  The most direct and strongest evidence for the presence of dark energy is
provided by the measurement of galaxy distances using SNe Ia. This result is
based on the assumption that the corrected brightness of SN Ia through the
empirical standardization would not evolve with look-back time. Recent studies
have shown, however, that the standardized brightness of SN Ia is correlated
with host morphology, host mass, and local star formation rate (SFR),
suggesting a possible correlation with stellar population property. To
understand the origin of these correlations, we have continued our
spectroscopic observations to cover most of the reported nearby early-type host
galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained
the most direct and reliable estimates of population age and metallicity for
these host galaxies. We find a significant correlation between SN luminosity
(after the standardization) and stellar population age at a 99.5 % confidence
level. As such, this is the most direct and stringent test ever made for the
luminosity evolution of SN Ia. Based on this result, we further show that the
previously reported correlations with host morphology, host mass, and local SFR
are most likely originated from the difference in population age. This
indicates that the light-curve fitters used by the SNe Ia community are not
quite capable of correcting for the population age effect, which would
inevitably cause a serious systematic bias with look-back time. Notably, taken
at face values, most of the Hubble residual used in the discovery of the dark
energy appears to be affected by the luminosity evolution.
