2005ApJ...626L..11F

We have obtained high-quality Keck optical spectra at three epochs of the Type Ia supernova 1997ex, whose redshift z is 0.361. The elapsed calendar time between the first two spectra was 24.88 days, and that between the first and third spectra was 30.95 days. In an expanding universe where 1+z represents the factor by which space has expanded between the emission and detection of light, the amount of aging in the supernova rest frame should be a factor of 1/(1+z) smaller than the observed-frame aging; thus, we expect SN 1997ex to have aged 18.28 and 22.74 days between the first epoch and the second and third epochs, respectively. The quantitative method for determining the spectral-feature age of an SN Ia reveals that the corresponding elapsed times in the supernova rest frame were 16.97±2.75 and 18.01±3.14 days, respectively. This result is inconsistent with no time dilation with a significance level of 99.0%, providing evidence against ``tired light'' and other hypotheses in which no time dilation is expected. Moreover, the observed timescale of spectral evolution is inconsistent with that expected in the ``variable mass theory.'' The result is within ∼1 σ of the aging expected from a universe in which redshift is produced by cosmic expansion.