Cosmological constant

The cosmological constant Λ is the term in the Einstein field equations that represents a uniform energy density permeating all of spacetime, causing the expansion of the universe to accelerate. Einstein introduced it in 1917 to permit static cosmological solutions, then discarded it after Hubble's 1929 discovery of cosmic expansion. Its resurrection began in 1998 with supernova observations showing that the universe's expansion is accelerating — a result now confirmed by cosmic microwave background data and large-scale structure surveys. The cosmological constant is mathematically equivalent to a perfect fluid with negative pressure equal to its energy density, and in quantum field theory it is identified with the energy of the vacuum itself.

The cosmological constant poses the most severe fine-tuning problem in physics. Quantum field theory predicts a vacuum energy density from zero-point fluctuations that exceeds the observed value by roughly 120 orders of magnitude. This discrepancy has driven proposals ranging from supersymmetry (which would cancel vacuum contributions) to anthropic reasoning (which treats Λ as a parameter varying across a multiverse) to modifications of general relativity that make the effective Λ dynamical rather than fixed. None of these proposals is experimentally confirmed, and the problem remains open.

The cosmological constant is physics at its most uncomfortable. It is a term that Einstein inserted by hand, rejected in shame, and that returned with observational credentials better than almost any other prediction in cosmology. Its small but nonzero value tells us either that we do not understand quantum field theory in curved spacetime, or that the universe is far less tidy than our aesthetic preferences demand. The former is a problem for theorists. The latter is a problem for anyone who believed nature shared their taste for simplicity.