Distorted galaxies challenge the theory of gravity

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An international team of astronomers recently studied the Fourneau cluster galaxies to test the ΛCDM model (or Lambda CDM) – the standard model that describes the universe as homogeneous and isotropic, consisting of ordinary matter, dark matter and dark energy – and modified Newtonian dynamics ( or MOND), a controversial alternative to general relativity. They identified disturbances in the cluster’s dwarf galaxies that suggest a different theory of gravity.

The majority of astronomers believe that the mysterious dark matter, which is the basis of the ΛCDM model, is the only way to account for certain phenomena, such as the mass and velocity of galaxies, or simply their coherence. This standard model of cosmology implies that most galaxies are surrounded by a halo of dark matter particles capable of exerting a strong gravitational pull on nearby objects. Despite decades of research, the existence of dark matter has never been proven, and various alternative theories have been proposed.

One of them is the theory of modified Newtonian dynamics (or MOND theory). To test these different models, a team of astronomers took an interest in the Fourneau cluster of dwarf galaxies, located about 62 million light-years from Earth. Dwarf galaxies are small, faint galaxies that are often located inside or near larger galaxies or galaxy clusters. Because of their low surface luminosities, they are particularly sensitive to tidal forces generated by more massive galaxies; they are therefore perfect objects of study to test the theories of gravity.

Dwarf galaxies without dark matter?

We present an innovative way to test the Standard Model based on the extent to which dwarf galaxies are perturbed by “gravity” from nearby large galaxies “Explains in a press release Elena Asencio, PhD candidate at the University of Bonn and first author of the study reporting the discovery. The degree of disturbance expected for these galaxies depends on the presumed law of gravity and the presence or absence of dark matter.

Observations suggest that several of the cluster’s dwarf galaxies had undergone severe deformations, as if the cluster’s environment had disturbed them. However, this is completely at odds with the standard model, because in theory the dark matter halos of dwarf galaxies should partly protect them from the tidal forces generated by the surrounding galactic cluster.

The team first looked at the expected level of perturbation from the dwarfs: this can be determined by their internal properties and their distance from the cluster’s powerful center of gravity. For example, large galaxies with low stellar mass, as well as galaxies near the center of the cluster, are more easily disrupted or even destroyed. The researchers compared their results with the size of the disturbances observed in images taken by the European Southern Observatory’s Very Large Telescope. This is where an inconsistency arose.

The comparison showed that if one wants to explain the observations with the Standard Model, the Fourneau cluster dwarfs should already be destroyed by gravity at the center of the cluster, even when the tide traveling on a dwarf is 64 times weaker than the gravity of the dwarf itself! “, explains Elena Asencio. The researcher points out that this observation is not only counterintuitive, but contradicts several studies which have shown that the external force needed to disrupt a dwarf galaxy is roughly equivalent to its own gravity.

The standard model repeatedly questioned

If these dwarf galaxies do not have halos of dark matter, how did they form? And above all, how were they able to resist the surrounding forces of attraction? Since the standard model was not suitable to explain the observed phenomenon, the team undertook to apply another theory, the MOND theory, which is based on a modification of Newton’s second law (which relates the mass of an object and the acceleration it receives , whose forces are applied to it). It eliminates the presence of dark matter haloes, but offers a correction whereby gravity is “amplified” at very low accelerations.

This pattern turned out to be much more consistent. ” Our results show a remarkable agreement between observations and MOND expectations for the level of perturbation of furnace dwarfs said Dr. Indranil Banik, a researcher at the University of St. Andrews and co-author of the study.

The team points out that this is not the first time that a study testing the effect of dark matter on the dynamics and evolution of galaxies has challenged the theory. ” The number of publications showing inconsistencies between observations and the dark matter paradigm continues to increase every year. It is time to start investing more resources in more promising theories said Dr. Pavel Kroupa, who heads the research group Stellar Populations and Stellar Dynamics at the University of Bonn.

These results obviously have major implications for fundamental physics. The team continues its investigations and expects to find other highly disturbed dwarf galaxies in other galactic clusters, inviting other research teams to explore this avenue.

Source: E. Asencio et al., Monthly Notices of the Royal Astronomical Society

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