  Free electrons deplete photons from type Ia supernovae through the (inverse)
Compton scattering. This Compton dimming increases with redshift and reaches
0.004 mag at $z=1$ and 0.01 mag at $z=2$. Although far from sufficient to
invalidate the existence of dark energy, it can bias constraint on dark energy
at a level non-negligible for future supernova surveys. This effect is
correctable and should be incorporated in supernova analysis. The Compton
dimming has similar impact on cosmology based on gamma ray bursts as standard
candles.
