First Year WMAP Observations

The results of the first year WMAP sky survey are full sky microwave maps in five frequency bands from 23 to 94 GHz. Calibration errors are <0.5% and the low systematic error level is well specified. The cosmic microwave background (CMB) is separated from the foregrounds using the multifrequency data. The 2≤l≤900 anisotropy power spectrum is cosmic variance limited for l<354 with a signal-to-noise ratio > 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. 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 old. The matter density is Ω_{mh² = 0.135^+0.008-0.009, the baryon density is Ωbh² = 0.0224 ± 0.0009, and the total mass-energy of the universe is Ωtot = 1.02 ± 0.02. For iMAP data alone, ns0.99± 0.04. The lack of CMB fluctuation power on the largest angular scales reported by COBE and confirmed by iMAP is intriguing. iMAP continues to operate, so results will improve.}