6.1.1. The Radio Background
In the centimeter to decameter range there is a well known background of radiation due to galactic synchrotron and thermal processes. This was discovered when the first radio telescope was built (Reber 1933). In addition there is a very large population of extragalactic radio sources which are usually active galaxies. Deep radio surveys can be used like deep optical surveys, described below, to construct the source count vs flux relation in an attempt to measure the geometry of the Universe (e.g., ). This is one of the classical cosmological tests. We have not discussed tests like this because they require an accurate understanding of the evolution of the sources (e.g., the radio or optical luminosity evolution of galaxies). Understanding the luminosity evolution of galaxies is extremely model dependent and hence the use of source counts as a function of apparent flux (or better yet, redshift) currently has little constraining power.
At shorter wavelengths, of course, the spectrum is completely dominated by the CMB. This spectrum has now been established by COBE to be that of a blackbody to very high precision (see Figure 1-5). There is no longer any room for extra components as was once thought to be the case in the late 80's when balloon measurements hinted at a possible sub-mm excess of radiation in the total spectrum (see Matsumoto et al. 1988). When this component was first hinted at, theorists were quick to construct (mostly for conference proceedings) fairly elaborate models of high-redshift dust, heated by QSOs or the first generation of massive stars in proto galaxies (see Bernstein et al. 1989, Lahav et al. 1990, Ostriker and Thompson 1989, Heisler and Ostriker 1988). This dust would be heated to temperatures of 20-40K and its thermal spectrum would be greatly redshifted and appear as the sub-mm excess. If real, this sub-mm excess would indicate a possibly large population of intergalactic baryons in the form of dust. Clearly that population would have cosmological implications. The spectrum determined by COBE now completely precludes this possibility and serves as a reminder that its very important to properly characterize any suspected cosmological background before attempting to model it.