To be sure there have been advances in all cosmological observations over the past three years, but the most dramatic improvements have come from observations of the CMB anisotropy. First came the BOOMERanG results which gave us high resolution and high signal-to-noise maps of the anisotropy (deBernardis et al. 2000). The data analysis improved considerably over the three years (e.g., Netterfield et al. 2002) culminating in the Ruhl et al. (2003) analysis. Concurrent with BOOMERanG was the MAXIMA experiment (Hanany 2000, Lee 2001). MAXIMA gave similar results though over a smaller fraction of the sky. For many, the greatest advance from these experiments was not so much the new measurement of curvature, but rather the ability to probe b and m with better than 30% precision using the CMB. In more general terms, they gave new and strong evidence that the concordance model in Figure 1 was correct.
In the year before the WMAP release (Bennett et al. 2003), the Archeops team published (Benoit 2002) results from a map that covered roughly 30% of the sky. The goal of the experiment, in addition to serving as a test bed for the Planck HFI instrument, was to bridge the angular range from the 7° COBE resolution to 1° resolution. The ACBAR experiment (Kuo et al. 2002), done from the South Pole, was aimed at pushing to angular scales beyond what WMAP could reach. Its resolution is 0.08° as opposed to WMAP's 0.21°. BOOMERanG, MAXIMA, Archeops, and ACBAR achieved their greatest sensitivity at 150 GHz using radiometers based on the Berkeley/Caltech/JPL spiderweb bolometers (Bock et al. 1996) with passbands defined by filters developed by Peter Ade and colleagues at Cardiff.
Great strides were made in CMB interferometry during the past three years. The three primary instruments were DASI (Halverson et al. 2002), VSA (Grainge 2003), and CBI (Mason 2003, Pearson 2003). All were based on broadband 30 GHz HEMT amplifiers designed by Marian Pospieszalski at the National Radio Astronomy Observatory (Pospieszalski 1992). Results from DASI first complemented and extended the framework that was becoming evident. After adding the polarization capability, DASI discovered the intrinsic polarization in the CMB at the predicted level (Kovac et al. 2002). This was an important piece of evidence that decoupling occurred as predicted. The VSA interferometer gave similar results to DASI though over a wider range in l. The CBI interferometer clearly observed the suppression of the anisotropy at l > 1000 due to Silk damping and the finite thickness of the decoupling surface. CBI also showed hints of observing the formation of non-linear structure at l > 2000, though more investigation is needed as emphasized by the CBI team.
Though the advances since IAU XXIV by ground and balloon based CMB experiments were tremendous, the results from WMAP are in a different category. Not only did WMAP have the unprecedented stability achievable only from deep space, but it mapped the entire sky. The systematic error limits achieved on multiple different aspects of the experiment and analysis were roughly an order of magnitude (sometimes two orders) improvement over what had been achieved previously. The data are so clean that 99% of the time ordered data goes into the final map. There is a low level of filtering and a 1% transmission imbalance is corrected, but other than this no other sytematic error corrections or selection criteria are applied. Finally, all the data from the experiment are publicly available so they may be checked. For a description of the WMAP mission see the article by Chuck Bennett in these proceedings.
The new CMB observations have narrowed the CMB swath in Figure 1 by roughly an order of magnitude. More importantly, they have told us that adiabatic scale invariant fluctuations seeded the formation of cosmic structure and that the contents of the universe are baryons, some form(s) of dark matter, and some form(s) of dark energy. A snapshot of all the CMB anisotropy data as of July 2003 has been compiled by Bond, Contaldi, & Pogosyan (2003) and a version is shown as a Grand Unified Spectrum (GUS) in Figure 2.
Figure 2. The Grand Unified Spectrum based on Bond, Contaldi, & Pogosyan (2003). The y axis shows the fluctuation power per logarithmic interval in l. The x-axis may be converted to angular scale by deg = l / 200. This spectrum is derived from the combination of 28 anisotropy experiments as of July 2003. The first and second peaks from the acoustic oscillations are clearly evident, the third peak is almost resolved, and the damping tail at l > 1000 is evident. The line is the best fit model.
This IAU is a particularly good time to take stock of where we are. Another chapter in the study of the CMB has been finished with the release of the first year WMAP data. Sadly, Dave Wilkinson, a pioneer of the CMB field for 35 years and a founder of both the WMAP and COBE satellite missions, died in the end of 2002 after battling cancer for 17 years. Fortunately Dave saw the WMAP maps in their full glory. The MAP satellite was renamed in his honor. Figure 3 is from the WMAP launch and shows three of CMB science's pioneers.
Figure 3. David Wilkinson (left), Dick Bond (center) and Rashid Sunyaev (right) at the WMAP launch, June 2001. They are standing in front of a Saturn V rocket (WMAP used a Delta II). Prof. Rashid Sunyaev was the recipient of the 2003 Gruber Prize in Cosmology which was presented at the IAU symposium.