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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 has a positive second derivative, so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.〔(Is the universe expanding faster than the speed of light? ) (see final paragraph)〕 In 1998 two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team simultaneously obtained results suggesting a totally unexpected acceleration in the expansion of the Universe by using distant type Ia supernovae as standard candles. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery.〔''Cosmology'', Steven Weinberg, Oxford University Press, 2008〕 Confirmatory evidence has been found in baryon acoustic oscillations and other new results about the clustering of galaxies. ==Background== Since Hubble's discovery of the expansion of the Universe in 1929, the Big Bang model has become the accepted explanation for the origin of our Universe. The Friedmann equation defines how the energy in the Universe drives its expansion. : where the pressure P is defined by the cosmological model chosen. (see explanatory models below) Physicists at one time were so assured of the deceleration of the Universe's expansion that they introduced a so-called deceleration parameter .〔Ryden, Barbara. "Introduction to Cosmology." Physics Today: 77. Print.〕 Current observations point towards this deceleration parameter being negative. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「accelerating universe」の詳細全文を読む スポンサード リンク
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