definition

In physical cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda) was proposed by Albert Einstein 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, as he had based his theory on the idea that the universe is unchanging.[1] However, the discovery of cosmic acceleration in the 1990s has renewed interest in a cosmological constant. Equation The cosmological constant appears in Einstein's modified field equation in the form of R_{\mu \nu} -\frac{1}{2}R\,g_{\mu \nu} + \Lambda\,g_{\mu \nu} = {8 \pi G \over c^4} T_{\mu \nu} where R and g pertain to the structure of spacetime, T pertains to matter and energy (thought of as affecting that structure), and G and c are conversion factors that arise from using traditional units of measurement. When lambda 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, vac (and an associated pressure). In this context it is commonly defined with a proportionality factor of 8: lambda = 8vac, where unit conventions of general relativity are used (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.) source: http://en.wikipedia.org/wiki/Cosmological_constant

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