We present full sky microwave maps in five frequency bands ( 23 to 94 GHz ) from the WMAP first year sky survey .
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 multifrequency data .
The sky maps are consistent with the 7 ^ { \circ } full-width at half-maximum ( 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 % CL ) .
The 2 \leq l \leq 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 \tau = 0.17 \pm 0.04 , which implies a reionization epoch of t _ { r } = 180 ^ { +220 } _ { -80 } Myr ( 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 t _ { 0 } = 13.7 \pm 0.2 \mbox { Gyr } old .
Decoupling was t _ { dec } = 379 ^ { +8 } _ { -7 } \mbox { kyr } after the Big Bang at a redshift of z _ { dec } = 1089 \pm 1 .
The thickness of the decoupling surface was \Delta z _ { dec } = 195 \pm 2 .
The matter density of the universe is \Omega _ { m } h ^ { 2 } = 0.135 ^ { +0.008 } _ { -0.009 } , the baryon density is \Omega _ { b } h ^ { 2 } = 0.0224 \pm 0.0009 , and the total mass-energy of the universe is \Omega _ { tot } = 1.02 \pm 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 Ly- \alpha forest .
This may be accounted for with a running spectral index of scalar fluctuations , fit as n _ { s } = 0.93 \pm 0.03 at wavenumber k _ { 0 } = 0.05 Mpc ^ { -1 } ( l _ { eff } \approx 700 ) , with a slope of dn _ { s } / d \ln { k } = -0.031 ^ { +0.016 } _ { -0.018 } in the best-fit model .
( For WMAP data alone , n _ { s } = 0.99 \pm 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 \% \mbox { CL } ) .

Inflation theory is supported with n _ { s } \approx 1 , \Omega _ { tot } \approx 1 , Gaussian random phases of the CMB anisotropy , and superhorizon fluctuations implied by the TE anticorrelations at decoupling .
An admixture of isocurvature modes does not improve the fit .
The tensor-to-scalar ratio is r ( k _ { 0 } = 0.002 \mbox { 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 .