Seasons of Dark Matter Freeze-In Shaped by the Weather of the Early Universe

Abstract

Quantifying the imprints of freeze-in dark matter (DM) on cosmological structures requires knowledge of its phase-space distribution. We investigate how variations in the cosmological history before nucleosynthesis, the "weather" of that epoch, give rise to distinct "seasons" in the DM momentum distribution that govern its warmness. Studying decay-driven production across diverse cosmological histories, we map how these conditions shape DM phase-space properties. Our study quantifies how the early universe composition plays a key role in determining the mass bound on freeze-in DM.

Figures (3)

• Figure 1: Cosmological histories considered in this work with a new species $\Phi$ redshifting slower (left panel) or faster (right panel) than radiation. The blue and red lines denote the energy density of $\Phi$ and radiation, respectively, both as a function of the scale factor $a$. Semi-analytical solutions for the different phases are provided in the legend. See text for further description.
• Figure 2: Numerical solutions for $(M, m_\chi) = (1\,\mathrm{TeV}, 30\,\mathrm{keV})$ with $g_{{\cal B}_1}\Gamma_{{\cal B}_1}$ fixed to reproduce the observed DM relic density. PSDs, normalized to the present relic density, are shown as functions of the normalized DM momentum $p_0 / T_\chi(t_0)$. Colored lines denote the benchmark scenarios in Tab. \ref{['tab:solutions']}, and the black dotted line shows a MB thermal PSD.
• Figure 3: Lower bound on the DM mass as a function of the parent mass $M$ for two-body decays. The solid black line shows the result for a standard cosmological history, while the shaded gray region indicates the range obtained by varying the cosmological background as discussed in the main text.