  We present multiepoch spectra of 13 high-redshift Type Ia supernovae (SNe Ia)
drawn from the literature, the ESSENCE and SNLS projects, and our own separate
dedicated program on the ESO Very Large Telescope. We use the Supernova
Identification (SNID) code of Blondin & Tonry to determine the spectral ages in
the supernova rest frame. Comparison with the observed elapsed time yields an
apparent aging rate consistent with the 1/(1+z) factor (where z is the
redshift) expected in a homogeneous, isotropic, expanding universe. These
measurements thus confirm the expansion hypothesis, while unambiguously
excluding models that predict no time dilation, such as Zwicky's "tired light"
hypothesis. We also test for power-law dependencies of the aging rate on
redshift. The best-fit exponent for these models is consistent with the
expected 1/(1+z) factor.
