An introduction to the cosmic microwave background radiation

I first review the physical processes which produce the observed blackbody spectrum of the cosmic microwave background radiation (CMBR). Some of these same processes may leave small amplitude distortions in the spectrum; these are examined next. I then review the physical processes which can produce fluctuations in the CMBR from point to point on the sky (these remarks parallel those in Chapter 7. by Lineweaver here). Several other mechanisms can induce a dipole and/or quadrupole moment into the CMBR; these are reviewed briefly. Next I describe spectral measurements of the CMBR and assess their consequences. I begin with a few radio astronomical fundamentals, Like antenna temperature and beam patterns. I then describe how spectral measurements of the CMBR are made, and the need for precise calibration. Next, I discuss potential sources of error in such measurements, particularly foreground emission from the atmosphere and the Galaxy. I then summarize our current knowledge of the spectrum of the CMBR. The available measurements set sharp constraints on spectral distortions. These limits, in turn, constrain a number of energy-releasing processes both early in the Universe and in the "local" intergalactic medium. The final section of this Chapter deals with searches for fluctuations in the temperature of the CMBR on angular scales of arcseconds to 180 degrees I describe how such differential measurements are made, and mention the advantages of making true maps of the CMBR. On large scales, COBE provides the best measurements. I present the newest dipole and quadrupole measurements, and talk briefly about cosmic variance. The COBE measurements are also used to normalize theoretical predictions for smaller-scale anisotropies. The present observational situation on CMBR fluctuations on smaller angular scales, 0.degrees 3-3 degrees, is given next (to complement Chapter 7.). Interferometric techniques are now being applied to CMBR studies. They offer several advantages, which I review. I also present recent results on both sub-degree and sub-arcminute angular scales. Finally, I look briefly at the future of such searches.