The graph plots luminosity against time for two types of superluminous supernova (R and I; see ref. 9) and for the traditional types (supernovae (SNe) type Ia and the core-collapse subtypes Ib, Ic and II). Magnitude is a logarithmic measure of an object's luminosity, with one unit of magnitude corresponding to a factor of 2.5 in luminosity. The intrinsic (absolute) magnitude is shown on the left and the observed magnitude for a supernova at redshift (z) 4 is shown on the right. Time is shown in days from the supernova's peak luminosity, with time at the supernova's location at the bottom and time as observed on Earth for a redshift-4 supernova at the top. Cooke et al.3 have discovered two superluminous supernovae, at redshifts 2.05 and 3.9, by adding together images from the CFHT Legacy Survey in monthly blocks. This approach does not result in loss of time resolution when looking back at high redshifts. Cosmological time dilation means that observers see the Universe evolve more slowly, by a factor of 1 + z, than they would if they were at the object's location. The authors obtained an effective time sampling of the real variation in luminosity five times shorter than is actually observed for the redshift-3.9 supernova.