Supernova tests of the timescape cosmology
  The timescape cosmology has been proposed as a viable alternative to
homogeneous cosmologies with dark energy. It realises cosmic acceleration as an
apparent effect that arises in calibrating average cosmological parameters in
the presence of spatial curvature and gravitational energy gradients that grow
large with the growth of inhomogeneities at late epochs. Recently Kwan, Francis
and Lewis [arXiv:0902.4249] have claimed that the timescape model provides a
relatively poor fit to the Union and Constitution supernovae compilations, as
compared to the standard Lambda CDM model. We show this conclusion is a result
of systematic issues in supernova light curve fitting, and of failing to
exclude data below the scale of statistical homogeneity, z < 0.033. Using all
currently available supernova datasets (Gold07, Union, Constitution, MLCS17,
MLCS31, SDSS-II, CSP, Union2), and making cuts at the statistical homogeneity
scale, we show that data reduced by the SALT/SALT-II fitters provides Bayesian
evidence that favours the spatially flat Lambda CDM model over the timescape
model, whereas data reduced with MLCS2k2 fitters gives Bayesian evidence which
favours the timescape model over the Lambda CDM model. We discuss the questions
of extinction and reddening by dust, and of intrinsic colour variations in
supernovae which do not correlate with the decay time, and the likely impact
these systematics would have in a scenario consistent with the timescape model.
