In 1998 Dr. Riess led a study for the High-z Team which provided the first direct and published evidence that the expansion of the Universe was accelerating and filled with Dark Energy (Riess et al. 1998, AJ, 116, 1009), a result which, together with the Supernova Cosmology Project's result, was called the Breakthrough Discovery of the Year by Science Magazine in 1998.
On the ten year anniversary of this discovery, Symmetry Magazine reprinted the key page from his lab notebook (shown below) showing the first indication, that the expansion of the Universe was accelerating.
He followed this work with a number of studies to test the susceptibility of this measurement to contamination by unexpected types of dust or evolution. To this aim, Dr. Riess led the Hubble Higher-z Team beginning in 2002 to find 25 of the most distant supernovae known with the Hubble Space Telescope, all at redshift greater than 1. This work culminated in the first highly significant detection of the preceding, decelerating epoch of the Universe and helped to confirm the reality of acceleration by disfavoring alternative, astrophysically-motivated explanations for the faintness of supernovae (Riess et al. 2004, ApJ, 607, 655).
This work also began characterizing the time-dependent nature of dark energy. It has been identified by NASA as the #1 Achievement of the Hubble Space Telescope to date.
This work and many other contributions led to Dr. Riess being awarded the Nobel Prize in Physics in 2011. He shared the award with astrophysicists Saul Perlmutter and Brian Schmidt.