Many astronomical sources are capable of emitting UV light with photon energies in the range h 10-20 eV through various thermal and non-thermal emission processes. The diffuse cosmic background radiation at ultraviolet 1000 - 2500 Å wavelengths is therefore of interest for a wide variety of topics. These include the study of the properties of interplanetary and interstellar dust grains, thermal line emission from the interstellar and intergalactic gas, the integrated light of galaxies, and radiative decay of exotic cosmological particles.
Reviews of the topic of the diffuse UV background have been given by Davidsen, Bowyer & Lampton (1974), Paresce & Jakobsen (1980), and most recently by Bowyer (1991) and Henry (1991). Not least thanks to the experimental efforts of the Berkeley group, the rather confusing observational situation surrounding the far-ultraviolet background has lately become much clearer. There is today good evidence that the diffuse ultraviolet background is largely dominated by dust scattering and other interstellar emission processes occurring within the Milky Way (Martin & Bowyer 1990; Martin, Hurwitz & Bowyer 1990; Hurwitz, Bowyer & Martin 1991). Although there remains some disagreement as to the relative strengths of the various interstellar contributors (cf. Henry, this conference), there is general consensus that the intensity of any quasi-isotropic and therefore possibly extragalactic component to the background in the 1300 - 2000 Å range is approximately I 100 photons s-1 cm-2 sr-1 Å-1, give or take a factor of ~ 2-3.
There are at present three candidate extragalactic sources of UV radiation that could conceivably produce a diffuse background flux of this intensity: i) diffuse thermal emission from the intergalactic medium; ii) the integrated UV light of galaxies and quasars; and iii) radiative decay of massive neutrinos. In what follows, each of these emission sources is discussed in turn.