Warm and Fuzzy Dark Matter: Free Streaming of Wave Dark Matter

TL;DR

This paper explores how wave or fuzzy dark matter with relativistic wavenumbers exhibits free streaming effects similar to warm or hot particle dark matter, affecting cosmological fluctuations and observational bounds.

Contribution

It introduces the concept of free streaming in wave dark matter, analyzing its effects on cosmological fluctuations and observational constraints, extending the understanding beyond cold dark matter models.

Findings

Free streaming causes damping of fluctuations.

Relativistic wave dark matter impacts isocurvature perturbations.

Observable effects depend on the warmth of the dark matter.

Abstract

Wave or fuzzy dark matter that is produced with relativistic wavenumbers exhibits free streaming effects analogous to warm or hot particle dark matter with relativistic momenta. Axions produced after inflation provide such a warm or mildly relativistic candidate, where the enhanced suppression and observational bounds are only moderately stronger than that from wave propagation of initially cold axions. More generally, the free streaming damping also impacts isocurvature fluctuations from generation in causally disconnected patches. As coherent spatial fluctuations free stream away they leave incoherent and transient superpositions in their wakes. These multiple wave momentum streams are the wave analogue of particle phase space fluctuations or directional collisionless damping of massive neutrinos or hot dark matter. The observable impact on both adiabatic and isocurvature fluctuations of fuzzy dark matter can differ from their cold dark matter counterparts due to free streaming depending on how warm or hot is their momentum distribution.