Improved Distances to Type Ia Supernovae with Multicolor Light-Curve Shapes: MLCS2k2

We present an updated version of the multicolor light-curve shape method to measure distances to Type Ia supernovae (SNe Ia), incorporating new procedures for K-correction and extinction corrections. We also develop a simple model to disentangle intrinsic color variations and reddening by dust and expand the method to incorporate U-band light curves and to more easily accommodate prior constraints on any of the model parameters. We apply this method to 133 nearby SNe Ia, including 95 objects in the Hubble flow (cz ≥ 2500 km s^-1), which give an intrinsic dispersion of less than 7% in distance. The Hubble flow sample, which is of critical importance to all cosmological uses of SNe Ia, is the largest ever presented with homogeneous distances. We find that the Hubble flow SNe with H₀d_SN ≥ 7400 km s^-1 yield an expansion rate that is 6.5% ± 1.8% lower than the rate determined from SNe within that distance, and this can have a large effect on measurements of the dark energy equation of state with SNe Ia. Peculiar velocities of SN Ia host galaxies in the rest frame of the Local Group are consistent with the dipole measured in the cosmic microwave background. Direct fits of SNe Ia that are significantly reddened by dust in their host galaxies suggest that their mean extinction law may be described by R_V ≃ 2.7, but optical colors alone provide weak constraints on R_V.