Unimodular Diffusion and Interacting Vacuum Cosmology
Year: 2026
Type: preprint
Abstract: We investigate the correspondence between unimodular diffusion cosmology and interacting dark sector models at the background and linear perturbation levels. In the diffusion framework, the effective cosmological constant becomes time dependent, $Λ(t)$, sourced by a diffusion current. We show that at the background level this framework can be mapped onto interacting dark energy models with $w=-1$ and energy transfer $Q$. Using two common parameterizations, $Q = ξH ρ_{\rm de}$ and $Q = ξH ρ_{\rm dm}$, and data from supernovae, DESI BAO, cosmic chronometers, and CMB distance priors, we find $ξ= -0.0197 \pm 0.0076$ for the vacuum-coupled case, while the matter-coupled case gives a best-fit $ξ= 0.0018$ with comparable goodness of fit. At the level of linear perturbations, however, the diffusion framework is consistent only with interacting vacuum models having homogeneous energy transfer ($Q \propto ρ_{\rm de}$ with $δQ=0$), thereby breaking the degeneracy with more general interacting dark energy scenarios. Including redshift-space distortion data, we obtain $ξ= -0.0147 \pm 0.0075$, consistent with $Λ$CDM ($ξ=0$) at $2σ$. The inferred clustering amplitude is $S_8 = 0.782 \pm 0.026$ for the diffusion model, compared to $S_8 = 0.77 \pm 0.025$ for $Λ$CDM under the same dataset, indicating a modest but non-negligible impact on structure growth.
Source: arXiv (Cornell University)
Authors: Kashyap, Gopal, Singh, Naveen K.
Institution: Vellore Institute of Technology University
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7140178122
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