Structure Formation, Backreaction and Weak Gravitational Fields

Abstract

There is an ongoing debate in the literature as to whether the effects of averaging out inhomogeneities (``backreaction'') in Cosmology can be large enough to account for the acceleration of the scale factor in the FLRW models. In particular, some simple models of structure formation studied in the literature seem to indicate that this is indeed possible, and it has also been suggested that the perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes nonlinear. In this work we attempt to clarify the situation to some extent, using a fully relativistic model of pressureless spherical collapse. We find that whereas averaging during structure formation can lead to acceleration via a selective choice of averaging domains, the acceleration is not present when more generic domains are used for averaging. Further, we show that for most of the duration of the collapse, matter velocities remain small, and the perturbed FLRW form of the metric can be explicitly recovered, in the structure formation phase. We also discuss the fact that the magnitude of the average effects of inhomogeneities depends on the scale of averaging, and while it may not be completely negligible on intermediate scales, it is expected to remain small when averaging on suitably large scales.

20 pages, 14 figures; v3 -- Added material, scale dependence of backreaction highlighted; 2 figures and 1 reference added; conclusions unchanged; version accepted for publication in JCAP

References in corpus (6)

• Cosmic clocks, cosmic variance and cosmic averages
• Exact solution to the averaging problem in cosmology
• On the onset of cosmological backreaction
• Gravitational energy as dark energy: Concordance of cosmological tests
• Cosmological Backreaction from Perturbations
• The Spatial Averaging Limit of Covariant Macroscopic Gravity - Scalar Corrections to the Cosmological Equations

Cited by in corpus (8)

• On the curvature of the present-day Universe
• Cosmological equivalence principle and the weak-field limit
• Description of our cosmological spacetime as a perturbed conformal Newtonian metric and implications for the backreaction proposal for the accelerating universe
• Cosmic Inhomogeneities and the Average Cosmological Dynamics
• LTB solutions in Newtonian gauge: from strong to weak fields
• Averaging Robertson-Walker Cosmologies
• Backreaction of Cosmological Perturbations in Covariant Macroscopic Gravity
• Nonlinear Structure Formation, Backreaction and Weak Gravitational Fields