First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Preliminary maps and basic results

C. L. Bennett, M. Halpern, G. Hinshaw, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, L. Page, D. N. Spergel, G. S. Tucker, E. Wollack, E. L. Wright, C. Barnes, M. R. Greason, R. S. Hill, E. Komatsu, M. R. Nolta, N. Odegard, H. V. Peiris, L. VerdeJ. L. Weiland

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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 < fNL < 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 tr = 180-80+220 Myr (95% CL) after the big bang at a redshift of zr = 20-9 +10 (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 tdec = 379-7+8 kyr after the big bang at a redshift of zdec = 1089 ± 1. The thickness of the decoupling surface was Δzdec = 195 ± 2. The matter density of the universe is Ωmh2 = 0.135-0.009 +0.008, the baryon density is Ωbh2 = 0.0224 ± 0.0009, and the total mass-energy of the universe is Ωtot = 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 ns = 0.93 ± 0.03 at wavenumber k0 = 0.05 Mpc -1 (ℓeff ≈ 700), with a slope of dns/d ln k = -0.031-0.018+0.016 in the best-fit model. (For WMAP data alone, ns = 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 ns ≈ 1, Ωtot ≈ 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.

Original languageEnglish (US)
Pages (from-to)1-27
Number of pages27
JournalAstrophysical Journal, Supplement Series
Issue number1
StatePublished - Sep 2003

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cosmic microwave background
  • Cosmology: observations
  • Dark matter
  • Early universe
  • Instrumentation: detectors
  • Space vehicles: instruments

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