The Greek letter representing the cosmological constant in Einstein’s field equations is capital lambda, written Λ. It appears as the term Λgμν, where it contributes to the relationship between spacetime geometry and matter.
Albert Einstein introduced the cosmological constant in 1917. At the time, many scientists—including Einstein—expected the universe to be static. Because gravity would make a static universe collapse, he added Λ as a counteracting term. After observations showed that the universe is expanding, Einstein reportedly regarded the extra term as unnecessary.
The idea returned to cosmology in the late twentieth century. Observations of distant Type Ia supernovae in 1998 showed that cosmic expansion is accelerating. A positive cosmological constant provides one explanation: empty space itself has a uniform energy density and negative pressure. In modern cosmology, this effect is commonly associated with dark energy.
Lambda is not the same as the density parameter ΩΛ. Λ is a constant in the equations, while ΩΛ is a dimensionless quantity describing the fraction of the universe’s present energy budget associated with it. Other forms of dark energy could produce similar acceleration without being a strict cosmological constant.