 | GW-induced freeze-in of dark matter for a GW background with a broken power-law spectrum. Colored lines show the phase transition temperature $T_*$ and DM mass $M$ required to explain the observed DM density. We have assumed a spectral index $m=3$ below the peak frequency, $q_\text{peak}$, while above the peak we have used $n=1$ (green, e.g.\ from bubble collisions in a first-order phase transition), $n=5/3$ (blue, e.g.\ from turbulence), and $n=4$ (orange, e.g.\ from sound waves) as benchmark values. Below the lines, GW-induced freeze-in can still contribute a fraction of the DM. The bottom edges of the shaded bands indicate where that fraction is 1\%. For comparison, we show in yellow the parameter region in which conventional cosmological production of supermassive fermions by the expansion of the Universe (``CGPP'') \cite{Kolb:2017jvz, Ema:2019yrd, Kolb:2023ydq} or graviton-mediated annihilation (``GMA'') \cite{Bernal:2018qlk, Clery:2021bwz} yield the correct relic density. Inside the gray area, fermions are massive already at the time $T_*$ of GW production, so our mechanism is not applicable. \emph{Gravitational-wave induced freeze-in can successfully explain the observed DM relic abundance in large swaths of parameter space, favoring $T_*$ well above the electroweak scale.} |
