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Dark matter could be even stranger than anyone thought, say cosmologists who suggest that this mysterious substance which makes up more than 80% of the mass of the universe could interact with itself.
“We live in an ocean of black matter, yet we know very little about what it could be, “said Flip Tanedo, assistant professor of physics and astronomy at the University of California Riverside, said in a press release.
Every attempt to explain dark matter using known physics has failed, and so Tanedo and his collaborators are developing exotic models that could better match the observations. They asked: what if dark matter interacts with itself through a continuum of forces operating in space with more dimensions than our usual three? It sounds crazy, but their model is able to better explain the behavior of stars in small galaxies than traditional, simple dark matter models. So it’s worth it.
Related: The 11 biggest unanswered questions about dark matter
Small galaxies, big problems
Even though cosmologists do not know the identity of dark matter, they do know some of its properties. All observations indicate that dark matter is made up of a new type of particle, hitherto unknown to physics. This particle floods every galaxy, representing more than 80% of their mass. This particle must not interact much with light, if at all (otherwise we would have already seen it in astronomical observations). And it must not interact much with normal matter, if at all (otherwise we would have seen it in particle collider experiments).
By taking these properties together, cosmologists are able to construct sophisticated computer simulations of the evolution of large structures in the universe. These simulations generally correspond to observations, with an interesting caveat. This simplified image of dark matter predicts that small galaxies should have very high densities of dark matter in their nucleus (known to cosmologists as the “cusp” model), but observations instead show that the density of dark matter is relatively flat, therefore the substance must be evenly distributed in the small galaxies (known as the “central model”).
This “core-cusp” problem has been a thorn in the side of dark matter studies for decades. A successful dark matter model must be able to account for the behavior of small and large galaxies, as well as all other dark matter observations. One of these models is called self-interacting dark matter, and as the name suggests, it predicts that dark matter will occasionally interact with itself, meaning that dark matter particles can sometimes bounce off each other. others or even annihilate themselves. This self-interaction smooths out regions of high density of dark matter, turning cusps into nuclei in small galaxies.
The heart of the matter
Problem solved, right? Not quite: Automatically interacting dark matter models struggle to match other observations, such as the galactic lens (when gravity a huge amount of matter distorts and amplifies the light of some galaxies behind it) and the growth of galaxies in the early universe.
However, these still poorly performing models are based on known physical interactions that take place via one of the four fundamental forces of nature. Electrons interact with each other by electromagnetic force. Quarks interact with each other through the a mighty force. Etc. But if the mere export of known physics to the realm of dark matter proves insufficient, it may be time to examine completely new forces.
Tanedo and his collaborators tried to do just that and described their work in an article published on June 1 in the Journal of High Energy Physics. Their new model dramatically expands the possible models of interacting dark matter, allowing unknown forces to come into play.
“The goal of my research program over the past two years is to extend the idea of ’talking’ dark matter to dark forces,” Tanedo said in the statement. “Over the past decade, physicists have come to realize that in addition to dark matter, hidden dark forces can govern dark matter interactions. These could completely rewrite the rules for how one should research dark matter. black matter.”
Tanedo’s approach to dark matter involves two surprising features. First, instead of a single force that binds dark matter particles together, the model includes an endless spectrum of new forces all working together. Second, the model requires an additional dimension to the universe, hence a four-dimensional space.
Think outside the universe
The infinite spectrum of forces, each represented by a new particle with a different mass, allows for great flexibility when constructing the theory of how dark matter particles might interact. And while there is no equivalent to such a theory in the everyday world of physics, astrophysicists already know that dark matter doesn’t necessarily follow the usual rules.
Related: The 12 strangest objects in the universe
In the theories that explain known physics, when two particles interact with each other, they do so by exchanging a single type of force-carrying particle. For example, two electrons bounce off each other exchanging photons, the carrier of the electromagnetic force. But this new model replaces that unique interaction with a continuum, or spectrum, of interactions, all working together to make the interaction happen.
“My research program targets one of the assumptions we make about particle physics: that the interaction of particles is well described by the exchange of more particles,” Tanedo said in the release. “While this is true for ordinary matter, there is no reason to assume this for dark matter. Their interactions could be described by a continuum of exchanged particles rather than the exchange of a single type of particle. by force. “
As for adding an extra dimension, Tanedo’s team borrowed a trick used in other theories of high-energy particle physics. Thanks to a remarkable, but not yet fully proven, concept known as AdS / CFT matching (“AdS” stands for Anti-de Sitter, which is a kind of space-time, and “CFT” stands for conformal field theory, which is a category of quantum theories), some physics problems that are extremely difficult to solve in our normal 3D space become much easier to solve when extended to four-dimensional space. .
Using this mathematical trick, Tanedo and his collaborators were able to figure out how the forces of dark matter would interact with each other. They could then translate their findings into the three dimensions of space and make predictions about how these forces would work in the real universe. They found that these forces behave very differently from the forces of nature we are used to.
“For the gravitational force or the electric force that I teach in my introductory physics class, when you double the distance between two particles, you reduce the force by a factor of four,” Tanedo said. “A continuum force, on the other hand, is reduced by a factor of up to eight.”
This modification of the self-interaction between dark matter particles has allowed researchers to build simulations that correspond to observations of small galaxies, giving them a dark matter profile of the “nucleus” type, rather than the “cusp” one. observed in traditional dark matter models. These results are similar to other automatically interacting dark matter models that also potentially reproduce “cuspy” centers, but this theory comes from a completely new theoretical direction that could have other observational consequences.
So there is a lot of work to be done. Cosmologists use dark matter to explain many different observations across the universe at a variety of scales. Further work will reveal whether this exotic theory matches the universe we see.
Originally posted on Live Science.
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