  UV and optical photometry of Type Ia supernovae (SNe Ia) at low redshift have
revealed the existence of two distinct color groups, NUV-red and NUV-blue
events. The color curves differ primarily by an offset, with the NUV-blue u-
color curves bluer than the NUV-red curves by 0.4 mag. For a sample of 23 low-z
SNe~Ia observed with Swift, the NUV-red group dominates by a ratio of 2:1. We
compare rest-frame UV/optical spectrophotometry of intermediate and high-z SNe
Ia with UVOT photometry and HST spectrophotometry of low-z SNe Ia, finding that
the same two color groups exist at higher-z, but with the NUV-blue events as
the dominant group. Within each red/blue group, we do not detect any offset in
color for different redshifts, providing insight into how SN~Ia UV emission
evolves with redshift. Through spectral comparisons of SNe~Ia with similar peak
widths and phase, we explore the wavelength range that produces the UV/OPT
color differences. We show that the ejecta velocity of NUV-red SNe is larger
than that of NUV-blue objects by roughly 12% on average. This velocity
difference can explain some of the UV/optical color difference, but differences
in the strengths of spectral features seen in meanspectra require additional
explanation. Because of the different b-v colors for these groups, NUV-red SNe
will have their extinction underestimated using common techniques. This, in
turn, leads to under-estimation of the optical luminosity of the NUV-blue
SNe~Ia, in particular, for the high-redshift cosmological sample. Not
accounting for this effect should thus produce a distance bias that increases
with redshift and could significantly bias measurements of cosmological
parameters.
