(Λ) A mathematical term introduced by A. Einstein into the equations of general relativity to obtain a solution corresponding to a static universe. This term describes a kind of pressure or (if it has the opposite sign) tension exerted by space itself, which can cause the Universe to expand or contract even in the absence of any matter. When the expansion of the Universe was discovered, Einstein came to regard his introduction of the cosmological constant as his ‘greatest blunder’. Recently, however, observations of distant supernovae and other observations have shown that the expansion of the Universe appears to be accelerating (see cosmology). Because gravity is an attractive force that would make the expansion decelerate, something like a cosmological constant must exist to counteract and even override it. Astronomers often attribute the observed acceleration to a force called dark energy. However, the true nature of dark energy remains one of the biggest questions in astronomy.
Subjects: Astronomy and Astrophysics.