The strange properties of dark matter may be beyond anyone’s imagination. Cosmologists believe this mysterious mass, which makes up more than 80 percent of the mass of the universe, can interact with each other, according to a new simulation study.
Dark matter particles may interact with each other
“We live in a sea of dark matter, but we know very little about what it might be,” study co-author Flip Tanedo, assistant professor of physics and astronomy at the University of California, Riverside, said in a statement.
Every attempt to explain dark matter with known physics has failed over the past few decades, so Tanedo and his collaborators hope to develop better models to match observations more precisely. One question they asked was what if dark matter interacted with itself with continuous forces in a more multidimensional space than three dimensions? It sounds crazy, but the model they’ve built does a better job of explaining the behavior of stars in small galaxies than the traditional simple dark matter model. So, this question is worth exploring further.
Small galaxy, big problem
Although cosmologists do not yet know the “identity” of dark matter, they do know some of its basic properties. All the observations suggest that dark matter is made of some new particle that is not known to physics. The particles fill every galaxy and make up more than 80 percent of the mass of the universe. They don’t interact much with light, and if they did, we might already see them in astronomical observations. And the particles certainly don’t interact significantly with normal matter, as we might expect to see them in LHC experiments if they did.
Combining these properties allows cosmologists to build sophisticated computer simulations to reconstruct the evolution of large structures in the universe. These simulations generally agree with observations, but there is an interesting caveat. This simplified dark matter model predicts that the cores of small galaxies should be very dense dark matter, which cosmologists call the “cusP” model; However, current observations suggest that the density of dark matter is relatively flat, so the mysterious material must be evenly dispersed throughout the small galaxy, a model known as the “core” model.
This peak-core problem has puzzled researchers in the dark matter field for decades. A successful dark matter model must be able to explain the behavior of galaxies large and small, as well as other observations of dark matter. One popular explanation for this problem is the Self-interacting Dark Matter model (SIDM). As the name suggests, the model predicts that dark matter will occasionally interact with itself, which means dark matter particles will sometimes collide with each other and even annihilate. This self-interaction flattens regions of high dark matter density, turning “spikes” into “cores” in small galaxies.
The heart of the matter
So, has the problem been solved? Not entirely, because the self-interacting dark matter model has trouble matching other observations, such as galactic lensing, where the gravity of a massive object distorts and magnifies light from a particular galaxy behind it, and the growth of galaxies in the early universe.
However, these notably flawed models are based on known physical interactions that operate through one of nature’s four fundamental forces. For example, electrons interact through electromagnetic forces, quarks interact through strong forces, and so on. But simply introducing known physics into the dark matter realm is not enough. Perhaps it is time to investigate entirely new forces.
Tanedo and his collaborators have tried to do just that, describing their work in a paper published today. Their new model greatly expands the possibilities for dark matter interactions and allows some unknown forces — known as the “dark force” — to come into play.
“For the past two years, the goal of my research project has been to expand the idea that dark matter ‘talks’ to the dark force,” Tanedo said in the statement. “Over the past decade, physicists have begun to realize that in addition to dark matter, the hidden dark force may control dark matter interactions. These dark forces could completely rewrite the rules for looking for dark matter.”
Tanedo et al. ‘s dark matter research contains two unexpected features. First, the model includes myriad new forces that work together, rather than a single force connecting dark matter particles. Second, the model needed to add a dimension to the universe, that is, instead of the familiar three dimensions, the universe would have a magical four.
Think outside the normal universe
The myriad new forces, each represented by a new particle of different mass, provide a great deal of flexibility in building theories about how dark matter particles interact. While there are no comparable theories in conventional physics, astrophysicists are well aware that dark matter doesn’t necessarily follow the usual rules.
In theories that explain known physical phenomena, two particles interact by exchanging a particle that carries a force. For example, two electrons collide by exchanging photons (carriers of electromagnetic force). Instead of a single interaction, however, the new model uses a continuum of interactions; All of these interactions work together to allow the self-interaction of dark matter particles to occur.
“My research project addresses an assumption we have about particle physics: that interactions between particles can be well described by the exchange of more particles,” Tanedo said in the statement. “While this is true of ordinary matter, there is no reason to assume that this is also true of dark matter. Their interactions may be described in terms of a continuum of exchanging particles, not just some type of force particle.”
To add that extra dimension, Tanedo’s team borrowed a trick from other theories of high-energy particle physics. Through the intriguing but not fully proven concept of “AdS/CFT duality” (” AdS “stands for anti-De Sitter space, a hypothetical cosmic space-time structure; “CFT” stands for conformal field theory, which falls under the category of quantum theory.) Some physical problems that are difficult to solve in normal three-dimensional space become much easier to solve in four-dimensional space.
Using this mathematical trick, Tanedo and his collaborators solved the problem of how the forces of dark matter interact with each other. They then translated the results into three dimensions and predicted how these forces would behave in the real universe, finding that they behave quite differently from the natural forces we’re used to.
“For gravity or electromagnetic force, which I teach in my introductory physics course, the force of gravity or electromagnetic force decreases by a factor of four when the distance between two particles is doubled,” Tanedo said. “By contrast, the continuous force decreases by as much as eight times.”
This modification of the self-interaction between dark matter particles allowed the researchers to build models that match observations of small galaxies, giving them a dark matter profile that resembles a “core” rather than the “spike” seen in traditional dark matter models. These results are similar to other self-interacting dark matter models that might reproduce the “spike” center, but this theory comes from a completely new theoretical direction that could lead to other observations.
So researchers have a lot of work to do. Cosmologists use dark matter to explain observations at many different scales in the universe, and further work will reveal whether this bizarre theory fits the universe as we see it.
