72 research outputs found
Light propagation through large-scale inhomogeneities in the Universe and its impact on cosmological observations
This paper analyses cosmological observations within inhomogeneous and exact
solutions of the Einstein equations. In some way the analyses presented here
can be freed from assumptions such as small amplitude of the density contrast.
The supernova observations are analysed using the Lema\itre-Tolman model and
the CMB observations are analysed using the quasispherical Szekeres model. The
results show that it is possible to fit the supernova data without the
cosmological constant. However if inhomogeneities of sizes and amplitudes as
observed in the local Universe are considered, their impact on cosmological
observations is small.Comment: 4 pages, to appear in the proceedings of the "Encuentros Relativistas
Espanoles - Spanish Relativity Meeting ERE07
Ricci focusing, shearing, and the expansion rate in an almost homogeneous Universe
The Universe is inhomogeneous, and yet it seems to be incredibly
well-characterised by a homogeneous relativistic model. One of the current
challenges is to accurately characterise the properties of such a model. In
this paper we explore how inhomogeneities may affect the overall optical
properties of the Universe by quantifying how they can bias the
redshift-distance relation in a number of toy models that mimic the real
Universe. The models that we explore are statistically homogeneous on large
scales. We find that the effect of inhomogeneities is of order of a few
percent, which can be quite important in precise estimation of cosmological
parameters. We discuss what lessons can be learned to help us tackle a more
realistic inhomogeneous universe.Comment: 22 pages, 8 figure
The effect of pressure gradients on luminosity distance - redshift relations
Inhomogeneous cosmological models have had significant success in explaining
cosmological observations without the need for dark energy. Generally, these
models imply inhomogeneous matter distributions alter the observable relations
that are taken for granted when assuming the Universe evolves according to the
standard Friedmann equations. Moreover, it has recently been shown that both
inhomogeneous matter and pressure distributions are required in both early and
late stages of cosmological evolution. These associated pressure gradients are
required in the early Universe to sufficiently describe void formation, whilst
late-stage pressure gradients stop the appearance of anomalous singularities.
In this paper we investigate the effect of pressure gradients on cosmological
observations by deriving the luminosity distance - redshift relations in
spherically symmetric, inhomogeneous spacetimes endowed with a perfect fluid.
By applying this to a specific example for the energy density distribution and
using various equations of state, we are able to explicitly show that pressure
gradients may have a non-negligble effect on cosmological observations. In
particular, we show that a non-zero pressure gradient can imply significantly
different residual Hubble diagrams for compared to when the
pressure is ignored. This paper therefore highlights the need to properly
consider pressure gradients when interpreting cosmological observations.Comment: Accepted for publication in Classical and Quantum Gravit
Bayesian analysis of the backreaction models
We present the Bayesian analysis of four different types of backreation
models, which are based on the Buchert equations. In this approach, one
considers a solution to the Einstein equations for a general matter
distribution and then an average of various observable quantities is taken.
Such an approach became of considerable interest when it was shown that it
could lead to agreement with observations without resorting to dark energy. In
this paper we compare the LambdaCDM model and the backreation models with SNIa,
BAO, and CMB data, and find that the former is favoured. However, the tested
models were based on some particular assumptions about the relation between the
average spatial curvature and the backreaction, as well as the relation between
the curvature and curvature index. In this paper we modified the latter
assumption, leaving the former unchanged. We find that, by varying the relation
between the curvature and curvature index, we can obtain a better fit.
Therefore, some further work is still needed -- in particular the relation
between the backreaction and the curvature should be revisited in order to
fully determine the feasibility of the backreaction models to mimic dark
energy.Comment: Extended analysis compared to v1. Matches published version
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