Tests of the accelerating universe with near-infrared observations of a high-redshift type Ia supernova

We have measured the rest-frame B-, V-, and I-band light curves of a high-r edshift type Ia supernova (SN 1a), SN 1999Q (z = 0.46), using the Hubble Sp ace Telescope (HST) and ground-based near-infrared detectors. A goal of thi s study is the measurement of the color excess, EB-I, a sensitive indicator of interstellar or intergalactic dust, which could affect recent cosmologi cal measurements from high-redshift SNe Ia. Our observations disfavor a 30% opacity of SN Ia visual light by dust as an alternative to an accelerating universe. This statement applies to both Galactic-type dust (rejected at t he 3.4 sigma confidence level) and grayer dust (grain size > 0.1 mu m, reje cted at the 2.3-2.6 sigma confidence level) as proposed by Aguirre. The res t-frame I-band light curve shows the secondary maximum 1 month after the B maximum typical of nearby SNe fa of normal luminosity, providing no indicat ion of evolution as a function of redshift out to z approximate to 0.5. An expanded set of similar observations could improve the constraints on any c ontribution of extragalactic dust to the dimming of high-redshift SNe Ia.