Self-interacting dark matter may solve three cosmic puzzles

A study led by UC Riverside physicist Hai-Bo Yu suggests that a new type of dark matter could explain three astrophysical puzzles across vastly different environments.

Published in Physical Review Letters, the study proposes that dense clumps of self-interacting dark matter (SIDM) - each about a million times the mass of the sun - can account for unusual gravitational effects observed in gravitational lenses, stellar streams, and satellite galaxies.

The gravitational lens system JVAS B1938+666. The black ring and central dot show an infrared image of a distant galaxy distorted by gravitational lensing. The orange emission shows radio waves from the same system. The inset highlights a small "pinch" in the image caused by an additional dense object - about a million times the mass of the Sun - shown as a white blob. Credit: Devon Powell, Max Planck Institute for Astrophysics, based on data from Keck/EVN/GBT/VLBA.

Dark matter, which makes up about 85% of the universe's matter, cannot be seen directly. The standard model assumes it is "cold" and collisionless, meaning particles pass through one another without interacting. This model struggles, however, to explain certain high-density structures observed in the universe.

Yu's work instead focuses on SIDM, in which dark matter particles collide and exchange energy. These interactions can trigger "gravothermal collapse," forming extremely dense, compact cores.

"The difference is like a crowd of people who ignore each other versus one where everyone is constantly bumping into one another," said Yu, a professor of physics and astronomy and deputy director of the Center for Experimental Cosmology and Instrumentation, to explain dark matter particles in the standard model and SIDM. "In SIDM, these interactions can dramatically reshape the internal structure of dark matter halos. Dark matter that interacts with itself can become dense enough to explain these observations."

The study shows that these dense SIDM clumps can simultaneously explain:

• An ultra-dense object in the gravitational lens system JVAS B1938+666, revealed through its powerful magnifying effect on distant galaxies;
• A striking spur-and-gap feature in the GD-1 stellar stream, as if an unseen, compact object tore through the stream and left a visible scar;
• The unusual star cluster Fornax 6 in the Fornax satellite galaxy of the Milky Way. A dense dark matter clump can act like an invisible gravitational trap, sweeping up passing stars and locking them into a tight, compact cluster.

"What's striking is that the same mechanism works in three completely different settings - across the distant universe, within our galaxy, and in a neighboring satellite galaxy," Yu said. "All show densities that are difficult to reconcile with standard model dark matter but arise naturally in SIDM."

The research was supported by the John Templeton Foundation and the U.S. Department of Energy.

The title of the paper is "Core-Collapsed SIDM Halos as the Common Origin of Dense Perturbers in Lenses, Streams, and Satellites."

Header image shows blue and red abstract nebula with bright stars. Credit: AntonioSolano/iStock/Getty Images Plus.

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