Measurements of the acoustic peaks in the CMB temperature spectrum have already shown that the Universe is nearly spatially flat and began with a nearly scale-invariant spectrum of curvature fluctuations, consistent with the simplest of inflationary models. In a remarkable confirmation of a longstanding prediction of Big Bang Nucleosynthesis, the CMB measurements have now verified that baryons account for about four percent of the critical density. Further, they suggest that the matter density is some ten times higher than this, implying the existence of non-baryonic dark matter and dark energy.
Future measurements of the morphology of the peaks in the temperature and polarization should determine the baryonic and dark matter content of the Universe with exquisite precision. Beyond the peaks, gravitational wave imprint on the polarization, the gravitational lensing of the CMB, and gravitational and scattering secondary anisotropies hold the promise of understanding the physics of inflation and the impact of dark energy on structure formation.
The once and future success of the CMB anisotropy enterprise rests on three equally important pillars: advances in experimental technique, precision in theory, and development of data analysis techniques. The remarkable progress in the field over the last decade owes much to the efforts of researchers in all three disciplines. That much more effort will be required to fulfill the bright promise of the CMB suggests that the field will remain active and productive for years to come.
W.H. thanks the hospitality of Fermilab where this review was written. W.H. was supported by NASA NAG5-10840 and the DOE OJI program. S.D. was supported by the DOE, by NASA grant NAG 5-10842 at Fermilab, by NSF Grant PHY-0079251 at Chicago.