What is Dark Energy?   Effects of Dark Energy   How is Dark Energy Detected?   What is a Type Ia Supernova?   Goals of SNfactory   SNfactory Collaboration   Role of CfCP

Dark energy refers to a mysterious form of energy density which causes an effective repulsive gravitational force, resulting in an accelerated expansion of the universe; by contrast, if the universe were filled only with conventional matter and radiation, the expansion would be slowing down. In the late 1990s, scientists studying the brightness of distant supernovae (exploding stars) were surprised to find evidence for dark energy and also that most of the total energy density (70%) in the universe is from dark energy; the remaining 30% is [mostly dark] matter. Shortly thereafter, additional evidence for dark energy was found in the pattern of temperature fluctuations in the 3 degree cosmic microwave background radiation and in the large-scale distribution of galaxies. Although the dark energy dominates the universe today, theory in fact predicts that its density should be 10¹²⁰ times larger, a puzzle known as the cosmological constant problem. There are currently a number of efforts to probe the nature of the dark energy with greater precision, and in particular determine whether the dark energy density is constant or evolves with cosmic time.