The diversity of type Ia supernovae from broken symmetries

Type Ia supernovae result when carbon-oxygen white dwarfs in binary systems accrete mass from companion stars, reach a critical mass and explode. The near uniformity of their light curves makes these supernovae good `standard candles' for measuring cosmic expansion(1-4), but a correction must be applied to account for the fact that the brighter ones have broader light curves(5). One-dimensional modelling, with a certain choice of parameters, can reproduce this general trend in the width-luminosity relation(6-8); but the processes of ignition and detonation have recently been shown to be intrinsically asymmetric(9-13), so parameterization must have its limits. Here we report multi-dimensional modelling of the explosion physics and radiative transfer, which reveals that the breaking of spherical symmetry is a critical factor in determining both the width luminosity relation and the observed scatter about it. The deviation from spherical symmetry can also explain the finite polarization detected in the light from some supernovae(14). The slope and normalization of the width-luminosity relation has a weak dependence on certain properties of the white dwarf progenitor, in particular the trace abundances of elements other than carbon and oxygen. Failing to correct for this effect could lead to systematic overestimates of up to 2 per cent in the distance to remote supernovae.