WIMP Freeze-Out in Diffusive Unimodular Gravity
We study a non-standard cosmology (NSC) scenario within the Unimodular Gravity (UG) framework, sourced by a scalar field $\phi$ that undergoes energy diffusion, parametrized by the diffusion parameter $x$, the initial energy densities rate $\kappa\equiv\rho_\phi/\rho_\gamma|_{\text{ini}}$, and the end-of-domination temperature $T_{\text{end}}$. We compare this UG+NSC scenario with standard NSC and $\Lambda$CDM cosmologies for WIMP Dark Matter (DM) production via the freeze-out mechanism. We find that energy diffusion reshapes the allowed $(m_\chi, \langle\sigma v\rangle)$ parameter space, where $m_\chi$ is the DM mass and $\langle \sigma v \rangle$ is the thermally averaged annihilation cross section, opening regions otherwise excluded by DM overproduction in $\Lambda$CDM, and shifting the mass and cross-section ranges accessible to WIMP candidates depending on $x$, $\kappa$, $T_{\text{end}}$, and the barotropic index $\omega$ of $\phi$. As a concrete application, we implement this framework for the Real Singlet Scalar WIMP and test its $(m_\chi, \lambda_{HS})$ parameter space against current direct detection bounds from the LZ experiment, showing that energy diffusion opens previously unconstrained regions in the Higgs-portal coupling $\lambda_{HS}$ and thereby alters the detectability prospects of this benchmark model in future searches.