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.

Messier’s catalog of 110 star clusters, nebulae, and galaxies is the most popular list of deep sky gems. The first edition of this stunning reference atlas was hailed as the most comprehensive, detailed, and beautiful account of the Messier objects then available and the second edition continues this trend. Each object is presented with:

• Updated historical information, including from new sources, featuring accounts and anecdotes from Messier and other prominent visual observers who followed him.

• Thoroughly researched astrophysical information, the results of an investigation of more than 500 recent scientific papers including, for the first time, fully consistent distance data from the Gaia space observatory.

• Extensive information on visual observing using the naked eye, binoculars, and amateur telescopes from modest sizes up to 20 inches aperture.

• New large-scale color photos from some of the world’s best amateurs displaying the objects’ splendor, as well as close-up images from the Hubble Space Telescope, for most objects, showing the fine details.

The introductory section includes an extensive biographical portrait of the life of Charles Messier, his observations and his telescopes, and his contemporaries, and a complete translation of Messier’s original catalog. There is also detailed information on how to observe the Messier objects and advice on how to conduct a Messier Marathon. For those seeking even more, the author provides the Herschel 100 list. In addition, many objects feature historical sketches from classical observers from the nineteenth century alongside the author’s modern deep-sky drawings.

Astronomers of all abilities will delight in the Atlas’s return as your guide to the sky’s finest objects.

Messier’s catalog of 110 star clusters, nebulae, and galaxies is the most popular list of deep sky gems. The first edition of this stunning reference atlas was hailed as the most comprehensive, detailed, and beautiful account of the Messier objects then available and the second edition continues this trend. Each object is presented with:

• Updated historical information, including from new sources, featuring accounts and anecdotes from Messier and other prominent visual observers who followed him.

• Thoroughly researched astrophysical information, the results of an investigation of more than 500 recent scientific papers including, for the first time, fully consistent distance data from the Gaia space observatory.

• Extensive information on visual observing using the naked eye, binoculars, and amateur telescopes from modest sizes up to 20 inches aperture.

• New large-scale color photos from some of the world’s best amateurs displaying the objects’ splendor, as well as close-up images from the Hubble Space Telescope, for most objects, showing the fine details.

The introductory section includes an extensive biographical portrait of the life of Charles Messier, his observations and his telescopes, and his contemporaries, and a complete translation of Messier’s original catalog. There is also detailed information on how to observe the Messier objects and advice on how to conduct a Messier Marathon. For those seeking even more, the author provides the Herschel 100 list. In addition, many objects feature historical sketches from classical observers from the nineteenth century alongside the author’s modern deep-sky drawings.

Astronomers of all abilities will delight in the Atlas’s return as your guide to the sky’s finest objects.

Messier’s catalog of 110 star clusters, nebulae, and galaxies is the most popular list of deep sky gems. The first edition of this stunning reference atlas was hailed as the most comprehensive, detailed, and beautiful account of the Messier objects then available and the second edition continues this trend. Each object is presented with:

• Updated historical information, including from new sources, featuring accounts and anecdotes from Messier and other prominent visual observers who followed him.

• Thoroughly researched astrophysical information, the results of an investigation of more than 500 recent scientific papers including, for the first time, fully consistent distance data from the Gaia space observatory.

• Extensive information on visual observing using the naked eye, binoculars, and amateur telescopes from modest sizes up to 20 inches aperture.

• New large-scale color photos from some of the world’s best amateurs displaying the objects’ splendor, as well as close-up images from the Hubble Space Telescope, for most objects, showing the fine details.

The introductory section includes an extensive biographical portrait of the life of Charles Messier, his observations and his telescopes, and his contemporaries, and a complete translation of Messier’s original catalog. There is also detailed information on how to observe the Messier objects and advice on how to conduct a Messier Marathon. For those seeking even more, the author provides the Herschel 100 list. In addition, many objects feature historical sketches from classical observers from the nineteenth century alongside the author’s modern deep-sky drawings.

Astronomers of all abilities will delight in the Atlas’s return as your guide to the sky’s finest objects.