cosmological constant

The cosmological constant (Λ or lambda) is a constant term included in Einstein's field equation (general relativity's equation) to counteract the inward force of gravity on the universe, providing a means by which the universe can remain static despite the pull of gravity from everything that the universe includes. Einstein initially included the term, presuming the universe was not contracting or expanding, but considered the term extraneous once the expansion of the universe was discovered and accepted.

The term and symbol (Λ) have been revived because its effect generally corresponds to dark energy (which is sometimes called Λ), a presumed outward force to explain the universe's more-recently-discovered expansion-history, which otherwise would not quite fit general relativity's prediction. Dark energy's equivalence to a constant term is a presumption, but that equivalence is a simple model that is still being explored as a working model with the potential to explain observation, the specific favorite working model being the Lambda-CDM model. When considering other models, in which dark energy does not act like a cosmological constant, the symbol Λ might possibly be considered a function of time (i.e., not a constant), or other terminology may be used for dark energy, such as in wCDM.