COSMO In the literature: open

Is dark matter self-interacting with a velocity-dependent cross section?

In plain words

If dark matter particles collide with each other, the centers of halos are smoothed into cores, and some cores later collapse into very dense centers. The collision strength may depend on speed, being larger in small galaxies than in clusters.

Precise statement

Measure the self-interaction cross section per unit mass $\sigma/m$ ($\mathrm{cm}^2/\mathrm{g}$) as a function of relative velocity v from about $3e6\ \mathrm{cm}/\mathrm{s}$ (dwarfs) to $1e8\ \mathrm{cm}/\mathrm{s}$ (clusters) using dwarf rotation curves, satellite central densities, strong-lens perturbers, cluster cores and cluster mergers. Kaplinghat, Tulin and Yu (2016, PRL 116, 041302) infer about $2\ \mathrm{cm}^2/\mathrm{g}$ at galaxy scales and $0.1\ \mathrm{cm}^2/\mathrm{g}$ at cluster scales. An answer is a nonzero $\sigma/m(v)$ detected at more than $3\sigma$, or an upper bound below $0.1\ \mathrm{cm}^2/\mathrm{g}$ at dwarf velocities.

What would settle it

A joint fit of dwarf, satellite, strong-lens and cluster data with self-interacting dark matter simulations that include gravothermal core collapse.

Status in the literature

Unverified note

The dense $1e6\,M_{\mathrm{sun}}$ perturber found by gravitational imaging in JVAS B1938+666 (2025) has been interpreted as a core-collapsed self-interacting halo, an interpretation that is debated.