The American Alsos Mission, a scientific intelligence-gathering task force, followed behind the advancing Allied armies in the west, looking for evidence of a German atomic bomb. Its scientific leader, the physicist Samuel Goudmit, quickly determined that the Germans were far removed from building nuclear weapons but also was misled by some documents and his own prejudices, convincing himself that the Germans, including his colleague Werner Heisenberg, had not understood how an atomic bomb would work. When Goudsmit returned to the United States, he began publishing books and articles using the German uranium work as an example of how the Nazis had ruined science through political and ideological control, mistakes that America must not repeat. Heisenberg responded by defending both his scientific work and conduct under Hitler. Goudsmit criticized both Heisenberg and Carl Friedrich von Weizsäcker for compromising with the Nazis. While Goudsmit eventually reconciled with Heisenberg, he never forgave Weizsäcker. Goudsmit had lost his parents in Auschwitz, and Weizsäcker’s father, a high-ranking official in the Foreign Ministry, had been convicted of war crimes.

In 1938 nuclear fission was discovered in Germany. Like their counterparts in other countries, German scientists brought the military potential of fission to the attention of officials and began researching isotope separation and nuclear reactors, the two paths to atomic bombs. During the Blitzkrieg phase of the war, powerful new weapons were not needed, so that the research had low priority and made modest progress. When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. Because the Americans, who had much greater resources, were apparently working on this, the Germans continued to as well. The steadily deteriorating state of the war made research more difficult, then impossible, as the scientists were focussed on their survival. After the war, the revelations of the Holocaust, and the atomic bombing of Japan, these scientists were criticized for collaborating with the Nazis and had to justify their work. The result was the legend of Copenhagen, a claim that they had in fact been trying to forestall all nuclear weapons.

In order to determine what really happened when Werner Heisenberg and Carl Friedrich von Weizsäcker met with Niels Bohr in occupied Copenhagen in September 1941, this visit has to be placed in several contexts. By this time the German uranium research had demonstrated that atomic bombs were probably feasible, even if not for Germany during the war. The summer 1942 German offensive against the Soviet Union had not yet begun to falter, although Heisenberg was nevertheless privately very anxious about the war. The Germans alienated Bohr and his colleagues by their participation in cultural propaganda and nationalistic and militaristic comments about the war. A comparison with Heisenberg’s other lecture trips abroad shows that he acted the same way in other places. Heisenberg’s subsequent efforts in 1942 to gain support from Nazi officials by both describing the power of atomic bombs and the threat that the Americans might get them first also do not fit with an attempt at Copenhagen to forestall all nuclear weapons. Instead the best explanation for the visit is Heisenberg and Weizsäcker’s fear of American atomic bombs falling on Germany.

When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. The project was transferred from Army Ordnance to the Reich Research Council, the institution responsible for mobilizing civilian research for the war effort. The scientists, who were now threatened with the loss of their exemptions from frontline service, began to “sell” their research. Although they did not promise to deliver atomic bombs, they did emphasize the tremendous power of such weapons and warned that the Americans, who had much greater resources, were apparently working on this. In the meantime a model nuclear reactor experiment had produced a neutron increase, which was interpreted as proof in principle that a nuclear reactor could be built. Several influential figures responsible for armament production now took a keen interest in uranium research and the powerful Minister of Armaments Albert Speer decided to generously support the project.

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.