If there is one thing that scientists believe is clear about dark matter, it is that its particles could not interact with those of ordinary matter, which forms planets, stars, and galaxies, except through gravity because it does not emit any type of electromagnetic radiation.

However, researchers at the Higher International School for Advanced Studies (SISSA) in Italy have discovered evidence of a direct connection between the two types of matter for the first time.

A recent study shows the presence of direct interaction between the basic particles that make up the dark matter halo and those that make up ordinary matter, which contradicts the current prevailing hypothesis used to describe the universe.

There is a large spherical region made up of dark matter particles at the center of spiral galaxies — both those close to us and those billions of light-years away.

This zone has two distinguishing features: a constant density out to a particular radius, which somehow extends over time while the density falls. This implies the existence of a direct connection between the elementary particles that comprise the dark matter halo and the protons, electrons, neutrons, and photons that comprise ordinary matter.

We believe this idea to be in direct opposition to the present prevailing explanation of the cosmos, known as Lambda-Cold Dark Matter, which states that cold dark matter particles are inactive and interact with other particles only gravitationally.

These significant discoveries were reported in a new study that looked at a huge number of distant galaxies, some seven billion light-years away, and was recently published in the Astronomy and Astrophysics journal.

The research, led by Gauri Sharma and Paolo Salucci of SISSA and Glen Van de Ven of the University of Vienna, took a fresh look at one of contemporary physics’ greatest riddles.

This new study, according to the authors, advances our understanding of dark matter, the elusive element in our universe that has been theorized based on its demonstrable effects on celestial bodies but has yet to be explicitly verified.

This is despite the fact that numerous specialized astrophysical observations and experiments have been set up in dedicated underground laboratories for the purpose.

Dark matter research in distant galaxies

Dark matter accounts for about 84 percent of the mass in the universe: “Its dominant presence throughout the galaxies arises from the fact that the stars and hydrogen gas are moving as if governed by an invisible element” says Gauri Sharma.

Up until now, attempts to study it have focused on galaxies near to our own: “In this study, however,” continues the researcher “for the first time, we were seeking to observe and determine the distribution of the mass of spiral galaxies with the same morphology of those nearby, but much further away and therefore earlier by some 7,000 million light-years. The idea is essentially that these progenitors of spiral galaxies like our own could offer fundamental clues into the nature of the particle at the heart of the mystery of dark matter.”

Paolo Salucci adds: “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 centre of a galaxy, this halo effectively has a region in which its density is constant.”

This feature has previously been identified in studies analyzing adjacent galaxies, some of which were also the work of SISSA.

However, new study has revealed that something entirely unexpected existed in this center region within the context of the so-called “standard model of cosmology.”

Sharma adds that “as a result of the contrast between the properties of nearby and distant spiral galaxies — that is, between today’s galaxies and their forebears from seven billion years earlier, we 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.”

If the dark matter particles in the Lambda-CDM model did not interact, this evidence would be very difficult to explain.

“In the research we recently published,” according to Sharma, “we offer evidence of direct interaction between dark matter and ordinary matter, that over time slowly builds up a region of consistent density from the centre of the galaxy outwards.”

There’s more, though.

A gradual but unstoppable process

“Amazingly, the above region with constant density expands over time. It’s a very slow process, but one that is inexorable” says Salucci.

One possible explanation?

“The simplest is that, in the beginning, when the galaxy was formed, the distribution of dark matter in the spherical halo was as predicted by the Lambda-CDM theory, with a density peak in the centre. Later on, the galactic disc that characterises spiral galaxies is formed, surrounded by a halo of extremely dense dark matter particles. As time passed, the effect of the interaction that we have posited meant that the particles were captured by the stars or expelled into the outer reaches of the galaxy.”

As explained in the article “Astronomy and Astrophysics,” this process would form a spherical zone of consistent density within the dark matter halo, with dimensions that expand correspondingly over time and eventually exceed those of the galactic stellar disc.

“The results of the study pose important questions for alternative scenarios that describe dark matter particles (aside from Lambda-CDM), such as Warm Dark Matter, Self-Interacting Dark Matter and Ultra Light Dark Matter” says Sharma.

“These models must also account for the clear time evolution registered of the above region. The properties of very distant galaxies in space and time offer cosmologists a genuine gateway to understanding the mysteries of dark matter.”

It is interesting to note, “that, in line with Nietzsche’s philosophy, the truth of this mystery may be revealed not by detailing the most beautiful scenario — the one that is mathematically most elegant, simple and anticipated as an expansion of long-verified theories — but rather through an “ugly” scenario determined by an inelegant and complicated observational phenomenology, from a neglected physical theory that is completely unrelated to that which is familiar to us,” adds Salucci.

Source: 10.1051/0004-6361/202141822

Image Credit: Getty

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