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5. WMAP Postscript

Three months after we submitted this Colloquium article, the WMAP Collaboration presented results from their first year of data. (1) The results were at the same time stunning and unsurprising. As several cosmologists put it, the biggest surprise was the lack of a surprise. Overlapping and precision measurements have elevated cosmology to a new maturity, where consistency is becoming a hallmark.

The angular power spectrum (cf Figure 9) was derived from five all-sky maps with maximum angular resolution of 0.2° (30 times that of COBE) at frequencies from 20 GHz to 100 GHz (reference). The measurements were calibrated from the Doppler shift of the CMB arising from Earth's motion around the sun, deltaT = (v / c)T0 appeq 0.27 mK (v / c = 10-4). WMAP's location a million miles from Earth helped keep systematics to below 0.5%. From ell = 2 to ell ~ 350 the measurements of the multipole amplitudes were limited by sample (or cosmic) variance. (Theories like inflation do not predict values for the individual multipoles, but rather the variance of the distribution from which they are draw. The fact that for a given ell only 2ell + 1 multipoles can be measured limits the precision with which the variance can be estimated.)

The WMAP results (Bennett et al., 2003) have sharpened and put on firmer footing a large number of cosmological parameters (see Table I). The consistency of WMAP-determined parameters with previous values was a strong indication of the increasing reliability of cosmological results and their error estimates. In particular, WMAP strengthened the case for dark matter by its measurement of the ratio of the total amount of matter to that in baryons, OmegaM / OmegaB = 6 ± 0.04, and the case for dark energy by showing that something like a cosmological constant is needed to "balance the books," OmegaX = 0.7 ± 0.04. WMAP made clear that our current consensus cosmology rests on a strong and diverse, interlocking set of measurements.

While WMAP has yet to map CMB polarization (though it is in the works), by detecting the cross correlation between polarization and temperature anisotropy it found the signature of the re-ionization from UV starlight of the first stars at a redshift z appeq 20 ± 10. This is consistent with the predictions of the CDM paradigm, and together with the SDSS quasars with redshifts greater than 6 this now nicely brackets the re-ionization history of the Universe: at z ~ 20 the fraction of free elections rose to around 50% and by z appeq 6 it exceeded 99.99%.

Two months after the WMAP results, a new compilation and analysis of over 200 type Ia supernovae was presented (Tonry et al., 2003), and the direct evidence for cosmic acceleration also grew stronger. In particular, if dark energy is assumed to have w = - 1 (like a cosmological constant), the supernovae data imply

Equation 4 (4)

1 When the results were announced, cosmologists were pleased to learn that the MAP satellite had been re-named the Wilkinson Microwave Anisotrophy Probe (WMAP) to honor David Wilkinson, a pioneer in the study of the CMB and a leader of the MAP project, who died in September 2002. Back.

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