QCD-scale modified-gravity universe

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.Comment: 24 pages; v7: published versio

Dynamics of the quantum vacuum: Cosmology as relaxation to the equilibrium state

“…[15,16] or (E QCD ) 6 /(E Planck ) 2 from Refs. [18,19]. However, the last two explanations of ρ vac,∞ may fare differently when compared with astronomical observations: the QCD explanation corresponds to a modified gravity theory and may already be ruled out by the astronomical observations, whereas the electroweak explanation corresponds to a standard ΛCDM model (now, with a calculated value of Λ) and appears to fit the current astronomical data well.…”

“…Actually, this Green's function at p 0 = 0 has been used in Refs. [18,19] to justify the presence of a running term |H(t)| (E QCD ) 3 in the vacuum energy density [here, H(t) is the Hubble parameter of the expanding universe] and to calculate the corresponding remnant vacuum energy density…”

“…Several scenarios have been proposed for this final (frozen) stage and the corresponding remnant vacuum energy density. These scenarios rely, for example, on new TeV-scale electroweak physics [14,15,16] or nonperturbative QCD dynamics [17,18,19,20,21] with a remnant vacuum energy density given by, respectively, ρ vac,∞ ∼ (E ew ) 8 /(E Planck ) 4 or ρ vac,∞ ∼ (E QCD ) 6 /(E Planck ) 2 . In Sec.…”