Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations:
  Power Spectra and WMAP-Derived Parameters
  (Abridged) We present the angular power spectra derived from the 7-year maps
and discuss the cosmological conclusions that can be inferred from WMAP data
alone. The third acoustic peak in the TT spectrum is now well measured by WMAP.
In the context of a flat LambdaCDM model, this improvement allows us to place
tighter constraints on the matter density from WMAP data alone, and on the
epoch of matter-radiation equality, The temperature-polarization (TE) spectrum
is detected in the 7-year data with a significance of 20 sigma, compared to 13
sigma with the 5-year data. The low-l EE spectrum, a measure of the optical
depth due to reionization, is detected at 5.5 sigma significance when averaged
over l = 2-7. The BB spectrum, an important probe of gravitational waves from
inflation, remains consistent with zero. The upper limit on tensor modes from
polarization data alone is a factor of 2 lower with the 7-year data than it was
using the 5-year data (Komatsu et al. 2010). We test the parameter recovery
process for bias and find that the scalar spectral index, ns, is biased high,
but only by 0.09 sigma, while the remaining parameters are biased by < 0.15
sigma. 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: Neff > 2.7 (95% CL). Also, using WMAP data
alone, the primordial helium mass fraction is found to be YHe = 0.28+0.14-0.15,
and with data from higher-resolution CMB experiments included, we now establish
the existence of pre-stellar helium at > 3 sigma (Komatsu et al. 2010).
