Introductory Remarks on The Extragalactic Infrared Background and its Cosmological Implications

I review the assumptions and observations that motivate the concept of the extragalactic cosmic background radiation, and the issues of energy accounts and star formation history as a function of galaxy morphological type that figure in the interpretation of the measurements of the extragalactic infrared background. 1. Fundamental Assumptions It is useful to begin by recalling the basic assumptions and observations that lead us to the concept of the extragalactic cosmic background radiation, as opposed to radiation surface brightness that may be some highly variable function of position and direction. Deep counts of objects detected at a broad range of wavelengths, from gamma ray sources to radio galaxies, are close to isotropic across the sky. It is an excellent bet therefore that the integrated radiation from observed sources plus those too faint to be detectable as individual objects also is quite to isotropic. This allows us to think of the local extragalactic radiation background as a function of one variable, the radiation energy density uν per interval of frequency ν. The fluctuations around the mean as a function of position in the sky are important too, as a measure of large-scale structure, but uν is the center of attention in these Proceedings. The argument for large-scale homogeneity — against a universe with a radial density gradient and us at the center — is less direct but I think persuasive; my review of the considerations is in Peebles (1993). If we are persuaded then we conclude that within our Hubble length space is filled with a near uniform sea of radiation with spectral energy density uν : the cosmic extragalactic background radiation. If the propagation of the radiation is described by a metric theory then it satisfies the Liouville or brightness theorem. If the metric describes a homogeneous isotropic spacetime then the geometry is fixed by the expansion factor a(t), a function of the proper world time t alone, together with the radius of curvature a(t)R of sections of constant time, where the comoving radius R is a constant. In this spacetime the radiation energy density u(t) = ∫ dν uν integrated over frequency at time t is an integral over the history of production and absorption of radiation, u(t) = ∫ t dt j(t) (a(t)/a(t)). (1)