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 has been revived because its effect generally corresponds to dark energy, the presumed outward force to explain the universe's more-recently-discovered expansion-history, which otherwise would not quite fit general relativity's prediction: the observed history of the expansion suggests something is counteracting the inward pull of the gravity of the universe's contents. The concept provides a simple, working-model of dark energy, and the term Λ (aka lambda) is now used for dark energy. The specific favorite such working model is termed Lambda-CDM model, which is being calibrated and tested against the ongoing collection of observation data. Alternative models, in which dark energy does not act like a cosmological constant (and the symbol Λ would be considered a function of time), are also compared against observational evidence. Such alternate models include wCDM and w0waCDM.