Cosmology with type Ia supernovae : the ultraviolet 'catastroph'? Evolution with redshift of type Ia supernovae in the ultraviolet domain

To extract useful cosmological information from type Ia supernovae (SNe Ia), standardization laws have to be determined and applied assuming that these objects do not intrinsically change with redshift. Recently, to improve that standardization, attention has been drawn towards the SNe Ia UV spectra. This resulted in (what seemed as) the discovery of two subpopulations of objects, grouped by their bluer or redder UV colors. To confirm the existence of these subpopulations, we significantly enlarged the number of studied SNe Ia (going from ~100 to ~700 objects). With such a sample, SNe Ia cannot be separated in two groups anymore but they rather follow a continuum of colors. More critically, we showed that the SNe Ia UV colors display a significant evolution with redshift, resulting in a potentially important impact on subsequent cosmological measurements.