A thorough taxonomy of explanations alternative to the orthodox explanation (predicated on the Hot Big Bang) is outlined and presented (including a diagram) in this chapter. Two basic groups are those predicated on the cosmological validity of relativistic field equations and their nonrelativistic radical alternatives. The first group includes explanations within variations on the Big Bang model (tepid and cold Big Bangs) and those aiming at regular astrophysical explanations (e.g., thermalization by grains or tired light hypothesis). The taxonomy reflects cosmological and astrophysical motivations, as well as explanations aiming to support a particular cosmological model or those aiming to explain the radiation as a regular astrophysical phenomenon. It is pointed out that the rest of the book analyzes technical details of explanations, predictions, and suggested tests, the historical context in which the explanations were devised, and explicit and implicit epistemic, metaphysical and methodological motivations for constructing them.

The chapter draws some epistemic lessons from forgotten alternatives, theoretical conjectures that led to them, observational refutations, and the roles they played in building orthodox consensus. Most alternatives have never been fully developed, and some examples suggest that the potential for their improvement should not be underestimated. Moreover, viable alternatives and criticisms in cosmology can arrive piecemeal, not necessarily as fully worked out models. Finally, less apparent general theoretical assumptions may always lurk in the background of any model, a reflection on which may improve it or lead to a new one. Features of old models can once again become attractive, as the field interrelates various observations and theoretical presuppositions. The chapter offers some examples of this. It then ranks the alternative explanations in terms of plausibility, persuasiveness, and possible fruitfulness of some of their features.