using current datasets'
---

Introduction {#sec:introduction}
============

By the end of the last century observations of type Ia supernovae (SNe Ia), used as standard candles, directly established the acceleration of the universe [@Riess1998; @Perlmutter1999], an awesome result, possibly only surpassed by the discovery of its very own expansion, around eighty years ago. They are still the backbone for the prevailing $\Lambda$CDM concordance model, which is furthermore corroborated by the combination of other probes, such as, e.g., cosmic microwave background anisotropies baryon acoustic oscillations and galaxy clustering [@Komatsu2011; @Eisenstein2005; @Percival2010; @Vikhlinin2009; @Mantz2010].

The sample standard deviation in the inferred, uncorrected absolute $B$ magnitude of typical sets of SNe Ia is of the order 0.4 mag (even after the exclusion of outliers such as the overluminous 1991T-like SNe Ia and the underluminous 1991bg-like SNe Ia) [@Phillips1993; @Vaughan1995; @Hamuy1996; @Contardo2000]. This amounts to a fractional uncertainty in luminosity of $-30\%$ and (under the assumption of negligible uncertainties in all other quantities) in luminosity distance of $20\%$. It is thus clear that SNe Ia are not exactly prima facie standard candles at all. However, even such a scatter, as compared to other categories of astrophysical sources (other supernova types, gamma-ray bursts, etc), is small 