In 1998, published observations of Type Ia supernovae ("one-A") by the High-z Supernova Search Team [5] followed in 1999 by the Supernova Cosmology Project [6] suggested that the expansion of theuniverse is accelerating.[7] Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background, gravitational lensing, and thelarge scale structure of the cosmos as well as improved measurements of supernovae have been consistent with the Lambda-CDM model.[8]
Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow the expansion history of the Universe to be measured by looking at the relationship between the distance to an object and its redshift, which gives how fast it is receding from us. The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, the absolute magnitude, is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme, and extremely consistent, brightness.
Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination of dark matter and baryonic matter.[9]
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