Astrophysical Plasma Instabilities induced by Long-Range Interacting Dark Matter

TL;DR

This paper investigates how plasma instabilities induced by long-range interacting dark matter could influence astrophysical phenomena and constrain dark matter properties, especially its charge-to-mass ratio, through effects on cosmic structures and previous limits.

Contribution

It demonstrates that plasma instabilities can significantly affect dark matter behavior and provide stringent constraints on millicharged dark matter, complementing previous astrophysical bounds.

Findings

Electromagnetic instabilities like Weibel can couple dark matter and baryons in cosmological settings.

Plasma instabilities could weaken existing spin-down constraints on millicharged dark matter.

Instability growth rates suggest strong limits on dark matter charge-to-mass ratios in astrophysical systems.

Abstract

If dark matter (DM) is millicharged or darkly charged, collective plasma processes may dominate momentum exchange over direct particle collisions. Plasma streaming instabilities can couple the momentum of the DM to counter-streaming baryons or other DM and result in the counter-streaming fluids coming to rest with each other, just as happens for baryonic collisionless shocks in astrophysical systems. While electrostatic plasma instabilities are highly suppressed by Landau damping when DM is millicharged, in the cosmological situations of interest, electromagnetic instabilities such as the Weibel can couple momenta, assuming that the linear instability saturates in the manner typically found for baryonic plasmas. We find that the streaming of DM in the pre-Recombination universe is affected more strongly by direct collisions than collective processes, validating previous constraints. However, when considering magnetized Weibel and Firehose instabilities, the properties of the Bullet Cluster merger are likely to be substantially altered if $[q_\chi/m_\chi] \gtrsim 10^{-12}-10^{-11}$, where $[q_\chi/m_\chi]$ is the charge-to-mass ratio of the DM relative to that of the proton. The Weibel growth rates are even faster in the case of a dark-$U(1)$ charge, potentially ruling out $[q_\chi/m_\chi] \gtrsim 10^{-14}$ in the Bullet Cluster system, in agreement with previous work. The strongest previous limits on millicharged DM (mDM) arise from considering the spin-down of galactic disks. We show that plasma instabilities or tangled background magnetic fields could lead to diffusive propagation of the DM, weakening these spin-down limits. Thus, plasma instabilities may place some of the most stringent constraints over much of the millicharged, and our results corroborate previous extremely stringent potential constraints on the dark-charged parameter space.