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 of the universe's contents. Einstein initially included the term, presuming the universe was not contracting or expanding, but he considered the term extraneous once the expansion of the universe was discovered and accepted. (That discovery did not reveal what triggered the outward expansion, but merely that the expansion is occurring: it was presumed that everything was currently effectively "coasting" away from each other, and with the passage of time, gravity would presumably reduce the rate of this expansion, possibly eventually stopping the expansion and bringing about contraction.)

The cosmological constant concept was later revived to account for the effects of dark energy, which is a term for whatever is causing the universe's more-recently-discovered expansion-history, which otherwise would not quite fit general relativity's prediction. This observed history of the expansion suggests something is counteracting the inward pull of the gravity of the universe's contents. The cosmological constant provides a simple working-model of dark energy, and the term Λ (aka lambda) is now used to mean dark energy. The currently-favored such working model is termed Lambda-CDM model, which is constantly calibrated and tested against the ongoing collection of observation data. Alternative models, in which dark energy does not act like a cosmological constant (in which Λ would represent a function of time), are also compared against observational evidence. Such alternate models include wCDM and w0waCDM.