Cosmology with matter diffusion

TL;DR

This paper proposes a cosmological model incorporating matter diffusion coupled with dark energy, constrained by observational data, and analyzes its effects on structure formation and cosmic microwave background signals.

Contribution

It introduces a diffusion-based cosmological model with a single new parameter, extending the standard Lambda-CDM framework and analyzing its observational viability.

Findings

Diffusion parameter constrained by observational data

Model affects CMB and matter power spectrum

Diffusion mechanism remains a viable extension of Lambda-CDM

Abstract

We construct a viable cosmological model based on velocity diffusion of matter particles. In order to ensure the conservation of the total energy-momentum tensor in the presence of diffusion, we include a cosmological scalar field $\phi$ which we identify with the dark energy component of the Universe. The model is characterized by only one new degree of freedom, the diffusion parameter $\sigma$. The standard $\Lambda$CDM model can be recovered by setting $\sigma=0$. If diffusion takes place ($\sigma >0$) the dynamics of the matter and of the dark energy fields are coupled. We argue that the existence of a diffusion mechanism in the Universe can serve as a theoretical motivation for interacting models. We constrain the background dynamics of the diffusion model with Supernovae, H(z) and BAO data. We also perform a perturbative analysis of this model in order to understand structure formation in the Universe. We calculate the impact of diffusion both on the CMB spectrum, with particular attention to the integrated Sachs-Wolfe signal, and on the matter power spectrum $P(k)$. The latter analysis places strong constraints on the magnitude of the diffusion mechanism but does not rule out the model.