The chapter reviews the notion of the initial conditions of the evolving universe and its relationship with the physical laws. It argues a key rift across various explanations of the CMB was based on different understandings of this relationship. Physicists were divided into two camps: those opting for initial conditions as extraneous to the laws, and those predicating initial conditions as following from or inherent to the laws.

Sir Martin Reese’s second model of 1978 was another Population III (pre-galactic stellar) explanation but with the motivation of deriving the photon-to-baryon ratio from known astrophysical processes. As the chapter explains, this motivation was interconnected with concerns about fine-tuning physical constants and cosmological parameters to enable a habitable universe. The non-primordial origin of the CMB Reese worked out with Bernard J. Carr was driven by adherence to the simplicity of a hypothesis. He expressed sympathy with Paul Dirac’s hypothesis of large number of coincidences that established relations between the age of the universe and atomic units, the gravitational constant and cosmic time, and the number of nucleons and cosmic time in terms of large dimensionless numbers. Dirac turned his initial hypothesis into a full-fledged but unusual and intriguing variant of the Big Bang model. The chapter presents some discussions of the model with respect to the precision of the measurements of the CMB.

Keywords: The steady-state theory of Bondi, Gold, and Hoyle of the late 1940s was very much alive at the time of the CMB discovery, and the discovery prompted new variants and reformulations. The chapter argues that the theory was motivated by the fear of the untenable changing of physical laws that the evolving universe enabled. This fear resulted in adherence to a “perfect cosmological principle” ensuring the homogeneity and isotropy of the universe at all times. Bondi and Gold’s version was a theoretical framework within Newtonian universes, while Hoyle’s was developed within the General Theory of Relativity without cosmic constant while introducing a universal scalar for (constant) creation of matter. The theory faced observational obstacles (especially with the newly discovered quasars), and thus required reworking. Reworking continued after the 1965 discovery of the CMB. Other radical unorthodoxies like plasma cosmologies and the closed stationary state cosmological model were not as original, but they too have a place – they ensure we understand that current rebels against allegedly outlandish inflationary cosmology had earlier counterparts.

The notion of the chaos in the early “chaotic cosmology” indicated that the same outcome of Big Bang would occur even under variations in the initial conditions, thereby avoiding the arbitrary and ad hoc nature of the initial conditions. In early 1970s, Reese argued the early universe may have not been much smoother than today, and inhomogeneity should follow from theory, not simply added as an ad hoc assumption. He also anticipated a related problem, later dubbed the “horizon problem,” where unconnected parts of the expanding universe somehow end up with the same curvature and entropy. Starting with these same problems, Zel’dovich independently developed a similar solution of corresponding initial fluctuations of baryon density on the one hand, and fluctuations in metrics on the other. These explanations, along with a variation by Zel’dovich and Sunyaev, prompted by the 1979 measured blackbody discrepancy of the CMB spectral shape, pushed the origin of the CMB to a time earlier than the orthodox model. The inflationary paradigm addressed these worries within the orthodoxy a few decades later.

Explanations of the CMB exhibited varied and often opposed epistemological motivations, and the models were correspondingly diverse. As the chapter clarifies, the explanations varied from subsidiaries to fully worked-out cosmological models, probing toy-models, and even the deliberate omission of modeling, relying on regular astrophysical insights alone. An admirable epistemic and observational diversity was achieved in the face of an emerging trend of ever-more centralized observational and theoretical programs that came to dominate much of physical science, including cosmology.

The idea that the basic features of the CMB were at least in part due to thermalization by cosmic dust was an auxiliary hypothesis to cold and tepid Big Bang explanations and later to the explanations within variants of the steady state model. David Layzer started developing his cold Big Bang views in the late 1960s, epistemically motivated by avoidance of Hot Big Bang ad hoc assumptions about initial conditions, while sticking to explanations based on regular known processes as much as possible. He argued for early favorable conditions in a cold Big Bang, which required the auxiliary of thermalization of the CMB by grains. Different physically plausible shapes of grains were devised, from hollow spheres to elongated ones, along with their different observationally plausible content. Explanations of the dust’s exact appearance during the evolution of the universe also differed.

It is tempting to think that the CMB, a remnant of the primordial fireball event, was conceived as smoking gun (or rather the smoke of a firing gun) evidence of the Hot Big Bang. Certainly, the work of some cosmologists was predicated on this assumption, but a number of others developed explanations based on variations on the Big Bang, and those who devised substantially different alternative explanations had various other motives. Moreover, the explanations involved both an historical (including the smoking gun) and a regular experimental mode of inquiry. This is, strictly speaking, even true of contemporary particle physics. Finally, although in principle, experimental and observational approaches to physical phenomena may be on a par epistemically, the physical limitations of studying the entire and unique universe puts cosmology in a far more challenging position than experimental fields of physics. The chapter argues this should prompt an especially cautious attitude to our understanding of the role of the alternatives.

As the chapter points out, standard cosmology textbooks offer a triumphant account of the Hot Big Bang model as the orthodoxy that was almost instantly accepted with the discovery of the CMB by Penzias and Wilson. Yet this historical account does not reveal the whole story of the orthodoxy’s gradual ascent and the all but forgotten, but at the time important, side roads. We argue that historical analysis must shun the triumphant partial account, but resist eliminating the current understanding as irrelevant for understanding the past. If approached in a balanced manner, the story of the CMB evidence often diverges from the values that unequivocally lent support to the orthodoxy, thus opening the space for alternative interpretations.

Due to the low angular resolution of sources in observations early after the discovery of the CMB, there was a possibility that the radiation’s uniformity and diffuse emission were produced by multiple unresolved very distant sources. This possibility was one of many similar dilemmas in other areas of astronomy and physics, and it was fairly quickly resolved. In the late 1960s, Gold and Pacini suggested the idea of unresolved sources was plausible, while Wolfe and Burbidge, working across the orthodoxy-alternatives divide, addressed the unexplained density of the radiation and its spectral shape by pointing to the possibility of unresolved sources potentially being observable at radio frequencies. However, observational tests demonstrated a lack of suitable objects radiating at predicted frequencies. The epistemic motivation for these and related models was to introduce minimal astrophysical assumptions to explain the nature of the CMB. There was also an anticipation of yet unknown astrophysical objects lurking in the background. Finally, although quickly refuted observationally, a clear and comprehensive model of Rowan-Robinson exhibited all the key features of such models while also anticipating key role of Active Galactic Nuclei in future research.

Sciences studying the deep past, including cosmology, reconstruct past phenomena from often meager remnants available in the present. This leads to prolonged periods of indecisiveness about discrepant theoretical explanations and models. The chapter argues that the notion of the so-called underdetermination of competing theories by evidence captures the epistemic situation that characterizes modern cosmology. The logical notion of underdetermination of competing theories predicated on total possible evidence is not so interesting in understanding and tracking details of the actual historical episodes. An historical notion of underdetermination more realistically assumes only partially equivalent evidential bases of competing theories. Protracted periods of underdetermination also question idealized notions of observational facts as opposed to speculative theories, as pointed out by Bondi in the 1950s. Prematurely establishing certain observations as immutable facts, which, in turn, eliminates various theoretical accounts, impedes the field that operates at the observational limit. The chapter argues that a qualified notion of these concepts is needed to approach historical analysis of cosmology properly.

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.

Narlikar and Wickramasinghe’s 1968 version of steady state theory made use of the dust grains hypothesis, while arguing that the precise measurements of the CMB spectral shape deviate from the black body shape. As critics pointed out, an unrealistic consequence of the model (CMB exceeding visible radiation by a factor of 100) was Wickramasinghe’s 1975 version; it included a very detailed account of elongated whiskers as thermalizers that compose cosmic dust. The 1990s version of steady state constructed by a larger group of authors – the usual proponents of the steady state – introduced the discrete creation of matter (“mini bangs”) in the cellular form of observed galactic structures. The motivation was to eliminate the epistemically problematic singularity of the Hot Big Bang, explain creation of matter as an inherent feature of physical laws, and introduce strong gravitational fields of galactic nuclei as the sites of synthesis of atomic nuclei (with a detailed description of the mechanisms of nucleosynthesis). The chapter discusses a number of combinations of steady state, Population III as sources, and dust grain thermalizers devised until early 2000.

The image of Darwin as a lone thinker, a theoretician who worked largely in isolation rather than a hands-on scientist, has no single origin but is stubbornly persistent. Modern accounts that do feature him as a practical researcher tend to emphasize the domestic setting of his work, focusing on experiments that can be replicated in a modern house, garden, or school. But contemporary evidence, in particular from Darwin’s extensive correspondence, demonstrates that he was an ingenious and innovative experimenter, keenly aware of advances in science, and often at the cutting edge both in the nature of his investigations and in the technologies he employed. Far from working alone on gathering facts and grinding out his theories, Darwin was expert at cultivating and exploiting contacts. He actively sought collaboration with all sorts of people around the world, both asking for their help and encouraging their own investigations. Although he rarely travelled after settling in the village of Down in Kent as a young married man, Darwin’s version of ‘working from home’ was far from solitary: he was surrounded not only by a large and happy family but by governesses, gardeners, friends, neighbors, and visitors, who acted as critics, assistants, editors, and even as research subjects.

The theories of Darwin and Wallace were similar, both seeing evolutionary change coming about as a function of a differential reproduction fueled by the pressure to succeed in the struggle for existence.  Darwin and Wallace came to their thinking independently.  The behavior of both men, in what could have been a tense situation, was exemplary.  Wallace sent his paper to Darwin, a postal journey from the Far East to England that took far longer than people expect.  This has led to beliefs that Darwin sat unfairly on the paper, perhaps using it to burnish his own work.  In truth, upon its receipt, Darwin quickly contacted his senior friends Lyell and Herschel, offering to let Wallace have full priority.  Lyell and Hooker arranged for Wallace’s paper and unaltered, pertinent abstracts of Darwin’s earlier writings to be published together in the journal of the Linnaean Society.  Both Darwin and Wallace always felt that matters had been dealt with speedily and honorably.  That said, there were significant differences in the thinking of Darwin and Wallace.  The latter was never comfortable with the metaphor of selection, he always embraced group selection in opposition to Darwin’s determined individual selectionist thinking, and most famously – notoriously – Wallace turned to spiritualism to explain human evolution, spurring Darwin to give an entirely naturalistic explanation is his Descent of Man.