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It is widely accepted that the field of cosmology is entering an era dubbed ``precision cosmology.'' Data directly relevant to the properties and evolution of the universe is flooding in by the terabyte (or soon will be). Such vast quantities of data were the purview only of high energy physics just a few years ago; now expertise from this area is being coopted by some astromers to help deal with our wealth of information. In the past decade, cosmology has gone from a data-starved science in which often highly speculative theories went unconstrained to a data-driven pursuit where many models have been ruled out and the remaining ``standard cosmology'' will be tested with stringent precision.

The cosmic microwave background radiation is at the center of this revolution. The radiation present today as a 2.7 K thermal background originated when the universe was denser by a factor of 109 and younger by a factor of around 5 × 104. The radiation provides the most distant direct image of the universe we can hope to see, at least until gravitational radiation becomes a useful astronomical data source. The microwave background radiation is extremely uniform, varying in temperature by only a few parts in 105 over the sky (apart from an overall dipole variation arising from our peculiar motion through the microwave background's rest frame); its departure from a perfect blackbody spectrum has yet to be detected.

The very existence of the microwave background provides crucial support for the Hot Big Bang cosmological model: the universe began in a very hot, dense state from which it expanded and cooled. The microwave background visible today was once in thermal equilibrium with the primordial plasma of the universe, and the universe at that time was highly uniform. Crucially, the universe could not have been perfectly uniform at that time or no structures would have formed subsequently. The study of small temperature and polarization fluctuations in the microwave background, reflecting small variations in density and velocity in the early universe, have the potential to provide the most precise constraints on the overall properties of the universe of any data source. The reasons are that (1) the universe was very simple at the time imaged by the microwave background and is extremely well-described by linear perturbation theory around a completely homogeneous and isotropic cosmological spacetime; and (2) the physical processes relevant at that time are all simple and very well understood. The microwave background is essentially unique among astrophysical systems in these regards.

The goal behind these lectures is to provide a qualitative description of the physics of the microwave background, an appreciation for the microwave background's cosmological importance, and an understanding of what kinds of constraints may be placed on cosmological models. These lectures are not intended to be a definitive technical reference to the microwave background. Unfortunately, such a reference does not really exist at this time, but I have attempted to provide pedagogically useful references to other literature. I have also not attempted to give a complete bibliography; please do not consider this article to give definitive references to any topics mentioned. A recent review of the microwave background with a focus on potential particle physics constraints is Kamionkowski and Kosowsky (1999). A more general review of the microwave background and large-scale structure with references to many early microwave background articles is White et al. (1994).

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