A model independent null test on the cosmological constant
  We use the Om statistic and the Genetic Algorithms (GA) in order to derive a
null test on the spatially flat cosmological constant model $\Lambda$CDM. This
is done in two steps: first, we apply the GA to the Constitution SNIa data in
order to acquire a model independent reconstruction of the expansion history of
the Universe $H(z)$ and second, we use the reconstructed $H(z)$ in conjunction
with the Om statistic, which is constant only for the $\Lambda$CDM model, to
derive our constraints. We find that while $\Lambda$CDM is consistent with the
data at the $2\sigma$ level, some deviations from $\Lambda$CDM model at low
redshifts can be accommodated.
