Hydrodynamics of Filtered Dark Matter: A Two-Component Approach

TL;DR

This paper models the hydrodynamics of Filtered Dark Matter during a first-order phase transition as a two-component fluid, revealing different behaviors in ballistic and LTE regimes and their effects on dark matter relic abundance.

Contribution

It introduces a novel two-component fluid framework for Filtered DM hydrodynamics, analyzing solution classes and their implications for dark matter relic abundance.

Findings

Hydrodynamics classified into detonation-like and deflagration-like branches.

Different behaviors of DM and radiation fluids in ballistic and LTE regimes.

Hydrodynamic effects significantly alter the predicted dark matter relic abundance.

Abstract

We study the hydrodynamics of the Filtered Dark Matter (Filtered DM) scenario during a first-order phase transition (FOPT). In this scenario, the bubble wall is highly reflective of the dark matter (DM) fluid but transparent to radiation, making the hydrodynamic problem fundamentally different from that of the electroweak FOPT. Motivated by this property, we formulate the hydrodynamics of this system as a two-component fluid composed of DM and radiation, and find that the solutions can be classified into detonation-like and deflagration-like branches in the ballistic regime and in the local thermal equilibrium (LTE) regime. In the ballistic regime, the energy--momentum of DM that cannot enter the wall appears as a reflected mode, while in the LTE regime, it relaxes into the energy--momentum of radiation. We find that this difference in the fate of the DM fluid that cannot enter the interior of the wall leads to different hydrodynamic behaviors in the DM and radiation fluids independently and, in particular, results in different existence conditions for solutions in the deflagration-like branch. Based on these results, we further revisit the impact of hydrodynamic effects on the relic abundance of Filtered DM and demonstrate the change in the abundance induced by hydrodynamic effects. In addition, we also discuss the non-conservation of the entropy current from the viewpoint of the two-fluid system, and briefly comment on the similarity between the Filtered DM system and information-thermodynamic systems.