Cosmological constraints on unimodular gravity models with diffusion

Abstract

A discrete space-time structure lying at about the Planck scale may become manifest in the form of very small violations of the conservation of the matter energy-momentum tensor. In order to include such kind of violations, forbidden within the general relativity framework, the theory of unimodular gravity seems as the simplest option to describe the gravitational interaction. In the cosmological context, a direct consequence of such violation of energy conservation might be heuristically viewed a "diffusion process of matter (both dark and ordinary)" into an effective dark energy term in Einstein´s equations, which leads under natural assumptions to an adequate estimate for the value of the cosmological constant. Previous works have also indicated that these kinds of models might offer a natural scenario to alleviate the Hubble tension. In this work, we consider a simple model for the cosmological history including a late time occurrence of such energy violation and study the modifications of the predictions for the anisotropy and polarization of the cosmic microwave background (CMB). We compare the model´s predictions with recent data from the CMB, supernovae type Ia, cosmic chronometers and baryon acoustic oscillations. The results show the potential of this type of model to alleviate the Hubble tension.