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In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is the value of the energy density of the vacuum of space. It was originally introduced by Albert Einstein in 1917 as an addition to his theory of general relativity to "hold back gravity" and achieve a static universe, which was the accepted view at the time. Einstein abandoned the concept after Hubble's 1929 discovery that all galaxies outside the Local Group (the group that contains the Milky Way Galaxy) are moving away from each other, implying an overall expanding universe. From 1929 until the early 1990s, most cosmology researchers assumed the cosmological constant to be zero. Since the 1990s, several developments in observational cosmology, especially the discovery of the accelerating universe from distant supernovae in 1998 (in addition to independent evidence from the cosmic microwave background and large galaxy redshift surveys), have shown that around 70% of the mass–energy density of the universe can be attributed to dark energy. While dark energy is poorly understood at a fundamental level, the main required properties of dark energy are that it functions as a type of anti-gravity, it dilutes much more slowly than matter as the universe expands, and it clusters much more weakly than matter, or perhaps not at all. The cosmological constant is the simplest possible form of dark energy since it is constant in both space and time, and this leads to the current standard model of cosmology known as the Lambda-CDM model, which provides a good fit to many cosmological observations as of 2014. ==Equation== The cosmological constant Λ appears in Einstein's field equation in the form of : where ''R'' and ''g'' describe the structure of spacetime, ''T'' pertains to matter and energy affecting that structure, and ''G'' and ''c'' are conversion factors that arise from using traditional units of measurement. When Λ is zero, this reduces to the original field equation of general relativity. When ''T'' is zero, the field equation describes empty space (the vacuum). The cosmological constant has the same effect as an intrinsic energy density of the vacuum, ''ρ''vac (and an associated pressure). In this context, it is commonly moved onto the right-hand side of the equation, and defined with a proportionality factor of 8: Λ = 8''ρ''vac, where unit conventions of general relativity are used (otherwise factors of ''G'' and ''c'' would also appear, i.e. Λ = 8 (G/c2)''ρ''vac = κ ''ρ''vac, where κ is Einstein's constant). It is common to quote values of energy density directly, though still using the name "cosmological constant", with convention 8 G = 1. (In fact, the true dimension of Λ is a length-2 and it has the value of ~1 10-52 m-2 or in reduced Planck units : ~3 10-122, calculated with the best present (2015) values of ΩΛ = 0.6911 ± 0.0062 and Ho = 67.74 ± 0.46 km/s / Mpc = 2.195 ± 0.015 10-18 s-1). A positive vacuum energy density resulting from a cosmological constant implies a negative pressure, and vice versa. If the energy density is positive, the associated negative pressure will drive an accelerated expansion of the universe, as observed. (See dark energy and cosmic inflation for details.) 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「Cosmological constant」の詳細全文を読む スポンサード リンク
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