In 1998, two teams studying distant Type Ia supernovae found evidence that cosmic expansion was accelerating rather than simply slowing under gravity. Comparing brightness, distance and redshift changed the cosmological picture. It ranks second for the scale of the unanswered question it exposed. Dark energy names the component used to explain acceleration in the standard model; the observation does not by itself reveal its underlying nature. The photograph illustrates a Type Ia supernova, not the full discovery dataset.
Source: Distant supernovae reveal accelerating expansion.
Image credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team · Public domain. Resized and JPEG compressed.
