This short chapter touches on the limitations of the SM. The SM does not include gravity, and it does not explain the major components of the mass–energy budget of the universe, dark matter and dark energy, the latter being probably the cosmological constant. CP violation in the quark sector is too small to explain the matter–antimatter asymmetry of the Universe, but, if confirmed, the non-SM CP violation in the neutrino sector might be large enough. The ‘strong CP violation’ problem might be solved with the existence of a very light particle, the axion; experiments are reaching the requested sensitivity. Supersymmetric particles present in some extensions of the SM have been searched for, but not found so far.

The SM contains too many free parameters: the masses of the fermions and of the bosons, and the mixing angles. The masses of the fermions, from neutrinos to the top quark, span 13 orders of magnitude. Why such big difference? Why is mixing small in the quark sector, and large in the neutrino sector? Why do the proton and the electron have exactly equal (and opposite) charges? Why are there just three families? Are there any spatial dimensions beyond the three we know? And so on.

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 classical electrodynamics (CED), the most important quantities are the electric and magnetic fields, which directly determine the forces. In quantum electrodynamics (QED), the potentials are the most important quantities; they determine the energy and momentum exchange between the EM field and matter. Gauge invariance, which in CED is just a mathematical curiosity, becomes fundamental in QED, ruling the gauge symmetry that determines the interaction itself.

The Lamb experiment that opened the way is discussed in detail.

Feynman diagrams are graphic representations of mathematical expressions of scattering or decay amplitudes. Without going into the mathematics, we use them to visually suggest the underlying physics. We show how the propagator describes virtual particles, and how uncertainty and relativity principles, joined, imply the existence of antimatter.

The fine-structure constant, which is the dimensionless expression of the electromagnetic charge, depends on the momentum transfer between the probe and the target charge in the scattering experiment we are performing. The ‘running’ of the coupling constants is a property of all the interactions.

The highest precision measurements and theoretical predictions of the magnetic moments of the electron and of the muon. The precision frontier to search for new physics.

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.