Dark Sound

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.