2003ApJS..148....1B

We present full-sky microwave maps in five frequency bands (23-94 GHz) from the Wilkinson Microwave Anisotropy Probe (WMAP) first-year sky survey. Calibration errors are less than 0.5%, and the low systematic error level is well specified. The cosmic microwave background (CMB) is separated from the foregrounds using multifrequency data. The sky maps are consistent with the 7° FWHM Cosmic Background Explorer (COBE) maps. We report more precise, but consistent, dipole and quadrupole values. The CMB anisotropy obeys Gaussian statistics with -58<f_NL<134 (95% confidence level [CL]). The 2≤ℓ≤900 anisotropy power spectrum is cosmic-variance-limited for ℓ<354, with a signal-to-noise ratio greater than 1 per mode to ℓ=658. The temperature-polarization cross-power spectrum reveals both acoustic features and a large-angle correlation from reionization. The optical depth of reionization is τ=0.17±0.04, which implies a reionization epoch of t_r=180⁺²²⁰_–80Myr (95% CL) after the big bang at a redshift of z_r=20⁺¹⁰_–9(95% CL) for a range of ionization scenarios. This early reionization is incompatible with the presence of a significant warm dark matter density.

A best-fit cosmological model to the CMB and other measures of large-scale structure works remarkably well with only a few parameters. The age of the best-fit universe is t₀=13.7±0.2 Gyr. Decoupling was t_dec=379⁺⁸_–7kyr after the big bang at a redshift of z_dec=1089±1. The thickness of the decoupling surface was Δz_dec=195±2. The matter density of the universe is Ω_mh²=0.135^+0.008_–0.009, the baryon density is Ω_bh²=0.0224±0.0009, and the total mass-energy of the universe is Ω_tot=1.02±0.02. It appears that there may be progressively less fluctuation power on smaller scales, from WMAP to fine-scale CMB measurements to galaxies and finally to the Lyα forest. This may be accounted for with a running spectral index of scalar fluctuations, fitted as n_s=0.93±0.03 at wavenumber k₀=0.05Mpc^–1 (ℓ_eff~700), with a slope of dn_s/dlnk=-0.031^+0.016_–0.018 in the best-fit model. (For WMAP data alone, n_s=0.99±0.04.) This flat universe model is composed of 4.4% baryons, 22% dark matter, and 73% dark energy. The dark energy equation of state is limited to w←0.78 (95% CL). Inflation theory is supported with n_s~1, Ω_tot~1, Gaussian random phases of the CMB anisotropy, and superhorizon fluctuations implied by the temperature-polarization anticorrelations at decoupling. An admixture of isocurvature modes does not improve the fit. The tensor-to-scalar ratio is r(k₀=0.002Mpc^–1)<0.90 (95% CL). The lack of CMB fluctuation power on the largest angular scales reported by COBE and confirmed by WMAP is intriguing. WMAP continues to operate, so results will improve.