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, 
T = (v /
c)T0
0.27 mK
(v / c = 10-4).
WMAP's location a million miles from Earth helped keep systematics to
below 0.5%. From 
= 2 to
~ 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
only
2 + 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,
M /
B = 6
± 0.04, and the case for dark energy
by showing that something like a cosmological
constant is needed to "balance the books,"
X = 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 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
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
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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.