Does dark matter follow the same laws as ordinary matter? The mystery surrounding this invisible and hypothetical component of our Universe, which neither emits nor reflects light, remains complete.
A research team led by the University of Geneva (UNIGE) wanted to know whether, on a cosmological scale, this matter "falls" in the same way as ordinary matter or if other forces come into play. Their results, published in
Nature Communications, suggest similar behavior, while leaving open the possibility of an as-yet-unknown interaction. This advance sheds a little more light on the properties of this elusive matter, which is five times more abundant than ordinary matter.
Ordinary matter obeys four well-known forces: gravitation, electromagnetism, the strong force, and the weak force, the last two acting on an atomic scale. But what about dark matter? Invisible and elusive, it could be subject to the same laws or governed by a fifth, still unknown force.
To unravel this mystery, a team led by UNIGE asked whether, on the scale of the cosmos, this matter "falls" in the same way as ordinary matter into gravitational wells. Under the influence of the gravitation of celestial bodies, the space occupied by our Universe deforms, generating "wells." Ordinary matter—here planets, stars, or galaxies—falls into them according to well-known physical laws, such as Einstein's famous theory of general relativity or the Euler equations. But what about dark matter?
"To answer this question, we compared the speeds at which galaxies move in the Universe with the depth of the gravitational wells," explains Camille Bonvin, associate professor in the Department of Theoretical Physics in the Faculty of Science at UNIGE, co-author of the study.
"If dark matter is not subject to a fifth force, then galaxies—which are mostly made of dark matter—will fall into the wells like ordinary matter, subject only to gravitation. On the other hand, if there is a fifth force acting on dark matter, it will influence the motion of galaxies, which will consequently fall into the wells in a different way. By comparing the depth of the wells with the speed of the galaxies, we can therefore test for the presence of such a force."
Map of the distribution of galaxies observed by the DESI collaboration, from which it is possible to accurately measure the speeds of galaxies.
Credit: Claire Lamman/DESI collaboration; custom colormap package by cmastro. Euler Equations Still Valid
By applying this approach to current cosmological data, the research team concluded that dark matter falls into gravitational wells in the same way as ordinary matter. And that it thus responds to the Euler equations.
"At this stage, however, these conclusions do not allow us to rule out the presence of an unknown force. But if this fifth force exists, it cannot be more than 7% of the strength of gravitation, in which case it would have appeared in our analyses," says Nastassia Grimm, first author of the study and former postdoctoral researcher in the Department of Theoretical Physics in the Faculty of Science at UNIGE, who recently joined the Institute of Cosmology and Gravitation at the University of Portsmouth.
These initial results mark a major advance in characterizing mysterious dark matter. The next step will be to clarify the existence, or not, of a fifth force governing it.
"Future data from the latest experiments, such as LSST and DESI, will be sensitive to a force as small as 2% of gravitation. They should therefore allow us to learn even more about the behavior of dark matter," concludes Isaac Tutusaus, researcher at ICE-CSIC and IEEC, associate professor at IRAP, Midi-Pyrénées Observatory, University of Toulouse, co-author of the study.