Supernovae Observations of the Accelerating Universe

Over the past three decades, supernovae observations have been the primary tool used in measuring both the current expansion rate and the expansion history of the universe. Supernovae are preferred because they are the best known standard candle and they can be observed in the early and recent universe. But because they are rare and unpredictable, they are difficult to measure. In the late 1990’s, two independent groups developed wide-filed imaging methods to systematically discover supernovae type 1a, and measure their magnitude and redshift. Their results showed that the expansion of the universe is accelerating. This discovery has many implications on cosmological theory, and requires the existence of some form of dark energy or cosmological constant to supply the repulsive force that leads to acceleration.