Seven-year wilkinson microwave anisotropy probe (WMAP*) observations: Power spectra and WMAP-derived parameters

摘要

The WMAP mission has produced sky maps from seven years of observations at L2. We present the angular power spectra derived from the seven-year maps and discuss the cosmological conclusions that can be inferred from WMAP data alone. With the seven-year data, the temperature (TT) spectrum measurement has a signal-to-noise ratio per multipole that exceeds unity for l < 919; and in band powers of width Δl = 10, the signal-to-noise ratio exceeds unity up to l = 1060. The third acoustic peak in the TT spectrum is now well measured by WMAP. In the context of a flat ACDM model, this improvement allows us to place tighter constraints on the matter density from WMAP data alone, Ω_mh~2 = 0.1334~(+0.0056)_(-0.0055), and on the epoch of matter-radiation equality, z_(eq) = 3196 ~(+134)_(-133). The temperature-polarization (TE) spectrum is detected in the seven-year data with a significance of 20σ, compared to 13s with the five-year data. We now detect the second dip in the TE spectrum near l ～ 450 with high confidence. The TB and EB spectra remain consistent with zero, thus demonstrating low systematic errors and foreground residuals in the data. The low-lEE spectrum, a measure of the optical depth due to reionization, is detected at 5.5σ significance when averaged over l = 2-7: l(l+1)C_l~(EE)/(2π) = 0.074~(+0.034)_(-0.025)μK~2 (68% CL). We now detect the high-l, 24 ≤ l ≤ 800, EE spectrum at over 8σ. The BB spectrum, an important probe of gravitational waves from inflation, remains consistent with zero; when averaged over l = 2-7, l(l + 1)C_l~(BB)/(2π) < 0.055 μK ~2 (95% CL). The upper limit on tensor modes from polarization data alone is a factor of two lower with the seven-year data than it was using the five-year data. The data remain consistent with the simple ACDM model: the best-fit TT spectrum has an effective χ~2 of 1227 for 1170 degrees of freedom, with a probability to exceed of 9.6%. The allowable volume in the six-dimensional space of ACDM parameters has been reduced by a factor of 1.5 relative to the five-year volume, while the ACDM model that allows for tensor modes and a running scalar spectral index has a factor of three lower volume when fit to the seven-year data. We test the parameter recovery process for bias and find that the scalar spectral index, n_s, is biased high, but only by 0.09σ, while the remaining parameters are biased by <0.15σ. The improvement in the third peak measurement leads to tighter lower limits from WMAP on the number of relativistic degrees of freedom (e.g., neutrinos) in the early universe: N_(eff) > 2.7 (95% CL). Also, using WMAP data alone, the primordial helium mass fraction is found to be Y _(He) = 0.28~(+0.14)_(-0.15), and with data from higher-resolution cosmic microwave background experiments included, we now establish the existence of pre-stellar helium at >3σ. These new WMAP measurements provide important tests of big bang cosmology.