Dark Sound

TL;DR

This paper introduces 'Dark Sound', a collective excitation mode in axion dark matter condensates, characterized by a linear dispersion relation influenced by self-interactions and gravity, with potential observable signatures.

Contribution

It reveals the existence of a stable, sound-like collective mode in axion dark matter, termed Dark Sound, with a detailed analysis of its spectrum, stability, and observational implications.

Findings

Dark Sound exhibits a linear dispersion relation with velocity ~10^{-12}c.

Gravity induces Jeans instability at large scales, affecting Dark Sound stability.

Potential observable signatures include quantized modes and noise features in dark matter clumps.

Abstract

We discuss the axion dark matter (DM) condensate and the consequences the interactions of dark matter would have on the spectrum of collective modes. We find that DM self-interactions change the spectrum of excitations from a quadratic to a linear-like dispersion with velocity $v_s$ which is set by the interactions, but dominated by gravity. For typical DM densities and interactions we find $v_s \sim 10^{-12}c$. This sound-like mode corresponds to DM density oscillations just like in any other Bose liquid, hence we call it {\em Dark Sound} (DS). The DS mode is well defined and describes stable density oscillations at intermediate length scales $k \geq k_{\text{min}} \sim 10^{4}\mathrm{lyr^{-1}}$. In the extreme long wavelength limit gravity dominates and leads to Jeans instability of the sound mode at the scale of clump formation $ k \leq k_{\text{min}}$. We also discuss the possible observable consequences of the DS, including quantized DS modes inside clumps, their characteristic energy, and noise features that might facilitate the observation of DM.