QCD-scale modified-gravity universe
  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.
