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.

The toolbox for subnuclear physics.

How the Lorentz transformations can be found from basic properties of space-time, independently of electromagnetism, as in the usual presentations. Lorentz-invariance is a common property of all the fundamental interactions.

Clear discussion of the fundamental concepts of energy, momentum and mass; of their relations; and of their transformations between reference systems, in particular the laboratory and centre of mass frames.

The sources of high-energy particles, cosmic rays and the different types of accelerators. The progress of our knowledge is fully linked to the experimental ‘art’ of detector design and development. Detectors are made of matter, solid or liquid, or gaseous. The interactions of charged and neutral high-energy particles with matter are described. The principal types of detector and the principles of their operation are introduced.

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 fascinating new world inside the nucleon, of quarks, gluons and colour, the nuclear strong force. How quantum chromodynamics (QCD) was discovered: probing the nucleons with scattering experiments and with increasing energy e+e− colliders, where quarks and gluons appear as hadronic jets.

The colour charges are three. Being the gauge of QCD non-Abelian, the gluons, not only the quarks, are ‘coloured’. How colour charges bind three quarks or a quark–antiquark pair forming hadrons that have zero overall colour charges.

The QCD coupling constant runs as the fine-structure constant, but with increasing momentum transfer, it decreases, instead of growing. Quarks become ‘free’, when they are very close to each other. Only a very small fraction of the proton mass is due to the quark masses, 99% being the energy of the colour field. The QCD vacuum, the status of minimum energy, a very active medium indeed, beautiful to study.

When matter first appeared in the universe, in the first microsecond after the Big Bang, quarks and gluons moved freely in a hot ‘soup’, the quark–gluon plasma. It is created in the laboratory in the ultra-relativistic heavy ion colliders and theoretically analysed with lattice QCD

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.