Dark matter, constituting an estimated 85% of the universe’s matter, remains enigmatic due to its nonluminous nature and elusive properties. While normal matter interacts with light, making it observable, dark matter’s inability to be directly seen poses a significant detection challenge. A theory called “self-interacting dark matter” (SIDM) suggests that dark matter particles interact through a dark force, engaging in strong collisions near a galaxy’s center.
In research published in The Astrophysical Journal Letters, a team led by Hai-Bo Yu, a professor at the University of California, Riverside, reveals that SIDM offers a compelling explanation for two astrophysical puzzles at opposing extremes.
“The first is a high-density dark matter halo in a massive elliptical galaxy,” Yu explains. “The halo, detected through strong gravitational lensing, exhibits a density inconsistent with the prevailing cold dark matter theory. The second puzzle involves ultra-diffuse galaxies with extremely low dark matter halo densities, challenging the cold dark matter theory.”
Dark matter halos surround galaxies, and gravitational lensing occurs when light from distant galaxies bends around massive objects. Ultra-diffuse galaxies, with minimal luminosity, present a distribution of stars and gas that is widely dispersed.
To demonstrate SIDM’s explanatory power, the team conducted high-resolution simulations of cosmic structure formation, considering dark matter self-interactions on relevant mass scales for both strong lensing halos and ultra-diffuse galaxies.
Ethan Nadler, a postdoctoral fellow at the Carnegie Observatories and USC, elaborates, “Self-interactions induce heat transfer in the halo, leading to varied central halo densities. Some halos exhibit higher central densities, while others show lower densities compared to their cold dark matter counterparts.”
The team contends that these puzzles challenge the standard cold dark matter paradigm and position SIDM as a compelling candidate for reconciling the extremes. Daneng Yang, a postdoctoral scholar at UCR, notes, “No other explanations are available in the literature. Now there is an intriguing possibility that dark matter may be more complex and vibrant than we expected.”
The research highlights the significance of probing dark matter through astrophysical observations, using computer simulations of cosmic structure formation. Yu anticipates increased studies in this area, particularly with the wealth of data expected from upcoming astronomical observatories such as the James Webb Space Telescope and Rubin Observatory.
Since 2009, Hai-Bo Yu’s work has played a pivotal role in promoting SIDM within the particle physics and astrophysics communities.