Do Type Ia Supernovae prove Lambda > 0 ?
  The evidence for positive cosmological constant $\Lambda$ from Type Ia supernovae is reexamined.   Both high redshift supernova teams are found to underestimate the effects of host galaxy extinction. The evidence for an absolute magnitude- decay time relation is much weakened if supernovae not observed before maximum light are excluded. Inclusion of such objects artificially supresses the scatter about the mean relation.   With a consistent treatment of host galaxy extinction and elimination of supernovae not observed before maximum, the evidence for a positive lambda is not very significant (3-4 $\sigma$). A factor which may contribute to apparent faintness of high z supernovae is evolution of the host galaxy extinction with z.   The Hubble diagram using all high z distance estimates, including SZ clusters and gravitational lens time-delay estimates, does not appear inconsistent with an $\Omega_o$ = 1 model.   Although a positive $\Lambda$ can provide an, albeit physically unmotivated, resolution of the low curvature implied by CMB experiments and evidence that $\Omega_o <$ 1 from large-scale structure, the direct evidence from Type Ia supernovae seems at present to be inconclusive. 
Michael Rowan-Robinson