In this Wikipedia article there can be much read about type-Ia supernovae. One can read:
This type Ia category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.
Could it be, however, that this (hypothesized) stability of the peak luminosity value is not constant in time? Let's assume the laws of physics didn't change since the big bang (this seems pretty obvious, but some physicists say these laws vary from place to place and from time to time).
This would imply that these "standard candles" can't be used for measuring the expansion rate of the Universe in the past? This, on its turn, could mean that dark energy may not exist after all (contrary to dark matter, the effects of which are certain, contrary to its Nature).