At the center of spiral galaxies—those near to us but also those billions of
light-years away—there is a vast spherical region made up of dark matter
particles. This region has two defining characteristics: a density that is
constant out to a certain radius that amazingly expands over time, while the
density decreases. This suggests the existence of a direct interaction
between the elementary particles that make up the dark matter halo and those
that make up ordinary matter—protons, electrons, neutrons, and photons. We
anticipate that this hypothesis is in direct conflict with the current
prevailing theory used to describe the universe—known as Lambda-Cold Dark
Matter—which posits that particles of cold dark matter are inert and do not
interact with any other particle except gravitationally.
These important findings have been reported in a new study, recently
published in the Astronomy and Astrophysics journal, that studied a large
number of distant galaxies, some seven billion light-years away. The study,
conducted by Gauri Sharma and Paolo Salucci from SISSA, together with Glen
Van de Ven from the University of Vienna, took a new look at one of the
greatest mysteries of modern physics. According to the authors, this new
research represents a step forward in our understanding of dark matter, the
elusive element in our universe which has been theorized based on its
demonstrable effects on heavenly bodies, but which is yet to be directly
proven. This is despite any number of targeted astrophysical observations
and experiments set up for the purpose in dedicated underground
laboratories.
Studying dark matter in distant galaxies
Dark matter makes up approximately 84% of the mass in the cosmos: "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"
explains Gauri Sharma. Up until now, attempts to study it have focused on
galaxies near to our own: "In this study, however," she explains, "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 seven billion 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 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." This trait had already been observed in studies
examining nearby galaxies, some of which were also the work of SISSA.
The new research has revealed, however, that this central region had
something that was wholly unexpected within the context of the so-called
"standard model of cosmology." Sharma says 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."
This evidence is very difficult to be explained if the dark matter particles
did not interact, as posited in the Lambda-CDM model. "In the research we
recently published," says 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 center of the galaxy outwards." But
there's more.
A slow yet inexorable process
"Amazingly, the above region with constant density expands over
time. It's a very slow process, but one that is inexorable" states 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 center.
Later on, the galactic disc that characterizes 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." This process would create a spherical region of
consistent density within the dark matter halo, with dimensions that
increase proportionately over time and finally reach those of the galactic
stellar disc, as described in the article in Astronomy and Astrophysics.
"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," says Salucci.
Reference:
G. Sharma et al, Observational evidence of evolving dark matter profiles since
z 1, Astronomy & Astrophysics (2022).
DOI: 10.1051/0004-6361/202141822