Cosmological acceleration from structure formation

We discuss the Buchert equations, which describe the average expansion of an inhomogeneous dust universe. In the limit of small perturbations, they reduce to the Friedmann-Robertson-Walker equations. However, when the universe is very inhomogeneous, the behaviour can be qualitatively different from the FRW case. In particular, the average expansion rate can accelerate even though the local expansion rate decelerates everywhere. We clarify the physical meaning of this paradoxical feature with a simple toy model, and demonstrate how acceleration is intimately connected with gravitational collapse. This provides a link to structure formation, which in turn has a preferred time around the era when acceleration has been observed to start.Comment: 6 pages, awarded honorable mention in the 2006 Gravity Research Foundation essay competitio

On ekpyrotic brane collisions

We derive the five-dimensional metrics which describe a non-singular boundary brane collision in the ekpyrotic scenario in the context of general relativity, taking into account brane tension. We show that the metrics constrain matter created in the collision to have negative energy density or pressure. In particular, the minimal field content of heterotic M-theory leads to negative energy density. We also consider bulk brane-boundary brane collisions and show that the collapse of the fifth dimension is an artifact of the four-dimensional effective theory.Peer reviewe

A primer on the ekpyrotic scenario

This is an introduction to the ekpyrotic scenario, with an emphasis on the two contexts of brane cosmology and primordial universe scenarios. A self-contained introduction to brane cosmology and a qualitative overview and comparison of the inflationary, pre-big bang and ekpyrotic scenarios are given as background. The ekpyrotic scenario is then presented in more detail, stressing various problems

The effect of structure formation on the expansion of the universe

Observations of the expansion rate of the universe at late times disagree by a factor of 1.5-2 with the prediction of homogeneous and isotropic models based on ordinary matter and gravity. We discuss how the departure from linearly perturbed homogeneity and isotropy due to structure formation could explain this discrepancy. We evaluate the expansion rate in a dust universe which contains non-linear structures with a statistically homogeneous and isotropic distribution. The expansion rate is found to increase relative to the exactly homogeneous and isotropic case by a factor of 1.1-1.3 at some tens of billion of years. The timescale follows from the cold dark matter transfer function and the amplitude of primordial perturbations without additional free parameters.Comment: 6 pages, 1 figure. Awarded Honorable Mention in the 2008 Gravity Research Foundation essay competition. More extended treatment of the topics can be found in arXiv:0801.2692v

Backreaction as an alternative to dark energy and modified gravity

The predictions of homogeneous and isotropic cosmological models with ordinary matter and gravity are off by a factor of two in the late universe. One possible explanation is the known breakdown of homogeneity and isotropy due to the formation of non-linear structures. We review how inhomogeneities affect the average expansion rate and can lead to acceleration, and consider a semi-realistic model where the observed timescale of ten billion years emerges from structure formation. We also discuss the relation between the average expansion rate and observed quantities.Non peer reviewe

Light propagation and the average expansion rate in near-FRW universes

Peer reviewe

Vacuum energy and dynamical symmetry breaking in curved spacetime

Essay written for the Gravity Research Foundation 2012 Awards for Essays on Gravitation HIP-2012-08-T

Backreaction: directions of progress

Peer reviewe

Dark energy from back-reaction

We consider the effect of inhomogeneities on the expansion of the Einstein–de Sitter universe. We find that the back-reaction of linear scalar metric perturbations results in apparent dark energy with a mixture of equations of state between 0 and -4/3. We discuss the possibility that back-reaction could account for present-day acceleration.Peer reviewe

Backreaction in the Lemaître–Tolman–Bondi model

We study backreaction analytically using the parabolic Lemaître–Tolman–Bondi universe as a toy model. We calculate the average expansion rate and energy density on two different hypersurfaces and compare the results. We also consider the Hubble law and find that backreaction slows down the expansion if measured with proper time, but speeds it up if measured with energy density.Peer reviewe