A (giant) void is not mandatory to explain away dark energy with a Lemaître-Tolman model

¹ Laboratoire Univers et Théories (LUTH), Observatoire de Paris, CNRS, Université Paris-Diderot, 5 place Jules Janssen, 92190 Meudon, France e-mail: marie-noelle.celerier@obspm.fr
² Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch 7701, South Africa
³ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00 716 Warszawa, Poland

Received: 2 November 2009
Accepted: 3 May 2010

Abstract

Context. Lemaître-Tolman (L-T) toy models with a central observer have been used to study the effect of large scale inhomogeneities on the SN Ia dimming. Claims that a giant void is mandatory to explain away dark energy in this framework are currently dominating.

Aims. Our aim is to show that L-T models exist that reproduce a few features of the ΛCDM model, but do not contain the giant cosmic void.

Methods. We propose to use two sets of data – the angular diameter distance together with the redshift-space mass-density and the angular diameter distance together with the expansion rate – both defined on the past null cone as functions of the redshift. We assume that these functions are of the same form as in the ΛCDM model. Using the Mustapha–Hellaby–Ellis algorithm, we numerically transform these initial data into the usual two L-T arbitrary functions and solve the evolution equation to calculate the mass distribution in spacetime.

Results. For both models, we find that the current density profile does not exhibit a giant void, but rather a giant hump. However, this hump is not directly observable, since it is in a spacelike relation to a present observer.

Conclusions. The alleged existence of the giant void was a consequence of the L-T models used earlier because their generality was limited a priori by needless simplifying assumptions, like, for example, the bang-time function being constant. Instead, one can feed any mass distribution or expansion rate history on the past light cone as initial data to the L-T evolution equation. When a fully general L-T metric is used, the giant void is not implied.