Phantom dark energy from non-local infrared modifications of General Relativity

TL;DR

This paper explores a non-local infrared modification of Einstein's equations that naturally produces a dynamical dark energy consistent with observations, predicting a phantom-like equation of state without introducing a cosmological constant.

Contribution

It introduces a non-local gravity model that explains dark energy and predicts its properties without free parameters, aligning with current cosmological data.

Findings

Reproduces observed dark energy density without a cosmological constant

Predicts a phantom-like equation of state for dark energy

No ghost instabilities propagate as physical degrees of freedom

Abstract

We discuss the cosmological consequences of a model based on a non-local infrared modification of Einstein equations. We find that the model generates a dynamical dark energy that can account for the presently observed value of $\Omega_{\rm DE}$, without introducing a cosmological constant. Tuning a free mass parameter $m$ to a value $m\simeq 0.67 H_0$ we reproduce the observed value $\Omega_{\rm DE}\simeq 0.68$. This leaves us with no free parameter and we then get a pure prediction for the EOS parameter of dark energy. Writing $w_{\rm DE}(a)=w_0+(1-a) w_a$, we find $w_0\simeq-1.04$ and $w_a\simeq -0.02$, consistent with the Planck data, and on the phantom side. We also argue that non-local equations of the type that we propose must be understood as purely classical effective equations, such as those derived in semiclassical gravity for the in-in matrix elements of the metric. As such, any apparent ghost instability in such equations only affects the classical dynamics, but there is no propagating degree of freedom associated to the ghost, and no issue of ghost-induced quantum vacuum decay.