LambdaCDM

Lambda-CDM (Lambda Cold Dark Matter) is the standard model of cosmology that describes the evolution of the universe from the early times to the present. It combines general relativity with the cosmological principle, modeling the large-scale geometry as a Friedmann–Lemaître–Robertson–Walker spacetime. The model posits that most of the matter-energy content is in the form of a cosmological constant (Lambda), which drives dark energy, and cold dark matter (CDM), with ordinary baryonic matter and radiation contributing a smaller fraction.

Key parameters commonly used in Lambda-CDM include the present-day matter density Omega_m, the dark energy density Omega_Lambda (with Omega_k = 0 in the flat case and Omega_m + Omega_Lambda = 1), the Hubble constant H0, and parameters describing primordial fluctuations such as the amplitude and tilt of the power spectrum (often noted as A_s and n_s), along with the clustering amplitude sigma8 in some analyses. In flat Lambda-CDM, the curvature is constrained to be zero, simplifying the parameter set.

Dynamics and predictions are obtained from the Friedmann equations, with contributions from baryons, cold dark matter, radiation, and Lambda. The model describes a universe that was radiation-dominated, then matter-dominated, and is currently undergoing accelerated expansion due to dark energy. It yields testable signatures in the cosmic microwave background, large-scale structure, and baryon acoustic oscillations.

Observational status: Lambda-CDM is the concordance model, consistent with measurements from the Planck satellite, Type Ia supernovae, and BAO. Some tensions exist, notably the H0 discrepancy between early-Universe inferences and local distance measurements, and mild tensions in clustering amplitude from weak lensing.

History and significance: The model emerged in the late 1990s following evidence for cosmic acceleration and remains the foundational framework in cosmology, with extensions such as wCDM allowing the dark energy equation of state to vary.