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\nhigh-redshift type Ia supernova (SN Ia), SN 1999Q (z=0.46), using HST and\nground-based near-infrared detectors.\n A goal of this study is the measurement of the color excess, E_{B-I}, which\nis a sensitive indicator of interstellar or intergalactic dust which could\naffect recent cosmological measurements from high-redshift SNe Ia. Our\nobservations disfavor a 30% opacity of SN Ia visual light by dust as an\nalternative to an accelerating Universe. This statement applies to both\nGalactic-type dust\n (rejected at the 3.4 sigma confidence level) and greyer dust (grain size >\n0.1 microns; rejected at the 2.3 to 2.6 sigma confidence level) as proposed by\nAguirre (1999). The rest-frame $I$-band light cur ve shows the secondary\nmaximum a month after B maximum typical of nearby SNe Ia of normal luminosi ty,\nproviding no indication of evolution as a function of redshift out to z~0.5. A\nn expanded set of similar observations could improve the constraints on any\ncontribution of extragalactic dust to the dimming of high-redshift SNe Ia.\n