QCD-scale modified-gravity universe

TL;DR

This paper presents a simplified QCD-scale modified gravity model that predicts observable deviations from standard LambdaCDM cosmology, offering a new approach to understanding cosmic acceleration without dark energy.

Contribution

It introduces a simplified, empirically testable QCD-scale modified gravity model using constant gluon condensate and cold dark matter, with numerical solutions and predictions for observable quantities.

Findings

Predicted E_{G}(z) differs from LambdaCDM by about ±10%.

Numerical solutions provide reliable predictions for observable universe quantities.

The model does not require additional scaling analysis, simplifying boundary condition treatment.

Abstract

A possible gluon-condensate-induced modified-gravity model with f(R) \propto |R|^{1/2} has been suggested previously. Here, a simplified version is presented using the constant flat-spacetime equilibrium value of the QCD gluon condensate and a single pressureless matter component (cold dark matter, CDM). The resulting dynamical equations of a spatially-flat and homogeneous Robertson-Walker universe are solved numerically. This simple empirical model allows, in fact, for a careful treatment of the boundary conditions and does not require a further scaling analysis as the original model did. Reliable predictions are obtained for several observable quantities of the homogeneous model universe. In addition, the estimator E_{G}, proposed by Zhang et al. to search for deviations from standard Einstein gravity, is calculated for linear sub-horizon matter-density perturbations. The QCD-scale modified-gravity prediction for E_{G}(z) differs from that of the LambdaCDM model by about \pm 10 % depending on the redshift z.