Dimming of Supernovae and Gamma Ray Busts by Compton Scattering and Its Cosmological Implications

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. Subject headings: cosmology: distance scale–theory 1. COMPTON DIMMING OF TYPE IA SUPERNOVAE Type Ia supernovae (SNe Ia) are standardizable as cosmological standard candles to measure cosmological distance and thus infer the expansion history of the universe. Current observations on SNe Ia have enabled the discovery of the late time acceleration of the universe (Riess et al. 1998; Perlmutter et al. 1999). This discovery has profound impact on fundamental physics, leading to either a dominant dark energy component with equation of state w ≡ P/ρ < −1/3 or significant deviations from general relativity at around Hubble scale. Ongoing and planned supernova surveys have the power to significantly improve these cosmological constraints and hopefully clarify the role of the cosmological constant in our universe (Albrecht et al. 2006). Various astrophysical processes, besides the possible intrinsic evolution in SN luminosity, can degrade the standard candle merit of SNe Ia by altering the supernova flux. An incomplete list include gravitational lensing magnification (Holz 1998), peculiar velocity (Hui & Greene 2006), dust extinction and incomplete K-correction. If not handled correctly, they can not only increase statistical errors, but also systematically bias the cosmological constraints. In this letter, we point out a new source of systematical errors, relevant for precision cosmology. The universe is (almost) completely ionized after z ∼ 6-10. Free electrons in the universe treat all low energy photons (hν ≪ mec) equally and Compton scatter off them with equal probability exp(−τ), where τ is the Thompson optical depth