New discovery points to alternative theory of gravity

The dwarf galaxy NGC1427A flies through the Fornax galaxy cluster and undergoes turbulence that would not have been possible if this galaxy were surrounded by a heavy, stretching dark matter halo, as required by standard cosmology. Credit: ESO

The disturbances in the dwarf galaxies of one of the groups of galaxies closest to Earth suggest a different theory of gravity.

Dwarf galaxies are small, faint galaxies often found in or near larger galaxies or galaxy clusters. As a result, they can be affected by the effects of the attraction of their larger companions.

“We present an innovative way to test the Standard Model based on how much dwarf galaxies are disturbed by gravitational tides from large nearby galaxies,” said Elena Ascencio, Ph.D. Student at the University of Bonn and lead author of the story.

Tides occur when gravity from one object pulls different areas of another object differently. These are comparable to Earth’s tides, which form when the Moon exerts a stronger pull on the side of the Earth that faces the Moon.

The Fornax Cluster is home to a large number of dwarf galaxies. Recent observations indicate that many of these dwarfs appear distorted as if the cluster environment had disturbed them. “Such disturbances in Fornax dwarfs are not expected according to the standard model,” said Pavel Krupa, a professor at the University of Bonn and Charles University in Prague. “This is because, according to the Standard Model, the dark matter halos of these dwarfs should partially shield them from the tides stirred up by the mass.”

The scientists examined the expected amount of the dwarfs’ turbulence, which is determined by their internal characteristics and their distance from the gravitationally strong cluster’s center. Large, low-mass galaxies, as well as galaxies close to the center of the cluster, are easy to jam or destroy. They matched the results to the amount of turbulence shown in images taken by the European Southern Observatory’s VLT Survey Telescope.

“The comparison showed that if one wanted to explain the observations in the Standard Model,” said Elena Ascencio, “Fornax dwarfs must indeed be destroyed by gravity from the center of mass even when the tides they raise on the dwarf are sixty-four times weaker than the dwarf’s self-gravity.” This is not only self-evident, but it also contradicts previous studies, which found that the external force needed to disturb a dwarf galaxy is roughly the same as the dwarf’s self-gravitational force.

Contradiction with the Standard Model

From this, the authors conclude that, in the Standard Model, the observed morphology of Fornax dwarfs cannot be explained in a self-consistent manner. They repeated the analysis using Milgromian dynamics (MOND). Rather than assuming dark matter halos surrounding galaxies, MOND theory proposes a correction for Newtonian dynamics in which gravity experiences a boost in a low-acceleration regime.

“We were not sure that dwarf galaxies would be able to survive in the harsh environment of a galactic crowd at MOND, due to the absence of protective dark matter halos in this model – admitted Dr Indranel Panik of the University of St Andrews – “but our results show a remarkable agreement between the observations and MOND’s forecast of the Fornax dwarf disorder level.”

Aku Venhola of the University of Oulu (Finland) and Stephen Miski of the European Southern Observatory, who are co-authors of the study, said.

This is not the first time that a study examining the effect of dark matter on the dynamics and evolution of galaxies has concluded that observations are best explained when they are not surrounded by dark matter. “The number of publications showing the incompatibility between observations and the dark matter model is increasing every year. It is time to start investing more resources in more promising theories,” said Pavel Krupa, a member of the interdisciplinary research areas “Modeling” and “Matter” at University of Bonn.

Dr Hongsheng Zhao from the University of St Andrews added: “Our results have major implications for basic physics. We expect to find more disordered dwarfs in other groups, which is a prediction that other teams should verify.”

Reference: “Distribution and morphology of Fornax Cluster dwarf galaxies suggest they lack dark matter” By Elena Ascencio, Indranil Banek, Stephen Miski, Aku Venhola, Pavel Krupa and Hong Sheng Zhao, June 25, 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac1765


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