3.2. The Hubble diagram with SNIa
As we have already discussed in section 2.3, the Hubble diagram of supernovae SNIa can be used not only to determine the Hubble constant (at relatively low redshifts) but also to trace the curvature of the Hubble relation at high redshifts (see  and references therein).
The two groups working laboriously on this subject (SCP and HZT) have found consistent results, by which the distant SNIa's are dimmer on average by 0.2 mag than what expected in a flat EdS model, which translates in them being ~ 10% further away than expected (, ). This implies that we live in an accelerating phase of the expansion of the Universe, a fact that supports a non-zero cosmological constant. The confidence intervals that their results put in the m - plane are shown in Fig.8.
Figure 8. Confidence intervals for (m - ) from the SCP and HZT results (from  with permission).
These results can be quantified by the following expression :
Together with the CMB fluctuation spectrum results we obtain:
However, since our understanding of the physics of SNIa'a is not complete (cf. , ) there could be some systematic effect, correlated with distance (eg. evolution), which could explain the dimming of the distant SNIa's and thus alleviate the > 0 interpretation. In Fig.9 we show the distance modulus residuals after subtracting an open m = 0.3 Hubble relation. The systematic distant-dependent effect mimics the accelerated expansion Hubble relation out to z ~ 0.8 -1. Beyond z ~ 1 the two relations depart due to the fact that the accelerated phase has to first pass from a decelerating one (see discussion in section 1.3) and this could provide a strong test for the possible distant dependent systematics. In fact, the recent discovery of the furthest known supernova (SN 1997ff) at a redshift of z ~ 1.7 , has provided evidence of the decelerating phase of the presently accelerating Universe (however, more very high-z supernovae are necessary to confirm this extraordinary result).
Figure 9. Distance modulus residuals after subtracting an open m = 0.3 Hubble relation (straight dashed line). The flat = 0.7 model is the thin curved line while the systematic effect is the thick label line (from  with permission).