The gravitational force in the Universe under which it has evolved from an almost uniform state at the Big Bang until now, when matter is concentrated in galaxies, stars and planets, is provided by what is called the “dark matter”.
But despite the essential role that this additional material plays, we know almost nothing about its nature, behavior and composition, which is one of the fundamental problems in modern physics. In a recent article by Letters of Astronomy and Astrophysics, scientists from the Instituto de Astrofísica de Canarias (IAC) / University of La Laguna (ULL) and the National University of the Northwest of the Province of Buenos Aires (Junín, Argentina) have shown that the dark matter of galaxies follows a “maximum entropy” distribution, which informs its nature.
Dark matter makes up 85% of the matter in the Universe, but its existence only appears on astronomical scales. That is, due to its weak interaction, the net effect can only be noticed when it is present in huge amounts. Since it does not cool easily, the structures it forms are usually much larger than planets and stars. As the presence of dark matter appears only on a large scale, the discovery of its nature must probably be made by astrophysical studies.
To say that the distribution of dark matter is organized according to maximum entropy (which is equivalent to “maximum disorder” or “thermodynamic equilibrium”) means that it is in its most probable state. To achieve this “maximum disorder”, dark matter had to collide within itself, just like gas molecules, in order to reach the equilibrium in which its density, pressure and temperature are related. However, we don’t know how dark matter reached this kind of equilibrium.
“Unlike molecules in the air, for example, because the gravitational action is weak, dark matter particles should hardly collide with each other, so that the mechanism by which they reach equilibrium is a mystery, ”says Jorge Sánchez Almeida, researcher at IAC who is the first author of the article. “However, if they did collide, it would give them a very special nature, which would partly solve the mystery of their origin,” he adds.
The maximum dark matter entropy has been detected in dwarf galaxies, which have a higher dark matter to total matter ratio than more massive galaxies, so it’s easier to see the effect. However, researchers expect this to be general behavior in all types of galaxies.
The study implies that the distribution of matter in thermodynamic equilibrium has a much lower central density than astronomers have assumed for many practical applications, such as in the correct interpretation of gravitational lenses, or when designing experiments for detect dark matter by its self-annihilation.
This central density is fundamental for the correct interpretation of the curvature of light by gravitational lenses: if it is less dense, the effect of the lens is less. To use a gravitational lens to measure the mass of a galaxy, you need a model, if that model is changed, the measurement changes.
Core density is also very important for experiments that attempt to detect dark matter using its self-annihilation. Two particles of dark matter could interact and disappear in a process highly improbable, but which would be characteristic of their nature. For two particles to interact, they must collide. The probability of this collision depends on the density of the dark matter; the higher the concentration of dark matter, the higher the probability that the particles collide.
“For this reason, if the density changes, so will the expected rate of production of self-annihilations, and since the experiments are designed on the prediction of a given rate, if that rate was very low, the experiment is unlikely to produce a positive result, ”says Sánchez Almeida.
Finally, the thermodynamic equilibrium of dark matter could also explain the luminosity profile of galaxies. This luminosity decreases with distance from the center of a galaxy in a specific way, the physical origin of which is unknown, but for which researchers are working to show that it is the result of a balance at maximum entropy.
SIMULATION VERSUS OBSERVATION
The density of dark matter at the center of galaxies has been a mystery for decades. There is a large gap between the predictions of the simulations (a high density) and what is observed (a low value). Astronomers have proposed many types of mechanisms to resolve this major disagreement.
In this article, the researchers showed, using basic physical principles, that the observations can be reproduced by assuming that dark matter is in equilibrium, that is, it has maximum entropy. The consequences of this result could be very important because they indicate that dark matter has interchanged its energy with itself and / or with the remaining “normal” (baryonic) matter.
“The fact that equilibrium was reached in such a short time, relative to the age of the Universe, could be the result of some type of interaction between dark and normal matter in addition to the gravity, ”suggests Ignacio Trujillo, IAC researcher and co-author of this article. “The exact nature of this mechanism needs to be explored, but the consequences could be fascinating to understand exactly what is this component that dominates the total amount of matter in the Universe.”
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