Dark Matter Breakthrough Allows Probing Three of the Most Popular Theories, All at the Same Time

Two numerical simulations predicting the distribution of dark matter around a galaxy similar to our Milky Way. The left panel assumes that dark matter particles were moving fast in the early universe (warm dark matter), while the right panel assumes that dark matter particles were moving slowly (cold dark matter). The warm dark matter model predicts many fewer small clumps of dark matter surrounding our Galaxy, and thus many fewer satellite galaxies that inhabit these small clumps of dark matter. By measuring the number of satellite galaxies, scientists can distinguish between these models of dark matter. (Images from Bullock & Boylan-Kolchin, Annual Review of Astronomy and Astrophysics 2017, based on simulations by V. Robles, T. Kelley, and B. Bozek)

Observations of dwarf galaxies around the Milky Way have yielded simultaneous constraints on three popular theories of dark matter.

A team of scientists led by cosmologists from the Department of Energy’s SLAC and Fermi national accelerator laboratories has placed some of the tightest constraints yet on the nature of dark matter, drawing on a collection of several dozen small, faint satellite galaxies orbiting the Milky Way to determine what kinds of dark matter could have led to the population of galaxies we see today.

The new study is significant not just for how tightly it can constrain dark matter, but also for what it can constrain, said Risa Wechsler, director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at SLAC and Stanford University. “One of the things that I think is really exciting is that we are actually able to start probing three of the most popular theories of dark matter, all at the same time,” she said.

Dark matter makes up 85 percent of the matter in the universe and interacts very weakly with ordinary matter except through gravity. Its influence can be seen in the shapes of galaxies and in the large-scale structure of the universe, yet no one is sure exactly what dark matter is. In the new study, researchers focused on three broad possibilities for the nature of dark matter: relatively fast-moving or “warm” dark matter; another form of “interacting” dark matter that bumps off protons enough to have been heated up in the early universe, with consequences for galaxy formation; and a third, extremely light particle, known as “fuzzy dark matter,” that through quantum mechanics effectively stretches out across thousands of light years.

To test those models, the researchers first developed computer simulations of dark matter and its effects on the formation of relatively tiny galaxies inside denser patches of dark matter found circling larger galaxies.

“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC. For instance, if dark matter moves a bit too fast or has gained a little too much energy through long-ago interactions with normal matter, those galaxies won’t form in the first place. The same goes for fuzzy dark matter, which if stretched out enough will wipe out nascent galaxies with quantum fluctuations.

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