The Wilkinson microwave anisotropy probe

Collaboration has released its analysis (WMAP V) of five years of data from the orbiting satellite, which was launched in 2001. The results appear in seven papers available at http://wmap.gsfc.nasa.gov . WMAP measures the tiny point-to-point fluctuation of the cosmic microwave background’s 2.7-K blackbody temperature and its even tinier polarization. The CMB is a snapshot of the cosmos when it first became transparent with the cooling of the opaque primordial plasma half a million years after the Big Bang. WMAP V brings no surprises, but it sharpens the “concordance” picture painted by the three-year data (WMAP III, see Physics Today, May 2006, page 16 ) and complementary cosmological observation. WMAP V’s significantly better delineation of the third acoustic peak in the power spectrum of the temperature fluctuations now shows the expected effect of the three species of light neutrinos that cosmologists believe to have decoupled from the plasma about a minute after the Big Bang. That confirms the existence of a cosmic background of neutrinos that have been redshifted to such low energy in the present universe that they cannot be detected directly. The data also address epochs much later than the first moment of cosmic transparency. The neutral hydrogen that first rendered the cosmos transparent has since been almost completely reionized by UV starlight. With much improved signal-to-noise ratio, the WMAP V polarization data make it clear that the reionization took about half a billion years to complete. WMAP III had been consistent with much faster reionization. The polarization data also set a new upper limit on the cosmic gravity-wave background, a limit that begins to encroach on the range of candidate theories of inflation. And WMAP V resolves an incipient inconsistency: The fitted concordance-model parameters from WMAP III predicted a density-fluctuation amplitude for large-scale structure in the present cosmos that was two standard deviations lower than that determined from gravitational-lensing observations of the distribution of dark and ordinary matter. But now the WMAP V predictions are in good agreement with improved gravitational-lensing data.