Do we really see a Cosmological Constant in the Supernovae data?

The magnitude-redshift relation is one among the tools for a direct observational approach to cosmology. The discovery of high redshift Type Ia supernovae (SNIa), and their use as ``standard candles'', has resurrected interest in this approach. Recently collected data have been devoted to address the problem of measuring the cosmological parameters of the universe. Analysed in the framework of homogeneous models, they have yielded, as primary result, a strictly positive cosmological constant. However, a straight reading of the published measurements, conducted with no a priori idea upon which model would best describe our universe, at least up to redshifts $z\sim 1$, does not exclude the possibility of ruling out the Cosmological Principle - and cosmological constant - hypotheses. It is therefore here shown how the large scale homogeneity of this part of the universe can be tested on our past light cone, using the magnitude-redshift relation, provided sufficiently accurate data from sources at redshifts approaching $z=1$ would be available. An exemple of inhomogeneous model with zero cosmological constant reproducing the current observations is given. The presently published SNIa data can thus be interpreted as implying either a strictly positive cosmological constant in an homogeneous universe or large scale inhomogeneity with no constraint on ``standard candles'' at very high redshift is therefore urgently needed, to progress in both fondamental issues of the Cosmological Principle and cosmological constant