Cosmological constant problem

The cosmological constant problem is the mismatch between quantum-field estimates of vacuum energy and the small value of the cosmological constant inferred from cosmic expansion.

Vacuum energy: Energy associated with the lowest-energy state of quantum fields. Quantum-field calculations assign vacuum energy a value that appears to gravitate.
Quantum field theory: A framework describing particles and their interactions as excitations of fields. Its vacuum-state calculations supply the theoretical energy estimates in the problem.
Fine-tuning: The sensitivity of a physical outcome to precise parameter values or cancellations. The observed value requires an extreme cancellation between contributions in conventional estimates.
Lambda-CDM model: The standard cosmological model combining a cosmological constant with cold dark matter. It fits a wide range of observations while leaving the constant’s tiny value unexplained.
Cosmological constant: A constant term in Einstein’s gravitational field equations, equivalent to uniform energy density in empty space. Its measured value captures the small gravitational effect at the heart of the mismatch.
Quantum vacuum: The lowest-energy state of a quantum field, not generally an empty state without fluctuations. Its properties underlie estimates of vacuum energy.
Anthropic principle: The idea that observations are conditioned by the requirements for observers to exist. It can constrain the cosmological constant through the formation of galaxies and observers.
Quintessence: A hypothetical time-varying scalar field proposed as the cause of cosmic acceleration. Unlike a strict cosmological constant, it allows dark energy to evolve over time.
Einstein field equations: Equations relating spacetime curvature to matter, energy, and momentum. They show how vacuum energy can contribute to cosmic curvature.
Zero-point energy: The minimum energy permitted for a quantum system by its uncertainty relations. Summing field modes’ zero-point energies motivates common vacuum-energy estimates.

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