Gold-set, and SNLS data. The joint analysis yields 0.03 eV $ < \sum m_\nu < 0.54$ eV at the 95% confidence level. The preference for massive neutrinos vanishes when systematics are included.]{}

Introduction {#sec1}
============

The construction of a cosmological model [@Bondi1960; @Ellis1971], must take into account any progress in the understanding of the laws of physics. To date, the reference model $\Lambda$CDM is based on the standard model of particle physics, general relativity, and some additional hypothesis about the symmetries of the background spacetime (the Copernican principle) to which it is mandatory to add the two still unknown components of cold dark matter (CDM) and cosmological constant ($\Lambda$).

We know now that it is imperative to include the effects of massive neutrinos, which behave as a warm or hot dark matter component depending on their mass. Strong evidence of such particles, which have not yet been included in the standard model of particle physics but whose existance is supported by e.g. grand-unified theories [@GUT], emerges from experimental results on oscillations of atmospheric, solar, and accelerator- or reactor-produced neutrinos, such as Super-Kamiokande, K2K, MINOS, KamLAND [see @NuExpReview for a review]. These results indicate a mixing of the three known neutrinos 