Mimicking Dark Energy with the backreactions of gigaparsec inhomogeneities
Abstract
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 45 and z 1000 respectively, i.e. comoving gigaparsec scales, mimic a Dark Energy (DE) component that can reach DE ≈ 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 4 for a density contrast of 10-2 initially, relaxing slowly to w ≈ -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.