Self-interacting Warm Dark Matter

TL;DR

This paper investigates self-interacting warm dark matter (SI-WDM) as a combined solution to cold dark matter’s small-scale problems. It derives the Boltzmann equation in synchronous gauge with a relaxation-time collisional term and computes linear matter power spectra and CMB fluctuations using CMBFAST, for both hot-like and warm-like DM masses. The main finding is that self-interactions raise the small-scale power relative to non-interacting WDM—by about a factor of $\sim 1.6$—and shift the cut-off to smaller scales, while the CMB power spectrum remains effectively indistinguishable from CDM, implying limited CMB-based constraints. This suggests SI-WDM could better match dwarf-galaxy observations without spoiling CMB fits, though it requires unusually large cross sections, potentially realizable in a mirror-sector DM scenario.

Abstract

It has been shown by many independent studies that the cold dark matter scenario produces singular galactic dark halos, in strong contrast with observations. Possible remedies are that either the dark matter is warm so that it has significant thermal motion or that the dark matter has strong self interactions. We have combined these ideas to calculate the linear mass power spectrum and the spectrum of cosmic microwave background (CMB) fluctuations for self-interacting warm dark matter. Our results indicate that such models have more power on small scales than is the case for the standard warm dark matter model, with a CMB fluctuation spectrum which is nearly indistinguishable from standard cold dark matter. This enhanced small-scale power may provide better agreement with the observations than does standard warm dark matter.

Figures (2)

• Figure 1: Matter power spectra for two different dark matter particle masses. The dashed line is for no self-interaction, the triple-dot-dashed is for $\sigma_0 = 1.2 \times 10^{-38} \, {\rm cm}^2$, the dot-dashed for $\sigma_0 = 8 \times 10^{-36} \, {\rm cm}^2$, and the dotted is assuming complete pressure equilibrium. For reference we have plotted the spectrum for standard cold dark matter (full line).
• Figure 2: CMB power spectra for the same models as in Fig. 1. The curve labels are also identical to those in Fig. 1.