Right after the 1965 discovery of the CMB, F. Hoyle and his student J.N. Narlikar constructed a new version of the steady-state model, starting with Hoyle’s matter creation scalar field, and this model is the focus of the chapter. The creation of matter in the pockets near massive objects violated earlier adherence to inhomogeneity. The 1972 version of the model introduced an intriguing explanation of the CMB as a radiating of the boundary between the regions of the universe with positive and negative mass: any amount of matter entering such a boundary will act as a perfect thermalizer, with radiation of 3 kelvin reaching us from all directions. It was perhaps the first worked out model of the multi-universe. Hoyle and Narlikar argued for perfect thermalization, implying a black body spectrum. In this, their model was unlike many other unorthodoxies motivated by the erroneous measurements of 1979 indicating disagreement with the shape of the spectrum.

The chapter briefly discusses an alternative explanation of the CMB origin in the semipopular plasma cosmology of O. Klein, later advocated by others. The approach took the still mysterious observed matter–antimatter asymmetry as its starting point, arguing in favor of symmetry with slow annihilation that provides (in principle) the energy contained in the CMB. Later versions added a challenge to the dark matter hypothesis and its solution by pointing to the problem of equilibrated parts of the expanding universe. Although developed in some detail, this sort of explanation eventually had to draw on older ideas (e.g., tired-light hypothesis) in the face of the COBE mission results.

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 possibility of the multiverse bean with early steady-state theories postulating causally unconnected regions, a standard Big Bang where spatial cross-sections are flat or open, or even an eternal inflationary universe. These cosmological options present a philosophical challenge to a realist understanding of the universe that is addressed through a discussion of the CMB’s central relevance in it in the chapter.

The introductory chapter presents the key features of the historical and philosophical analysis of the cosmic microwave background (CMB) radiation, focusing on little known alternative explanations to the Hot Big Bang model that ultimately became the standard interpretation of the CMB. The book challenges the common perception of a swift consensus on the CMB’s explanation by revealing multiple valid alternative hypotheses that are largely forgotten today. It argues this has hindered a comprehensive understanding of the history and methodology of cosmology, as well as the ability to draw important philosophical lessons for contemporary cosmological research. The chapter emphasizes the need to understand the epistemic role of the CMB in cosmology and its implications for addressing criticisms of the field. It highlights the diverse range of alternative theories proposed and suggests revisiting these theories may yield valuable ideas and conjectures for modern cosmology. It emphasizes that overlooking alternatives can impede progress. The discussion covers various aspects, from the controversial beginnings of physical cosmology to the characterization of the orthodox interpretation of the CMB, epistemological and methodological concerns, and both moderate and radical alternatives, drawing lessons for the present and future of cosmological research.

The chapter provides a brief overview of the first three major eras, out of four, in the development of cosmology. The first era started with “prehistory” of cosmology in antiquity, continued with the major contributions of Newton and the nineteenth-century debates on thermodynamics conditions at the cosmic scale, and ended with a “quantum leap” in relevant observational capacities at the beginning of the twentieth century. The second era saw cosmology develop as a mathematical game of sorts, rather than a physical theory predicated on Einstein’s General Theory of Relativity. It was marked by Einstein’s static model of the universe and a static model by De Sitter. A cosmological revolution began in the third era (from 1929 to 1948), with the development of expanding models of the universe that captured its physical dynamics.

In this chapter, the two main ingredients of the contemporary cosmological paradigm, or the new standard cosmology, are initially presented as a thought experiment that brings us back to the initial singularity from which the physical universe sprang. The thought experiment follows the trajectory of the contraction of matter and radiation, darkening galaxies, and ever hotter universe to the first hundreds of seconds when a violent inflation of the universe took place that we can understand speculatively and perhaps observe indirectly through the structure of gravitational waves. After 300,000 years of expansion of the universe, very energetic photons disrupted positively charged nuclei and negatively charged electrons in forming atoms. At that point, the atoms were formed, and photons scattered off, traveling through space and slowly losing energy due to expansion of the universe. The structures (stars and galaxies) started forming. The accelerated, rather than uniform, expansion of the universe and dark energy driving it were postulated in 1998. This feature is not as established as the Hot Big Bang model, but observational evidence is accumulating. The chapter provides a box with the basic physical elements of the new standard cosmological model.

The chapter starts with a discussion of the minimal definition of anthropic reasoning that avoids the usual confusions – the biases due to us being evolved intelligent observers. Anthropic reasoning led to mundane important conclusions about some key parameters of the universe in the work of Hoyle and other pioneers. The chapter discusses a deep connection with the justifications of the violation of the cosmological principle. It also discusses Aguirre’s study of habitable regions in the parameter space of cold Big Bangs in the early 2000s. Finally, it briefly addresses some CMB problems that stem from anthropic reasoning and typicality of the observer.

In the early 1990s, Gnedin and Ostriker developed a framework for explaining the CMB that was not tied to any particular model. It was predicated on questioning the Hot Big Bang assumptions, especially the ad hoc assumption of dark nonbaryonic matter, while it opted for more “natural” regular astrophysical explanations and observed properties. Gnedin and Ostriker devised a complicated scheme of early interactions of baryonic matter and plasma based on regular physics, with the CMB its expected product. Yet this move required hypothesizing hidden baryonic matter, in galaxies or otherwise, and this had its own epistemic challenges. The chapter notes that this approach was abandoned with the dark energy postulation of the late 1990s.

The chapter discusses the “great controversy” of modern cosmology. The controversy began after World War II and lasted for a couple of decades. In the controversy, the proponents of various iterations of the steady-state theory of the universe collided with the pioneers of the emerging big-bang expanding universe theory. The latter theory triumphed, while establishing empirical standards of cosmological theories and breaking the stigma of cosmology as an unscientific subject that lurked in the science community. Parsimonious observational criteria were devised for the key cosmological parameters, including the age of the universe, source counts, redshift–magnitude relation, and redshift–angular size relationship. The chapter also discusses how the relation between redshift in the spectrum and magnitude was pioneered by Hubble and slowly perfected by tests on different celestial objects, from galaxies to Type Ia supernova stars.

In this chapter, we argue that if we are blinded by the constant stream of astrophysical and cosmological observations, we may forget that cosmology is the youngest of all the physical sciences. The 1965 discovery of the CMB radiation by Penzias and Wilson moved cosmology to the territory of firmly observational science from the domain of exclusively mathematical modeling, and the 1977 measurements of CMB’s anisotropies with detectors mounted on US spy aircraft opened its Big Science phase. A number of measurements of the CMB spectral shape by detectors mounted on rockets and balloons following the 1965 discovery led to fluctuating agreement with the values of the black body radiation spectrum. In particular, 1978–1979 measurements exhibited discrepancies that gave new impetus to the alternative explanations of the radiation. A series of satellite measurements since the early 1990s, with equipment similar to previous experiments but without atmospheric disturbances, led to the final phase of the convergence to the Hot Big Bang model.

N. C. Rana’s explanation, the focus of this chapter, is probably the most radical among the moderate alternative explanations of the CMB. It does not rely on the thermalization of Population III objects’ radiation, but on the thermalization of “normal” starlight at redshifts between 10 and 5. This explanation required strong starburst at these epochs and adequate cosmic dust to thermalize their radiation. A specialist on the latter, Rana modeled elongated metallic dust grains twice in order to achieve sufficient values of thermalization. His explanation avoided both the “horizon problem” and the isotropy problem (as isotropy of matter distribution implies isotropy of light distribution). His rather general model understandably underestimated the influence of small-angular scale fluctuations measured by COBE.

The chapter presents the key properties and the evolution of satellite measurements before diving into the history of various explanations of the CMB. The shape of the CMB spectrum reflecting the black body radiation and its unusual isotropy are the essential properties telling us about the origin, but its temperature is also dependent on other contingent factors. The chapter briefly discusses details of the physical process behind such properties and the relevance of their ever-more precise satellite measurements and presents these in diagrams.

Although there is little reason to discuss the CMB in relation to classical static models starting with Einstein’s model, a mid-2000s attempt by Sorrell deserves some attention and is discussed in the chapter. Motivated by the strong (perfect) cosmological principle of homogeneity and isotropy of the universe at all times, and drawing on the tired-light hypothesis of Crawford, Sorrell postulated a cosmic ether and a continuous nucleosynthesis that produces two kinds of particles composing it; we observe the resulting photons as the CMB. It is a toy-model with the goal of cautioning against an unconditional acceptance of orthodoxy.