7.4. Evolution of CMB data
The pace of progress in CMB experiments has maintained an astonishing rate for a decade. Following the 1992 COBE detection of fluctuations, 5 years of effort yielded the unclear picture of the first panel in figure 18, in which of order 10 experiments gave only vague evidence for a peak in 2 C. By the year 2000, this had been transformed to a clear picture of a peak at 200, although there was no model-independent evidence for higher harmonics. The present situation is much more satisfactory, with 3 peaks established in a way that does not require any knowledge of the CDM model.
Figure 18. Dramatic change took place in CMB power spectrum measurements around the turn of the 21st century. Although some rise from the COBE level was arguably known even by 1997, a clear peak around 200 only became established in 2000, whereas by 2003 definitive measurements of the spectrum at 800, limited mainly by cosmic variance, had been made by the WMAP satellite.
The WMAP results (Spergel et al. 2003) measure the power spectrum about as well as possible (i.e. hitting the limit of cosmic variance from a finite sky) up to the second peak. At smaller scales, however, there is still much scope for improvement, and the rate of advance is unlikely to drop in the future. The useful web page http://background.uchicago.edu/~whu/cmbex.html lists 14 ongoing experiments, as well as 19 completed ones.