Theoretical Overview of Cosmic Microwave Background Studies

The cosmic microwave background (CMB) is a gas of electromagnetic radiation left over from the Big Bang.  It has a very precisely thermal frequency spectrum with a temperature T=2.725 K.  The intensity is very nearly uniform across the sky, with variations of roughly one part in 100,000.  The CMB radiation was emitted roughly 13.8 billion years ago, when the Universe was only 380,000 years old.  Maps of the intensity of the CMB across the sky thus provide a snapshot of a spherical surface, of radius 14 billion light years, in the Universe when it was extremely young.  NASA's Wilkinson Microwave Anisotropy Probe (WMAP), and (soon) the European Space Agency's Planck satellite provide high-signal-to-noise maps of the CMB intensity with angular resolutions a fraction of a degree.  They thus provide an extremely detailed picture of the early Universe. By comparing the results of CMB measurements with theoretical models for the origin of the fluctuations, we have been able to derive a remarkably precise description of the early Universe, its evolution over time, and its contents.  We now have a precise inventory of the contents of the Universe (ordinary atomic  matter, dark matter, neutrinos, electromagnetic radiation, and dark energy), and we can map precisely the distribution of tiny primordial mass inhomogeneities that are seeds for the galaxies and galaxy clusters in the Universe today.  The observations provide strong evidence in favor of inflation, a period of accelerated expansion in the ultra-early-Universe, that in some sense set the Big Bang in motion.  Thus, the aim of CMB experiments now is to learn more about inflation. Specific targets in this effort include a particular pattern of CMB polarization, as well as characteristic higher-order correlations in the CMB temperature pattern.  There are also opportunities to learn more about the later Universe, when galaxies and galaxy clusters were forming, by looking for distortions to the CMB image from gravitational lensing by the intervening matter distribution.  In this talk I will sketch out how the contents and largest-scale structure of the Universe have been determined by CMB experiments.  I will briefly explain why they suggest a period of inflationary expansion in the early Universe.  I will then discuss the prospects for learning both about the early and later Universe with forthcoming experiments.  This talk will serve as an introduction to later talks in the series that will explain how the experiments are done, document recent experimental progress, and present several new results.