Perlmutter heads the Supernova Cosmology Project at Lawrence Berkeley National Laboratory. It was this team along with the competing High-z Supernova Search Team led by Riess and Schmidt, which found evidence of the accelerating expansion of the universe based on observing Type Ia supernova in the distant universe. Type Ia supernova occurs whenever a white dwarf star gains enough additional mass to pass above the Chandrasekhar limit, usually by stealing additional mass from a companion star. Since all Type Ia supernovae are believed to occur in essentially the same way, they form a standard candle whose intrinsic luminosity can be assumed to be approximately the same in all cases. By measuring the apparent luminosity of the explosion from Earth, researchers can then infer the distance to supernova. Comparing this inferred distance to the apparent redshift of the explosion allows the observer to measure both the distance and relative velocity of the supernova.