Type Ia Supernovae Yielding Distances with 3-4% Precision

The luminosities of Type Ia supernovae (SN), the thermonuclear explosions of white dwarf stars, vary systematically with their in-trinsic color and light-curve decline rate. These relationships have been used to calibrate their luminosities to within ∼ 0.14–0.20 mag from broadband optical light curves, yielding individual distances accurate to ∼ 7–10%. Here we identify a subset of SN Ia that erupt in environments having high ultraviolet surface brightness and star-formation surface density. When we apply a steep model extinction law, these SN can be calibrated to within ∼ 0.065–0.075 mag, cor-responding to ∼ 3–4 % in distance — the best yet with SN Ia by a substantial margin. The small scatter suggests that variations in only one or two progenitor properties account for their light-curve-width/color/luminosity relation. The disruption of a white dwarf by a runaway thermonuclear reaction can create a SN explosion luminous enough to be visible across the last ∼ 10 Gyr of the cosmic expansion. The successive discoveries that intrinsically brighter SN Ia have light curves that fade more slowly (1) and have bluer color (2) made it possible to determine the luminosity (intrinsic brightness) of individual SN Ia with ∼ 0.14–0.20 mag accuracy, using only the SN color and the shape of the optical light curve. Through comparison between the intrinsic and apparent brightness of each SN Ia, a distance to each explosion can be estimated. Employing the precision afforded by light-curve calibration, measurements of luminosity distances to redshift z. 0.8 SN Ia showed that the cosmic expansion is accelerating (3, 4). 1 a