Free Streaming in Warm Wave Dark Matter

We provide a framework for numerically computing the effects of free-streaming in scalar fields produced after inflation. First, we provide a detailed prescription for setting up initial conditions in the field. This prescription allows us to specify the power spectra of the fields (peaked on subhorizon length scales and without a homogeneous field mode), and importantly, also correctly reproduces the behaviour of density perturbations on large length scales consistent with superhorizon adiabatic perturbations. We then evolve the fields using a spatially inhomogeneous Klein-Gordon equation, including the effects of expansion and radiation-sourced metric perturbations. We show how gravity enhances, and how free streaming erases the initially adiabatic density perturbations of the field, revealing more of the underlying, non-evolving, white-noise isocurvature density contrast. Furthermore, we explore the effect of non-gravitational self-interactions of the field, including oscillon formation, on the suppression dynamics. As part of this paper, we make our code, ${\sf{Cosmic-Fields-Lite}}$ (${\sf{CFL}}$), publicly available. For observationally accessible signatures, our work is particularly relevant for structure formation in light/ultralight dark matter fields.
Comment: v2: added references, fixed typos. v1: 38 pages + references and appendices, 13 figures. See https://github.com/hypermania/Cosmic-Fields-Lite for our code $\sf{Cosmic-Fields-Lite}$ and https://www.youtube.com/playlist?list=PLecJrnvnk5c7Iaqi-Wq7xvqk1Msgxn5pk for videos of simulation results