WMAP precision data enable accurate testing of cosmological models. We find that the emerging standard model of cosmology, a flat Λ-dominated universe seeded by a nearly scale-invariant adiabatic Gaussian fluctuations, fits the WMAP data. For the WMAP data only, the best-fit parameters are h = 0.72 ± 0.05, Ω_bh² = 0.024 ± 0.001, Ω_mh² = 0.14 ± 0.02, τ = 0.166, n_s = 0.99 ± 0.04, and σ₈ = 0.9 ± 0.1. With parameters fixed only by WMAP data, we can fit finer scale cosmic microwave background (CMB) measurements and measurements of large-scale structure (galaxy surveys and the Lyα forest). This simple model is also consistent with a host of other astronomical measurements: its inferred age of the universe is consistent with stellar ages, the baryon/photon ratio is consistent with measurements of the [D/H] ratio, and the inferred Hubble constant is consistent with local observations of the expansion rate. We then fit the model parameters to a combination of WMAP data with other finer scale CMB experiments (ACBAR and CBI), 2dFGRS measurements, and Lyα forest data to find the model's best-fit cosmological parameters: h = 0.71, Ω_bh² = 0.0224 ± 0.0009, Ω_mh² = 0.135, τ = 0.17 ± 0.06, n_s(0.05 Mpc^-1) = 0.93 ± 0.03, and σ₈ = 0.84 ± 0.04. WMAP's best determination of τ = 0.17 ± 0.04 arises directly from the temperature-polarization (TE) data and not from this model fit, but they are consistent. These parameters imply that the age of the universe is 13.7 ± 0.2 Gyr. With the Lyα forest data, the model favors but does not require a slowly varying spectral index. The significance of this running index is sensitive to the uncertainties in the Lyα forest.

     By combining WMAP data with other astronomical data, we constrain the geometry of the universe, Ω_tot = 1.02 ± 0.02, and the equation of state of the dark energy, w < -0.78 (95% confidence limit assuming w ≥ -1). The combination of WMAP and 2dFGRS data constrains the energy density in stable neutrinos: Ω_νh² < 0.0072 (95% confidence limit). For three degenerate neutrino species, this limit implies that their mass is less than 0.23 eV (95% confidence limit). The WMAP detection of early reionization rules out warm dark matter.