New dark matter theory explains two puzzles in astrophysics

Considered to constitute 85% of the matter in the universe, dark matter is not luminous and its nature is not well understood. While normal matter absorbs, reflects and emits light, dark matter cannot be seen directly, making it harder to detect. A theory called self-interactive dark matter, or SIDM, proposes that dark matter particles themselves interact through a dark force, colliding strongly with each other near the center of a galaxy.

In a work published in Letters from the astrophysical journalA research team led by Hai-Bo Yu, professor of physics and astronomy at the University of California, Riverside, reports that SIDM can simultaneously explain two astrophysical puzzles in opposite extremes.

“The first is a high-density dark matter halo in a massive elliptical galaxy,” said Yu. “The halo was detected through the observation of strong gravitational lensing, and its density is so high that it is extremely unlikely in the prevailing cold dark matter theory. The second is that dark matter halos in ultra-diffuse galaxies have extremely low densities and are difficult to explain by cold dark matter theory.”

A dark matter halo is a halo of invisible matter that permeates and surrounds a galaxy or cluster of galaxies. Gravitational lensing occurs when light traveling through the universe from distant galaxies bends around massive objects. The cold dark matter paradigm/theory, or CDM, assumes that dark matter particles are collision-free. As their name suggests, ultra-diffuse galaxies have extremely low luminosity and the distribution of their stars and gas is spread out.

Yu was joined in the study by Ethan Nadler, a joint postdoctoral researcher at the Carnegie Observatories and the University of Southern California, and Daneng Yang, a postdoctoral researcher at UCR.

To show that SIDM can explain both astrophysical puzzles, the team conducted the first high-resolution simulations of the formation of cosmic structures with strong dark matter self-interactions at mass scales relevant to the powerful lensing halo and ultra-diffuse galaxies.

“These self-interactions lead to heat transfer within the halo, which diversifies the halo density in the central regions of galaxies,” Nadler said. “In other words, some halos have higher central densities, and others have lower central densities, compared to their MDP counterparts, with details depending on the history of cosmic evolution and the environment of individual halos.”

According to the team, these two conundrums pose a formidable challenge to the standard CDM paradigm.

“The CDM has the challenge of explaining these puzzles,” Yang said. “SIDM is arguably the ideal candidate to reconcile the two opposing extremes. No other explanation is available in the literature. There is now an intriguing possibility that dark matter is more complex and dynamic than previously thought.”

The research also demonstrates the power of probing dark matter through astrophysical observations, with the tool of computer simulations of the formation of cosmic structures.

“We hope that our work will encourage more studies in this promising area of ​​research,” said Yu. “This will be a particularly timely development given the expected influx of data in the near future from astronomical observatories, including the James Webb Space Telescope and the upcoming Rubin Observatory.”

Since around 2009, the work of Yu and his collaborators has helped popularize SIDM in the particle physics and astrophysics communities.