Dark matter behavior may conflict with our best theory of the universe

New research shows a direct interaction between dark matter particles and those that make up ordinary matter.
Paul Ratner

A new paper, published in the Astronomy and Astrophysics journal, discovered unexpected characteristics for the elusive dark matter that likely goes against our best theory of the universe – the Lambda-Cold Dark Matter model. 

What is dark matter?

Dark matter is a hypothetical form of matter that scientists believe makes up about 27 percent of all matter in the universe and accounts for about 84 percent of its mass, according to NASA. We haven’t been able to detect this type of matter directly yet as it does not emit electromagnetic radiation or scatter light. Still, scientists have inferred its existence from its gravitational effects. 

The Lambda-Cold Dark Matter model (ΛCDM), which came to the forefront in the late 1990s, considers various observations of the universe that previously appeared inconsistent and tries to create consensus explanations on the universe's energy breakdown, including the nature and role of dark matter. Based on the assumption that consensus theories regarding the Big Bang and general relativity are correct, the ΛCDM is considered the simplest model, describing such features as the cosmic microwave background (CMB), large-scale structure, and accelerating acceleration expansion of the universe, supernovae and more. 

The Lambda-CDM model suggests that dark matter particles are slow, cold, and inert and do not interact with other particles other than gravitationally. But suppose the team of scientists behind the new study is correct. In that case, there are huge, sphere-like regions in distant space, such as at the center of spiral galaxies, that are "taken over" by dark matter particles which directly interact with ordinary matter like protons, electrons, neutrons, and photons. 

Dark matter behavior may conflict with our best theory of the universe

The Lambda CDM Model of Cosmoslogy via NASA/ LAMBDA Archive / WMAP Science Team

Looking back in time and space

To reach their conclusion, Italian scientists Gauri Sharma and Paolo Salucci from SISSA (International School of Advanced Studies) and Glen Van de Ven from the University of Vienna analyzed a sizable group of distant galaxies with some as far as seven billion light-years from Earth. Studying objects so far away gave the scientists an understanding of how they appeared way back in the earlier days of the universe. 

Sharma explained in a press release that the mystery of dark matter “arises from the fact that the stars and hydrogen gas are moving as if governed by an invisible element". So far, most studies of dark matter have looked at nearby galaxies. Still, the work from Sharma’s group zeroed in on galaxies that were progenitors of spiral galaxies like our Milky Way to understand if they could provide essential clues about the particles “at the heart of the mystery of dark matter.” Paolo Salucci elaborated further that "by studying the movement of stars in approximately 300 distant galaxies, we discovered that these objects also had a halo of dark matter, and that, by starting out from the center of a galaxy, this halo effectively has a region in which its density is constant.” 

Dark matter behavior may conflict with our best theory of the universe

Spiral galaxies feature a "vast spherical region" of dark matter particles with density that changes as the region expands over time. Via Gauri Sharma

Going against the Standard Model of Cosmology

What they found in that central region of these faraway galaxies was unexpected to the scientists and may go against the established notions of the standard model of cosmology. By contrasting the known traits of the spiral galaxies that are closer to us with the distant “forebear” galaxies, essentially comparing their evolution over time, the scientists “could see that not only is there an unexplained region with a constant density of dark matter, but also that its dimensions increase over time as if being subjected to a process of ongoing expansion and dilution,” shared Sharma. 

This observation suggests that dark matter particles interact in some ways with regular visible or “baryonic” matter, which is at odds with the established Lambda-CDM model. Over time, this interaction would have created “a region of consistent density from the galaxy's center outwards." The gradual expansion of this region, as theorized by the scientists, can be explained by a rather simple process," explained Salucci.

One explanation is that the observed spiral galaxies could have started with dark matter distributed in the spherical halo as predicted by the Lambda-CDM theory, featuring a density peak in the center. Later, as the galactic disc that characterizes spiral galaxies was eventually formed, it was surrounded by "a halo of extremely dense dark matter particles." The interaction between dark matter and baryonic particles led to particles being “captured by the stars or expelled into the galaxy's outer reaches."

Such a process could be responsible for the shape and density of the regions observed by the scientists, leading to a potential rethink of what we know about dark matter. Suppose there is an interaction of the kind the paper suggests. In that case, alternative dark matter versions could consist of different types of dark matter, including “Warm Dark Matter, Self-Interacting Dark Matter, and Ultra Light Dark Matter,” posited Sharma.

Future research, including observation from the James Webb Telescope, is necessary to prove further the ideas proposed by the scientists and hopefully resolve the true nature of the enigmatic dark matter. The search is also likely to be helped by CERN’s Large Hadron Collider, which is going back online after a prolonged shut down for maintenance to search for dark matter, among other mysterious particles.

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