His research involves measurements of the cosmological framework with supernovae (exploding stars) and Cepheids (pulsating stars). Currently, he leads the SHOES Team in efforts to improve the measurement of the Hubble Constant and the HIgher-z Team to find and measure the most distant type Ia supernovae known to probe the origin of cosmic acceleration.

In 2011, he was named a co-winner of the Nobel Prize in Physics and was awarded the Albert Einstein Medal for his leadership in the High-z Supernova Search Team’s discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained "dark energy" filling the universe. The discovery was named by Science magazine in 1998 as "the Breakthrough Discovery of the Year."