1 pag. -- 235th AAS Meeting (Honolulu, HI)
High-redshift type Ia supernovae are crucial for constraining any time variation in dark energy. The Supernova Cosmology Project has finished executing a large (174 orbits, cycles 22-23) Hubble Space Telescope program, which has measured ~25 type Ia Supernovae above z~1 in the highest-redshift, most massive galaxy clusters known to date. We present the cosmology analysis, demonstrating substantial improvement to the uncertainty on the Dark Energy density above z~1.