Cosmological Constant

The cosmological constant, denoted Λ, is a term Einstein added to his field equations in 1917 of the form Λg_{μν}, representing a uniform energy density intrinsic to empty space. In the Friedmann equations it appears as a constant contribution Λc²/3 to the expansion rate. Unlike matter or radiation, the energy density associated with Λ does not dilute as the universe expands: stretch space and you get proportionally more vacuum energy. This unchanging density, combined with its strongly negative pressure (p = −ρc²), is what allows Λ to drive accelerated expansion through the second Friedmann equation.

Einstein introduced Λ to make a static universe possible, balancing the inward pull of matter. When Hubble's observations established that the universe was expanding, the static model became unnecessary, and Einstein is famously said to have called the constant his 'greatest blunder.' For decades it was set to zero. That changed in 1998, when measurements of distant Type Ia supernovae revealed that the cosmic expansion is accelerating — exactly the behaviour a positive Λ produces — and the constant returned as the leading description of dark energy.

The measured value, Λ ≈ 1.1 × 10⁻⁵² m⁻², corresponds to a vacuum energy density that is fantastically small by particle-physics standards. Attempts to compute it from quantum field theory, by summing the zero-point energies of the vacuum, overshoot the observed value by some 120 orders of magnitude — the largest known discrepancy between theory and observation in physics, sometimes called the cosmological constant problem. Whether dark energy is genuinely a constant Λ, or a dynamical field that only mimics one, remains an open question.

Introduced by Einstein in 1917 to permit a static universe, abandoned after Hubble's 1929 discovery of expansion, and revived in 1998 when supernova surveys revealed accelerating expansion. Lemaître had argued for its physical reality as a vacuum energy decades earlier.