EPM regression obtained for SN 2005cs in the two versions (the left plot shows the more conservative approach, where we adopted a flat prior for t₀, and the right plot shows the fit for the t₀ prior that is informed by the light curve). The x-axis shows the time elapsed since the explosion epoch t₀, MJD 53548.73, for both plots. The points show the evolution of Θ/v_ph for this supernova as inferred from spectral fitting. The shaded region shows the uncertainty of the fit.