A term originally introduced by Albert Einstein in his General Theory of Relativity to represent a constant energy density filling space uniformly. Einstein added the cosmological constant (denoted by the Greek letter Λ) to allow for a static universe, which was the prevailing belief in the early 20th century. However, after the discovery of the universe’s expansion by Edwin Hubble, Einstein reportedly called it his “biggest blunder.”
In modern cosmology, the cosmological constant has gained new significance as a possible explanation for dark energy—the mysterious force causing the accelerated expansion of the universe. It represents the energy of empty space, or vacuum energy, which exerts a repulsive gravitational effect that counteracts gravity on cosmic scales.
Today, the cosmological constant is a central part of the ΛCDM model (Lambda Cold Dark Matter model), the standard model of cosmology. This model successfully describes the composition and evolution of the universe, where Λ (dark energy) accounts for about 70% of the total energy density, driving the universe’s ongoing acceleration.