The Spectrum of the Universe
  The cosmic background (CB) radiation, encompassing the sum of emission from
all sources outside our own Milky Way galaxy across the entire electromagnetic
spectrum, is a fundamental phenomenon in observational cosmology. Many
experiments have been conceived to measure it (or its constituents) since the
extragalactic Universe was first discovered; in addition to estimating the bulk
(cosmic monopole) spectrum, directional variations have also been detected over
a wide range of wavelengths. Here we gather the most recent of these
measurements and discuss the current status of our understanding of the CB from
radio to $\gamma$-ray energies. Using available data in the literature we piece
together the sky-averaged intensity spectrum, and discuss the emission
processes responsible for what is observed. We examine the effect of
perturbations to the continuum spectrum from atomic and molecular line
processes and comment on the detectability of these signals. We also discuss
how one could in principle obtain a complete census of the CB by measuring the
full spectrum of each spherical harmonic expansion coefficient. This set of
spectra of multipole moments effectively encodes the entire statistical history
of nuclear, atomic and molecular processes in the Universe.
