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[1009.0273] Spatial and temporal tuning in void models for acceleration
Authors:Simon Foreman, Adam Moss, James P. Zibin, Douglas Scott
Abstract:There has been considerable interest in recent years in cosmological models in which we inhabit a very large, underdense void as an alternative to dark energy. A longstanding objection to this proposal is that observations limit our position to be very close to the void centre. By selecting from a family of void profiles that fit supernova luminosity data, we carefully determine how far from the centre we could be. To do so, we use the observed dipole component of the cosmic microwave background, as well as an additional stochastic peculiar velocity arising from primordial perturbations. We find that we are constrained to live within 80 Mpc of the centre of a void--a somewhat weaker constraint than found in previous studies, but nevertheless a strong violation of the Copernican principle. By considering how such a Gpc-scale void would appear on the microwave sky, we also show that there can be a maximum of one of these voids within our Hubble radius. Hence, the constraint on our position corresponds to a fraction of the Hubble volume of order 10^{-8}. Finally, we use the fact that void models only look temporarily similar to a cosmological-constant-dominated universe to argue that these models are not free of temporal fine-tuning.
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Maciej Bilicki

Joined: 12 May 2010
Posts: 19
Affiliation: University of Cape Town

PostPosted: September 03 2010  Reply with quote

This paper seems to put inhomogeneous models with a giant void around us into more trouble. Although the constraint on our position with respect to the center of the void is slightly loosened, but first of all the authors rule out other large voids within the observational volume and secondly, point out that our position would have to be fine-tuned to 1:108 at early times. What is more, it is argued that the fine tuning applies also to the time of observation, at least not worse than within ΛCDM and its issue that ΩmΩΛ only today.
It is however mentioned at the end that maybe a more sophisticated inhomogeneous model, beyond the spherical LTB, is needed. Still, it is claimed, void models would continue to face obstacles, such as some constraints from the CMB.
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