Phenomenological Gravitational Phase Transition: Reconciliation between the Late and Early Universe

TL;DR

This paper explores a phenomenological gravitational phase transition in the universe that transitions from an early to a late phase, aiming to reconcile discrepancies in cosmological measurements such as the Hubble constant and matter distribution.

Contribution

It introduces a new model with a transition in gravitational behavior characterized by a tanh function, and demonstrates its effectiveness in resolving multiple cosmological tensions.

Findings

A recent transition at redshift z_t ≈ 0.9 is preferred by data.

The transition relaxes the Hubble tension and matter distribution inconsistencies.

The model is consistent with various datasets and does not conflict with existing measurements.

Abstract

In this work we investigate whether a certain phenomenological extension to the general relativity (GR), in the form of a gravitational phase transition (GPT) in the Universe, can reduce the external $Planck$ tensions with the local Hubble measurements and the distribution of matter, characterized by $\sigma_8$, as well as its internal inconsistencies in the lensing amplitude and the low--high $\ell$ parameter estimates. We introduce new degrees of freedom into the background and the two scalar perturbation equations in the Newtonian gauge, with simultaneous transitions from an early gravitational phase equivalent to GR toward a late phase. We model the transition as a "tanh" parametrized by the transition redshift $z_{\rm t}$ and width $\alpha$, with amplitudes $\Lambda(z)$ and $(\mu(z),\gamma(z))$ for the background and perturbations respectively. We verify the consistency of the datasets used in this work in the GPT framework and confirm that the individual tensions do not require conflicting transitions. We find that the joint datasets prefer a recent transition at $z_{\rm t}\approx 0.9$ in the background and perturbed Einstein equations, driven mainly by the local Hubble measurement. This transition relaxes all the tensions considered in this work.