10 research outputs found
"Swiss-Cheese" Inhomogeneous Cosmology & the Dark Energy Problem
We study an exact swiss-cheese model of the Universe, where inhomogeneous LTB
patches are embedded in a flat FLRW background, in order to see how
observations of distant sources are affected. We find negligible integrated
effect, suppressed by (L/R_{H})^3 (where L is the size of one patch, and R_{H}
is the Hubble radius), both perturbatively and non-perturbatively. We
disentangle this effect from the Doppler term (which is much larger and has
been used recently \cite{BMN} to try to fit the SN curve without dark energy)
by making contact with cosmological perturbation theory.Comment: 35 pages, 6 figure
The effect of inhomogeneous expansion on the supernova observations
We consider an inhomogeneous but spherically symmetric Lemaitre-Tolman-Bondi
model to demonstrate that spatial variations of the expansion rate can have a
significant effect on the cosmological supernova observations. A model with no
dark energy but a local Hubble parameter about 15% larger than its global value
fits the supernova data better than the homogeneous model with the cosmological
constant. The goodness of the fit is not sensitive to inhomogeneities in the
present-day matter density, and our best fit model has Omega_M ~ 0.3, in
agreement with galaxy surveys. We also compute the averaged expansion rate,
defined by the Buchert equations, of the best fit model and show explicitly
that there is no average acceleration.Comment: minor corrections to match the version published in JCA
Gauss-Bonnet Cosmology with Induced Gravity and Non-Minimally Coupled Scalar Field on the Brane
We construct a cosmological model with non-minimally coupled scalar field on
the brane, where Gauss-Bonnet and Induced Gravity effects are taken into
account. This model has 5D character at both high and low energy limits but
reduces to 4D gravity in intermediate scales. While induced gravity is a
manifestation of the IR limit of the model, Gauss-Bonnet term and non-minimal
coupling of scalar field and induced gravity are essentially related to UV
limit of the scenario. We study cosmological implications of this scenario
focusing on the late-time behavior of the solutions. In this setup, non-minimal
coupling plays the role of an additional fine-tuning parameter that controls
the initial density of predicted finite density big bang. Also, non-minimal
coupling has important implication on the bouncing nature of the solutions.Comment: 33 pages, 12 figures, one table, revised and final version accepted
for publication in JCA
The Effect of Large-Scale Inhomogeneities on the Luminosity Distance
We study the form of the luminosity distance as a function of redshift in the
presence of large scale inhomogeneities, with sizes of order 10 Mpc or larger.
We approximate the Universe through the Swiss-cheese model, with each spherical
region described by the Tolman-Bondi metric. We study the propagation of light
beams in this background, assuming that the locations of the source and the
observer are random. We derive the optical equations for the evolution of the
beam area and shear. Through their integration we determine the configurations
that can lead to an increase of the luminosity distance relative to the
homogeneous cosmology. We find that this can be achieved if the Universe is
composed of spherical void-like regions, with matter concentrated near their
surface. For inhomogeneities consistent with the observed large scale
structure, the relative increase of the luminosity distance is of the order of
a few percent at redshifts near 1, and falls short of explaining the
substantial increase required by the supernova data. On the other hand, the
effect we describe is important for the correct determination of the energy
content of the Universe from observations.Comment: 27 pages, 5 figures Revised version. References added. Conclusions
clarifie
The effect of large scale inhomogeneities on the luminosity distance
We study the form of the luminosity distance as a function of redshift in the presence of large scale inhomogeneities, with sizes of order 10 Mpc or larger. We approximate the Universe through the Swiss-cheese model, with each spherical region described by the Lemaitre-Tolman-Bondi metric. We study the propagation of light beams in this background, assuming that the locations of the source and the observer are random. We derive the optical equations for the evolution of the beam area and shear. Through their integration we determine the configurations that can lead to an increase of the luminosity distance relative to the homogeneous cosmology. We find that this can be achieved if the Universe is composed of spherical void-like regions, with matter concentrated near their surface. For inhomogeneities consistent with the observed large scale structure, the relative increase of the luminosity distance is of the order of a few per cent at redshifts near 1, and falls short of explaining the substantial increase required by the supernova data. On the other hand, the effect we describe is important for the correct determination of the energy content of the Universe from observations. Ā© IOP Publishing Ltd
Light propagation and large-scale inhomogeneities
We consider the effect on the propagation of light of inhomogeneities with sizes of order 10 Mpc or larger. The Universe is approximated through a variation of the Swiss-cheese model. The spherical inhomogeneities are void-like, with central underdensities surrounded by compensating overdense shells. We study the propagation of light in this background, assuming that the source and the observer occupy random positions, so that each beam travels through several inhomogeneities at random angles. The distribution of luminosity distances for sources with the same redshift is asymmetric, with a peak at a value larger than the average one. The width of the distribution and the location of the maximum increase with increasing redshift and length scale of the inhomogeneities. We compute the induced dispersion and bias of cosmological parameters derived from the supernova data. They are too small to explain the perceived acceleration without dark energy, even when the length scale of the inhomogeneities is comparable to the horizon distance. Moreover, the dispersion and bias induced by gravitational lensing at the scales of galaxies or clusters of galaxies are larger by at least an order of magnitude. Ā© IOP Publishing Ltd
Modified brane cosmologies with induced gravity, arbitrary matter content, and a Gauss-Bonnet term in the bulk
We extend the covariant analysis of the brane cosmological evolution in order to take into account, apart from a general matter content and an induced-gravity term on the brane, a Gauss-Bonnet term in the bulk. The gravitational effect of the bulk matter on the brane evolution can be described in terms of the total bulk mass as measured by a bulk observer at the location of the brane. This mass appears in the effective Friedmann equation through a term characterized as generalized dark radiation that induces mirage effects in the evolution. We discuss the normal and self-accelerating branches of the combined system. We also derive the Raychaudhuri equation that can be used in order to determine if the cosmological evolution is accelerating. Ā© 2007 The American Physical Society