which is connected to the simple initial conditions and the negligible later gas accretion. We also analyse how the addition of more complex baryonic physics changes the main resulting properties of the models and find this to be negligibly so in the Milgromian framework.
Nils Wittenburg, Pavel Kroupa and Benoit Famaey "The formation of exponential disk galaxies in MOND." (February 5, 2020) Astrophysical Journal, http://arxiv.org/abs/2002.01941