The Cosmological Constant: Physical Mechanisms

In ΛCDM, Λ contributes a nearly uniform energy density while ordinary matter and dark matter dilute as volume expands. At early times matter dominated the expansion dynamics; at late times the constant Λ contribution becomes relatively more important.

A constant vacuum-like energy density has negative pressure in general relativity. Its contribution to the cosmological acceleration equation can drive accelerated expansion once matter density falls sufficiently low.

A constant-Λ model fits a wide combination of CMB, supernova, BAO, and large-scale-structure data remarkably well. Recent DESI analyses have motivated increased scrutiny of time-varying alternatives, but Λ remains the reference model against which extensions are compared.

Negative vacuum-like pressure changes the cosmological acceleration equation. Once matter thins sufficiently, the constant component drives accelerated expansion.