University of California Berkeley
Measurements of high-redshift type Ia supernovae (SNe Ia) have revealed that the expansion of the Universe is accelerating. A decade after this realization, our measurements of the nature of Dark Energy is dominated by systematics associated with supernovae. One such error is possible evolution between low and high-redshift SNe. As part of the ESSENCE survey, spectra of SNe are obtained to determine their type and redshift. A by-product of this effort is a large sample of low to moderate signal-to-noise SN Ia spectra. I present composite spectra of high-redshift SNe (and their low-redshift counterparts) using the cumulative signal of ~100 high-redshift SN spectra to assess small differences between the two samples.