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In as a modification of his original theory of General Relativity to achieve a Stationary Universe . Einstein abandoned the concept after the observation of the Hubble Redshift indicated that the universe might not be stationary. However, the discovery of Cosmic Acceleration in the 1990s has renewed interest in a cosmological constant. The cosmological constant Λ appears in Einstein's Modified Field Equation in the form : where R and G pertain to the structure of Spacetime , T pertains to matter (thought of as affecting that structure), and G and C are conversion factors which 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 ). Astronomical observations imply that the constant cannot exceed 10-46 km-2.1 The cosmological constant has the same effect as an intrinsic Energy Density of the vacuum, ''ρvac'' (and an associated Pressure ). In this context it is commonly defined with a Proportionality factor of 8π: Λ = 8π''ρvac'', where modern unit conventions of general relativity are followed (otherwise factors of G and c would also appear). It is common to quote values of energy density directly, though still using the name "cosmological constant". 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 empty space; see Dark Energy and Cosmic Inflation for details. In lieu of the cosmological constant, cosmologists often quote the ratio between the energy density due to the cosmological constant and the current , but the energy density due to the cosmological constant remains unchanged throughout the history of the universe. GENERAL RELATIVITY Einstein included the cosmological constant as a term in his developed his static theory, observations by Edwin Hubble indicated that the universe appears to be expanding; this was consistent with a cosmological solution to the ''original'' general-relativity equations that had been found by the mathematician Friedman . It is now thought that adding the cosmological constant to Einstein's equations does not lead to a static universe at equilibrium because the Equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting. Since it no longer seemed to be needed, Einstein abandoned the cosmological constant and called it the ''"biggest , such as Quintessence , but the cosmological constant is in most respects the most Economical solution. Thus, the current standard model of cosmology, the Lambda-CDM Model , includes the cosmological constant, which is measured to be on the order of 10-35s-2, or 10-47GeV4, or 10-29g/cm3, or about 10-120 in Reduced Planck Units . Some early generalizations of Einstein's gravitational theory, known as Classical Unified Field Theories , either introduced a cosmological constant on theoretical grounds or found that it arose naturally from the mathematics. For example, Sir Arthur Stanley Eddington claimed that the cosmological constant version of the vacuum field equation expressed the " Epistemological " property that the universe is "self- Gauging ", and Erwin Schrödinger 's pure- Affine theory using a simple Variational Principle produced the field equation with a cosmological term. COSMOLOGICAL CONSTANT PROBLEM A major outstanding from Particle Physics . One possible explanation for the small but non-zero value was noted by (also see Quintessence ). Critics note that these Multiverse theories, when used as an explanation for fine-tuning, commit the Inverse Gamblers Fallacy . As was only recently seen, by works of of the observable universe (see the Holographic Principle ). More recent work has suggested the problem may be indirect evidence of a Cyclic Universe predicted by String Theory . With every cycle of the universe ( Big Bang then eventually a Big Crunch ) taking about a Trillion (1012) years, "the amount of matter and radiation in the universe is reset, but the cosmological constant is not. Instead, the cosmological constant gradually diminishes over many cycles to the small value observed today." 'Cyclic universe' can explain cosmological constant , NewScientistSpace, 04 May 2006 Critics respond that, as the authors acknowledge in their paper, the model “entails tuning” to “the same degree of tuning required in any cosmological model.” and Turok, 1437 SEE ALSO
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