5 research outputs found
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
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Apparent and average acceleration of the Universe
In this paper we consider the relation between the volume deceleration
parameter obtained within the Buchert averaging scheme and the deceleration
parameter derived from the supernova observation. This work was motivated by
recent findings that showed that there are models which despite
have volume deceleration parameter . This opens the possibility
that backreaction and averaging effects may be used as an interesting
alternative explanation to the dark energy phenomenon.
We have calculated in some Lema\^itre--Tolman models. For those
models which are chosen to be realistic and which fit the supernova data, we
find that , while those models which we have been able to find
which exhibit turn out to be unrealistic. This indicates that
care must be exercised in relating the deceleration parameter to observations.Comment: 15 pages, 5 figures; matches published versio