The accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor a(t) has a positive second derivative, so that the velocity at which a distant galaxy is receding from us should be continuously increasing with time. The Supernova Cosmology Project and High-Z Supernova Search Team first discovered the accelerating expansion of the Universe through observations of distant (“High-Z”) supernovae in 1998.
