ABSTRACT. 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 ≤ ≤ 900 anisotropy power spectrum is cosmic-variance-limited for < 354, with a signal-to-noise ratio greater than 1 per mode to = 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 Myr (95% CL) after the big bang at a redshift of z_r = 20 (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 kyr 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, 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 = 0.05 Mpc^-1 (_eff 700), with a slope of dn_s/d ln k = -0.031 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 = 0.002 Mpc^-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.

Subject headings: cosmic microwave background; cosmology: observations; dark matter; early universe; instrumentation: detectors; space vehicles: instruments

^* WMAP is the result of a partnership between Princeton University and the NASA Goddard Space Flight Center. Scientific guidance is provided by the WMAP Science Team.

Print publication: Issue 1 (2003 September)
Received 2003 February 11, accepted for publication 2003 May 29