A cosmic equation of state for the inhomogeneous Universe: can a global far-from-equilibrium state explain Dark Energy?

TL;DR

This paper explores whether inhomogeneities and backreaction effects in the universe can lead to a far-from-equilibrium state that explains Dark Energy without a cosmological constant, suggesting a potential paradigm shift in cosmology.

Contribution

It proposes a cosmic equation of state linking inhomogeneities to Dark Energy, highlighting the significance of far-from-equilibrium states and backreaction effects in cosmological models.

Findings

Kinematical backreaction can be comparable to average density.

A far-from-equilibrium state may explain cosmic acceleration.

Global interpretation suggests a stationary universe with specific effective equation of state.

Abstract

A system of effective Einstein equations for spatially averaged scalar variables of inhomogeneous cosmological models can be solved by providing a `cosmic equation of state'. Recent efforts to explain Dark Energy focus on `backreaction effects' of inhomogeneities on the effective evolution of cosmological parameters in our Hubble volume, avoiding a cosmological constant in the equation of state. In this Letter it is argued that, if kinematical backreaction effects are indeed of the order of the averaged density (or larger as needed for an accelerating domain of the Universe), then the state of our regional Hubble volume would have to be in the vicinity of a far-from-equilibrium state that balances kinematical backreaction and average density. This property, if interpreted globally, is shared by a stationary cosmos with effective equation of state $p_{\rm eff} = -1/3 \rho_{\rm eff}$. It is concluded that a confirmed explanation of Dark Energy by kinematical backreaction may imply a paradigmatic change of cosmology.