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In physics, quintessence is a hypothetical form of dark energy, more precisely a scalar field, postulated as an explanation of the observation of an accelerating rate of expansion of the universe, rather than due to a true cosmological constant. The first example of this scenario was proposed by Ratra and Peebles (1988). The concept was expanded to more general types of time-varying dark energy and the term "quintessence" was first introduced in a paper by R.R.Caldwell, Rahul Dave and Paul Steinhardt. It has been proposed by some physicists to be a fifth fundamental force. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic, that is, it changes over time, unlike the cosmological constant which always stays constant. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Specifically, it is thought that quintessence became repulsive about ten billion years ago (the universe is approximately 13.8 billion years old).〔Christopher Wanjek; "Quintessence, accelerating the Universe?"; http://www.astronomytoday.com/cosmology/quintessence.html〕 == Scalar field == Quintessence is a scalar field with an equation of state where ''w''''q'', the ratio of pressure ''p''''q'' and density ''q'', is given by the potential energy and a kinetic term: : Hence, quintessence is dynamic, and generally has a density and ''w''''q'' parameter that varies with time. By contrast, a cosmological constant is static, with a fixed energy density and ''w''''q'' = −1. 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「Quintessence (physics)」の詳細全文を読む スポンサード リンク
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