Filippenko is the only person who was a member of both the
Supernova Cosmology Project and the
High-z Supernova Search Team, which used observations of extragalactic Type Ia
supernovae to discover the
accelerating universe and its implied existence of
dark energy. The discovery was voted the top science breakthrough of 1998 by
Science magazine and resulted in the 2011
Nobel Prize in Physics being awarded to the leaders of the two project teams. Filippenko developed and runs the
Katzman Automatic Imaging Telescope (KAIT), a fully
robotic telescope which conducts the
Lick Observatory Supernova Search (LOSS). During the years 1998–2008, it was by far the world's most successful search for relatively nearby supernovae, finding over 650 of them. His research, concentrating on optical spectroscopy, showed that many core-collapse supernovae result from massive stars with partially or highly stripped envelopes, helped establish the Type IIn subclass characterized by ejecta interacting with circumstellar gas, observationally identified the progenitors of some supernovae, revealed that many supernovae are quite aspherical, and showed that Type Ia supernovae exhibit considerable heterogeneity—crucial to the development of methods to calibrate them for accurate distance determinations. Filippenko's early work showed that the nuclei of most bright, nearby galaxies exhibit activity physically similar to that of quasars, driven by gas accretion onto a supermassive black hole. He is also a member of the
Nuker Team which uses the
Hubble Space Telescope to examine
supermassive black holes and determined the relationship between a
galaxy's central
black hole's mass and
velocity dispersion. In half a dozen X-ray binary stars, he provided compelling dynamical evidence for a stellar-mass black hole. His robotic telescope (KAIT) made some of the very earliest measurements of the optical afterglows of gamma-ray bursts. The Thompson-Reuters "incites" index ranked Filippenko as the most cited researcher in space science for the ten-year period between 1996 and 2006. == In the media ==