# Mimicking Dark Energy with the backreactions of gigaparsec inhomogeneities [CEA]

Spatial averaging and time evolving are non-commutative operations in General Relativity, which questions the reliability of the FLRW model. The long standing issue of the importance of backreactions induced by cosmic inhomogeneities is addressed for a toy model assuming a peak in the primordial spectrum of density perturbations and a simple CDM cosmology. The backreactions of initial Hubble-size inhomogeneities are determined in a fully relativistic framework, from a series of simulations using the BSSN formalism of numerical relativity. In the FLRW picture, these backreactions can be effectively described by two so-called morphon scalar fields, one of them acting at late time like a tiny cosmological constant. Initial density contrasts ranging from $10^{-2}$ down to $10^{-4}$, on scales crossing the Hubble radius between $z\sim 45$ and $z\sim 1000$ respectively, i.e. comoving gigaparsec scales, mimic a Dark Energy (DE) component that can reach $\Omega_{\mathrm{DE}} \approx 0.7$ when extrapolated until today. A similar effect is not excluded for lower density contrasts but our results are then strongly contaminated by numerical noise and thus hardly reliable. A potentially detectable signature of this scenario is a phantom-like equation of state $w< -1$, at redshifts $z\gtrsim 4$ for a density contrast of $10^{-2}$ initially, relaxing slowly to $w \approx -1$ today. This new class of scenarios would send the fine-tuning and coincidence issues of Dark energy back to the mechanism at the origin of the primordial power spectrum enhancement, possibly in the context of inflation.

S. Clesse, A. Roisin and A. Fuzfa
Thu, 23 Feb 17
30/48