Cosmological Constant Problem

The cosmological constant Λ — the term Einstein added to his field equations to describe the energy density of empty space, now identified with dark energy — carries one of the most embarrassing numbers in all of science. Observations constrain Λ to approximately 10⁻¹²² in Planck units, an extraordinarily small but non-zero value. Quantum field theory, which successfully describes all other known forces, predicts that vacuum fluctuations should contribute an energy density roughly 10¹²² times larger — a disagreement spanning 122 orders of magnitude, the largest discrepancy between theory and observation in physics.

ΛCDM simply inserts Λ as a free parameter fitted to data and offers no explanation for its value. Every quantum field that exists contributes vacuum energy; summing them yields a catastrophically large cosmological constant unless cancellations occur to extraordinary precision. No known symmetry or principle enforces such cancellation. Supersymmetry, the leading candidate for reducing the prediction, still overshoots by roughly 60 orders of magnitude even if it is exact. The cosmological constant problem is not a tension between two measurements — it is a fundamental incompatibility between the two pillars of modern physics: general relativity and quantum field theory.