  One of the biggest mysteries in cosmology is Dark Energy, which is required
to explain the accelerated expansion of the universe within the standard model.
But maybe one can explain the observations without introducing new physics, by
simply taking one step back and re-examining one of the basic concepts of
cosmology, homogeneity. In standard cosmology, it is assumed that the universe
is homogeneous, but this is not true at small scales (<200 Mpc). Since general
relativity, which is the basis of modern cosmology, is a non-linear theory, one
can expect some backreactions in the case of an inhomogeneous matter
distribution. Estimates of the magnitude of these backreactions (feedback)
range from insignificant to being perfectly able to explain the accelerated
expansion of the universe. In the end, the only way to be sure is to test
predictions of inhomogeneous cosmological theories, such as timescape
cosmology, against observational data. If these theories provide a valid
description of the universe, one expects aside other effects, that there is a
dependence of the Hubble parameter on the line of sight matter distribution.
The redshift of a galaxy, which is located at a certain distance, is expected
to be smaller if the environment in the line of sight is mainly high density
(clusters), rather than mainly low density environment (voids). Here we present
a test for this prediction using redshifts and fundamental plane distances of
elliptical galaxies obtained from SDSS DR8 data. In order to get solid
statistics, which can handle the uncertainties in the distance estimate and the
natural scatter due to peculiar motions, one has to systematically study a very
large number of galaxies. Therefore, the SDSS forms a perfect basis for testing
timescape cosmology and similar theories. The preliminary results of this
cosmological test are shown in this contribution.
