ABSTRACT
6.1 Extragalactic backgrounds
The final matter of cosmological interest to consider is one that is often neglected. We have earlier addressed the issue of the general baryonic content of the Universe, as measured by primordial nucleosynthesis and other techniques, but we have not addressed the issue of where these baryons are located. In general, there are two choices: baryons are either trapped inside a potential well, hence located in a galaxy, or they are distributed outside of galaxies. This is determined by the efficiency of galaxy formation. What percentage of the primordial hydrogen and helium gas managed to creep into some preexisting gravitational potential to ultimately fragment, forming stars inside of galaxies? Theory, regardless of the particular structure formation scenario, does not make a reliable prediction of this efficiency, so it has to be determined observationally. This chapter will consider the evidence regarding three possible repositories for baryonic material: (1) easy-to-detect galaxies, (2) hard-to-detect galaxies and (3) a distributed background that is detected via recombination radiation at some particular wavelength. Observationally, this entails determining the galaxy luminosity function (GLF) and searching for evolution with look-back time as well as trying to detect various radiation processes that are tracers of intergalactic atoms. At the very least, we wish to ascertain whether there are more baryons located inside galaxies compared to outside. And this section begins with a summary of various searches for diffuse extragalactic backgrounds that might arise from a significant intergalactic population of baryons.
