Omegalambda

OmegaLambda, commonly written as Omega_Lambda or ΩΛ, is the cosmological density parameter associated with dark energy in the standard model of cosmology. It represents the ratio of the energy density of dark energy to the critical density required to make the universe spatially flat. In the Lambda-CDM model, Omega_Lambda0 is about 0.7, indicating that roughly seven-tenths of the current energy budget of the universe comes from dark energy, with the remainder from matter (dark and baryonic). The critical density is defined as ρ_c = 3H^2/(8πG), where H is the Hubble parameter.

In a cosmological constant scenario, the dark energy density remains constant as the universe expands, leading to accelerated cosmic expansion. The parameter is related to the equation of state w, where for a true cosmological constant w = -1. Many analyses allow for time variation in dark energy, suggesting Omega_Lambda may evolve alongside other components, though observational data favor a behavior close to a constant with w near -1.

Observationally, Omega_Lambda is constrained by measurements of the cosmic microwave background, type Ia supernovae, and baryon acoustic oscillations; current estimates place Omega_Lambda0 around 0.69–0.71. The sum Omega_Lambda + Omega_m is close to unity, consistent with a spatially flat universe.

Although primarily used in cosmology, the symbol Omega_Lambda appears in literature as a shorthand for the dark energy density parameter and is a critical component of the standard cosmological model. See also: dark energy, cosmological constant, Omega_m, Omega_k, Lambda-CDM.