New dark matter theory explains two puzzles in astrophysics

Dark matter is nonluminous and its nature is poorly known. It is thought to comprise 85% of all matter in the universe. Dark matter is more difficult to detect than normal matter because it is invisible, whereas normal matter emits, reflects, and absorbs light. According to a hypothesis known as "self-interacting dark matter," or SIDM, dark matter particles fiercely collide with one another in a galaxy's core as a result of self-interacting through a dark force.

A research team led by Hai-Bo Yu, a physics and astronomy professor at the University of California, Riverside, finds in work published in The Astrophysical Journal Letters that SIDM may concurrently solve two astrophysical difficulties at opposing extremes.

"The first is a high-density dark matter halo in a massive elliptical galaxy," Yu explained. Strong gravitational lensing measurements allowed for the detection of the halo, whose density is so large that it is implausible under the dominant cold dark matter paradigm. The second is that the cold dark matter hypothesis struggles to explain the extraordinarily low concentrations of the dark matter halos of ultra-diffuse galaxies."

A galaxy or cluster of galaxies surrounded by an unseen halo of matter is called a dark matter halo. When light from far-off galaxies bends around enormous objects while traveling across the cosmos, it is known as gravitational lensing. The cold dark matter (CDM) hypothesis and paradigm postulates the collisionlessness of dark matter particles. Ultra-diffuse galaxies, as their name implies, are characterized by a very low brightness and a dispersed star and gas distribution.

Ethan Nadler, a postdoctoral researcher at the Carnegie Observatories and University of Southern California, and Daneng Yang, a postdoctoral scholar at UCR, collaborated with Yu on the project.

The group performed the first high-resolution simulations of cosmic structure creation with strong dark matter self-interactions on relevant mass scales for the strong lensing halo and ultra-diffuse galaxies in order to demonstrate that SIDM may explain the two astrophysical mysteries.

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

The two puzzles, in the team's opinion, present a serious threat to the conventional CDM paradigm.

"CDM is challenged to explain these puzzles," Yang stated. "SIDM is maybe the most convincing option to bring the two diametrically opposed sides together. There are no alternative explanations found in the literature. There is now a compelling prospect that dark matter could be livelier and more complicated than previously thought."

The study also shows how effective it is to use computer models of cosmic structure development to investigate dark matter using astrophysical data.

"We hope our work encourages more studies in this promising research area," Yu stated. "It 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 upcoming Rubin Observatory."

Since around 2009, Yu and associates' efforts have contributed to the acceptance of SIDM in the fields of particle physics and astronomy.