The Spectrum of the Universe
Ryley Hill, Kiyoshi W. Masui, Douglas Scott

TL;DR
This paper reviews the current understanding of the cosmic background radiation across the electromagnetic spectrum, summarizing recent measurements, emission processes, and the potential for a complete spectral census of the universe's history.
Contribution
It compiles recent observational data of the cosmic background across wavelengths and discusses methods to fully characterize its statistical properties through spectral analysis.
Findings
Sky-averaged intensity spectrum assembled from literature data
Identification of atomic and molecular line perturbations in the spectrum
Proposal for measuring the full spectrum of spherical harmonic coefficients
Abstract
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 -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…
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