Unraveling the nature of dark matter is a primary objective in modern physics, and although scientists have inferred its existence from its gravitational effects on galaxies, it remains one of the most puzzling elements of the Universe. Accounting for about 80% of all mass, this elusive substance does not emit, absorb, or reflect light, making it invisible to our telescopes.

Current popular models that describe the formation of structures within the Universe assume that whatever particles that make up dark matter interact with each other and other matter indirectly through gravity — that is, they are defined as “collisionless.” 

A team of researchers is challenging this assumption, suggesting instead that dark matter particles may actually interact with one another. This challenges the dominant theory of this mysterious entity and shakes up our understanding of the Universe.

In their study, the scientists focused on the distribution of dark matter in six dwarf satellite galaxies in the Milky Way and found discrepancies in the cluster’s properties, like unexpectedly low density in its galactic core, which contradicts with what the theory of collisionless dark matter predicts.

This, they argue, indicates inconsistencies between observational data and the most popular dark matter model, meaning there might just be an alternative theory.   

Gravitational signatures of dark matter

The gravitational forces created by dark matter within galaxies provide vital clues about its distribution. Earlier analyses published in the literature have shown that if dark matter is collisionless, its density should sharply increase towards the center of a galaxy, forming what is called a cusp. 

This occurs because in the absence of particle collisions, there are no forces acting to disperse or inhibit the movement of dark matter particles, allowing them to move freely under the influence of gravity. This would mean that  dark matter would accumulate at the center of a galaxy, where the gravitational pull is naturally the strongest. 

However, if dark matter particles do interact with one another, they may transfer energy to one another or collide, which can lead to dispersion. This interaction can prevent the particles from accumulating as tightly at the center, smoothing out the density profile and resulting in a shallower core.

In addition to their proximity to Earth, the six ultra-faint dwarf galaxies the researchers that were studied — Horologium I, Horologium II, Hydra II, Phoenix II, Sagittarius II, and Triangulum II — were chosen because the mass of their stars is hundreds to thousands of times smaller than the mass of dark matter.

The extremely low mass of stars in these galaxies, around 1,000 to 10,000 times the mass of the Sun, means that stellar activity doesn’t significantly alter the dark matter’s gravitational influence, making it much easier to study.

In contrast, when the contribution of ordinary matter is significant, say in a solar system with large stars, it can change the shape of dark matter halos, sometimes smoothing out the dense central region into a flatter core. This process has been suggested to explain the dark matter halos seen in other dwarf galaxies.

Using data from the Hubble Space Telescope, the team analyzed the distribution of stars in these galaxies to estimate the gravitational field generated by dark matter, and found the gravitational fields of these galaxies were consistent with a cored profile, not the cusp predicted by the conventional collisionless dark matter model.

“Six small [ultrafaint dwarf] galaxies do not reside in [collisionless dark matter gravitational] potentials, a conclusion supported with a confidence level greater than 97%,” the scientists wrote in their paper. “Simultaneously, the observed [galaxies] are consistent with potentials with an inner core as predicted by many alternatives to [collisionless dark matter theory].”

While the results raise questions about the adequacy of the collisionless dark matter model, more data is required before definitive conclusions can be made. Examining only six galaxies, though insightful, is insufficient to entirely dismiss the Standard Model.

More extensive observations and larger-scale data analysis will be necessary to resolve the issue and clarify the composition and properties of dark matter.

Implications for the future of dark matter research

These findings suggest that dark matter could be more intricate than previously believed. The Standard Model of collisionless dark matter has effectively explained many large-scale cosmic phenomena, such as the formation of galaxies, galaxy clusters, and the distribution of cosmic structures.

However, this new evidence indicates that dark matter particles might interact with each other in ways that go beyond the simple gravitational effects typically assumed. 

These interactions could potentially influence the dynamics of dark matter within galaxies and alter how galaxies form and evolve on smaller scales, hinting at a deeper and more nuanced understanding of dark matter’s role in shaping the Universe.

“To sum up, this work shows that six [ultrafaint dwarf] galaxies reside in cored gravitational potentials,” they concluded. “Since stellar feedback should be inoperative […], the best explanation seems to be that the [dark matter] deviates from the nature assumed in the standard [collisionless dark matter] cosmological model.

“The Standard Model provides an extremely good approximation to reality but is likely not the last theory. Studying the kind of galaxies analyzed here may provide a gateway to go beyond.”

Reference: Jorge Sánchez Almeida, et al, The Stellar Distribution in Ultrafaint Dwarf Galaxies Suggests Deviations from the Collisionless Cold Dark Matter Paradigm, The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad66bc

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