Cosmological constant

Article By:

Caldwell, Robert Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.800540

The constant gravitational source added to the general theory of relativity by Albert Einstein to balance the gravitational field equations necessary for a static universe. In formulas it is denoted Λ (lambda) and has units of curvature, or (length)⁻². The existence of the cosmological constant would mean that empty space gravitates as if it were filled by a static, uniform energy density ρ = Λc⁴/8πG and pressure p = −ρ, where G is Newton's constant of gravitation and c is the speed of light. It is speculated to be responsible for dark energy, because a sufficiently large cosmological constant would cause the accelerated expansion of spacetime. However, the physical origin of the cosmological constant is not understood. Quantum theory predicts the existence of the cosmological constant, but with an energy density that is approximately 10¹²⁰ times too large to be compatible with observations. This gross mismatch between theory and observation, known as the cosmological-constant problem, is a deep enigma. It is thought that a resolution requires a quantum theory of gravity. Testing for the existence of the cosmological constant is a major goal of current physics and astronomy research. See also: Accelerating universe; Dark energy; Quantum gravitation; Relativity

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