The chapter summarizes the historical trajectory of alternatives to the orthodox explanation of the CMB, which lacked strife or “great controversy” prior to the 1965 discovery. Various alternative explanations were discussed and criticized to differing extents, but no consensus emerged. The engagement and observational refutations, most decisively with the COBE results, came in gradually without significant social and professional jitters. Yet careful theoretical considerations of the alternatives and the challenges they offered prepared the way for later convergence when overwhelming empirical evidence in favor of the emerging orthodoxy still did not exist. Some of the most authoritative physics figures worked on alternatives because the wiggle room was quite wide. This was an epistemically responsible response to the prolonged state of underdetermination of theories by existing limited (and fluctuating) evidence. In a concluding discussion, the chapter compares specific challenges and limitations of cosmology to those of experimentally driven fields of physics.

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.

If the mass of a hadron is large enough, decays into final states that can be reached by strong interaction, that is, without violating any selection rule, are possible. The lifetime is then extremely short, of the order of a yoctosecond (10−24 s). These hadrons decay practically where they were born. We show how they are observed as ‘resonances’.

Hadrons, both baryons and mesons, were discovered in rapidly increasing numbers in the 1950s and early 1960s. How their quantum numbers, spin, parity and isospin were measured. Gradually it became clear that hadrons with the same spin and parity could be grouped in multiplets of the SU(3) symmetry. Proposal of the quark model and experimental verifications of its predictions. With increasing accelerator energies, more surprises were to come. The quarks are not only the three originally known, u, d and s, but three more exist, c, b and t. And more leptons were found, in total three ‘families’ of fundamental fermions, each with two quarks, a charged lepton and its neutrino.

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.

In modern physics, symmetries are a powerful tool to constrain the form of equations, namely the Lagrangian that describes the system. Equations are assumed to be invariant under the transformation of a given group, which may be discrete or a continuous Lie group. Classification of the various types of symmetry. The concept of spontaneous symmetry breaking. It will evolve into the Higgs mechanism, which gives origin to the masses of the vector bosons that mediate the weak interactions, of the quarks and of the charged leptons.

The discrete symmetries, in particular the parity and the particle–antiparticle conjugation operations and the corresponding quantum numbers.

An important dynamical symmetry of the hadrons, the invariance of the Lagrangian under rigid rotations in an ‘internal’ space, the isospin space. The unitary group is SU(2).

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.