2.2. Is the Universe accelerating ?

It is often mentioned that the present day data on the CMB excluded a model without a cosmological constant. Given the present-day quality of the data, and the anticipated accuracy one can hope from satellite experiments, this is a crucial issue. Actually, what's happen is that an Einstein-de Sitter is at the boarder of the 3 - contour in likelihood analysis. But this is not sufficient to claim that the model is excluded at 3 - ! Actually, a model without a cosmological constant provides a very acceptable fit to the data in term of a goodness of fit. Therefore, CMB data do not request a non-zero cosmological constant.

Figure 2. An example of an acceptable model to the CMB data without inclusion of a cosmological constant (left) The Hubble constant has been taken to a low value of 44 km/s/Mpc. An exemple of optimal model is shown, corresponding to the concordance model. Both provide an acceptable fit in term of goodness of fit. Courtesy of M. Douspis.

The possible detection of a cosmological constant from distant supernovae has brought the essential piece of evidence largely comforting the so-called concordance model: the apparent luminosity of distant supernovae now appears fainter, i.e. at larger distance, than expected in any decelerating universe (Reiss et al, 1998; Perlmutter et al., 1999) and can therefore be explained only within an accelerating universe (under the assumption of standard candle). Indeed a CDM model in a flat universe dominated by a cosmological constant is in impressive agreement with most of existing data: such a model is consistent with the HST measurement of the Hubble constant, the age of the Universe, the power spectrum and the amplitude of matter fluctuations as measured by clusters abundance and weak lensing on large scale, as well as most current measurements of the mass content on small scales obtained by various technics. The concordance model offers therefore a remarkable success for the CDM theory, but at the expense of the introduction of a non-zero cosmological constant.