What do the cosmological supernova data really tell us?
  Not much by themselves, aparently.
  We try to reconstruct the scale factor $a(t)$ of the universe from the SNe Ia
data, i.e. the luminosity distance $d_{L}(z)$, using only the cosmological
principle and the assumption that gravitation is governed by a metric theory.
In our hence "model-independent," or "cosmographic" study, we fit functions to
$d_{L}(z)$ rather than $a(t)$, since $d_{L}(z)$ is what is measured. We find
that the acceleration history of the universe cannot be reliably determined in
this approach due to the irregularity and parametrization-dependence of the
results.
  However, adding the GRB data to the dataset cures most of the irregularities,
at the cost of compromising the model-independent nature of the study slightly.
Then we can determine the redshift of transition to cosmic acceleration as
$z_{\rm t} \sim 0.50 \pm 0.09$ for a flat universe (larger for positive spatial
curvature).
  If Einstein gravity (GR) is assumed, we find a redshift at which the density
of the universe predicted from the $d_{L}(z)$ data is independent of curvature.
We use this point to derive an upper limit on matter density, hence a lower
limit on the density of dark energy. While these limits do not improve the
generally accepted ones, they are derived *only using the $d_{L}(z)$ data*.
