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.

Computer simulations have played a crucial role in modern cosmology, a role anticipated and pioneered by F. Hoyle. Their interrelation with theory and observation is invaluable in developing theoretical positions, testing general features of isotropy and homogeneity at various scales, and probing the anomalies. The simulations also make up for the inability to experiment with different structures of the universe and substantially different physical universes. Yet various selection effects and the numerical intractability of certain alternative explanations limit the value of simulations. This is a serious epistemic worry that can be addressed with the lessons learned from the alternative explanations of the CMB.

The principle of relativity requires that no interaction can propagate instantly. Gravitational waves (GW) must exist, propagating with the same speed as light. The specific characters of GW are predicted by Einstein’s general relativity (GR). After decades of efforts to develop detectors, on 11 February 2016, the LIGO and Virgo Collaboration published the discovery of a GW.

The elements of GR relevant for GW production, propagation and detection. How the GR field, which is the dimensionless metric tensor, differs from the other fundamental fields, which have physical dimensions. The instruments and the discovery. After the first observation, dozens of gravitational signals have been detected, the vast majority from merging black holes and one, on 17 August 2017, from the merger of neutron stars. In this case, electromagnetic signals are expected, and have been detected, providing unique information to astrophysics and to fundamental physics as well. The measurement of the speed of the GW and the establishment of a bound on the mass of the graviton.

G.F.R. Ellis’s late 1970s exploratory model, the topic of the chapter, reverse-engineered the current state of the universe into an inhomogeneous static state with two centers located at the opposite sides and our galaxy very close to one of them. This relativistic framework attempted to bypass the cosmological principle as an unjustified assumption while agreeing with the observations, including the measurements of the CMB, and offering alternative explanations of the key parameters (e.g., the observed redshift of galaxies has a gravitational origin). In the model, the CMB photons are continuously produced at one center (singularity) and annihilated at the other. Variations of the model were worked out, and another model similar in spirit was devised by Phillips in the 1990s. It introduced two singularities (poles) and two kinds of matter that circulate across the universe from one center to another. The hot plasma near the singularity acted as a perfect black body radiating the redshifted CMB. The chapter discusses the epistemic virtues of these models (e.g., the “natural” origin of CMB photons as in the orthodox approach) and their observational refutation and mentions a related model by P.C.W. Davis.

Oscillations between members of flavoured, electrically neutral meson pairs and the CP violation are phenomena strictly connected with the mixing. However, CP is more general, having been observed also in the decay of charged mesons.

CP violation was first observed in the neutral K system. We see the states of definite strangeness, those of definite CP and those with definite mass and lifetime. The oscillation between the former states, the mathematical expressions and the experimental evidence.

The oscillations and CP violation in the B0 system, and the beautiful experimental results obtained at dedicated high-luminosity electron–positron colliders, the ‘beauty factories’. Beauty physics at the dedicated experiment LHCb at LHC, in particular for the B0, that is not accessible to beauty factories. Examples of CP violation in B0. The recent discovery of CP violation in the charm sector.

How the many different measurements can be put together to test the SM with the unitary triangle.

Neutrinos are the most difficult particle to study, because they interact only via weak interactions. However, they have given revolutionary surprises, and it is with neutrinos that physics beyond the SM has been discovered. In the SM, neutrino masses are rigorously zero, but experiments show that they do have a mass. In the SM, neutrino flavour eigenstates are mass eigenstates; experiments show that they are mixtures of them. Two discoveries proved this. One is neutrino oscillations, discovered in atmospheric neutrinos, the other is the adiabatic flavour conversion in matter, discovered in solar neutrinos.

These were with natural neutrinos. Several experiments have been, and are being, performed with artificial neutrinos from reactors or accelerators to measure with increasing accuracy the neutrino mixing matrix and the mass spectrum. We found that the neutrino mixing is much larger than that of the quarks. Nobody knows why.

The SM assumes neutrinos to be different from antineutrinos, but no experimental proof of it exists. Neutrinos and antineutrinos may well be the same particle, a Majorana spinor. We see how this is searched for by looking for the extremely rare double beta decay.

Although developed within the relativistic framework, the cold and tepid Big Bang models were prime examples of moderate unorthodoxy that introduced alternative, very different initial conditions (i.e., photon to baryon ratio). As the chapter explains, they provided more plausible and “easier” conditions for the structure formation in the early universe, but they differed in terms of their theoretic, epistemic, and methodological motivations. Misner reversed-engineered the universe to more favorable initial conditions, Alfvén’s toy-model was motivated by the ad hoc nature of the Hot Big Bang model, and Carr and Reese emphasized the necessity of the early fluctuations the orthodoxy lacked in order to explain inhomogeneity in the current universe and the implausibility of cosmological entropy. The respective explanations of the CMB within these models relied in various ways on the so-called Population III objects’ radiation, a set of objects that formed fairly soon after the Big Bang. These non-cosmological explanations were partly motivated by the 1978 measurements that erroneously indicated lack of agreement with the black body spectrum shape, and they ran into real difficulties only with the COBE satellite data. The chapter contains a box with a technical explanation of the minimal comptonization parameter describing a redistribution of photons in reference to the black body spectrum.

The weak interaction was proposed by Fermi in 1933, to interpret the beta decay. The interaction Lagrangian is the product of two charged currents (CC) – one of the nucleons, one of the leptons. It was later discovered that parity and charge conjugation are not conserved and that the structure of the charged currents is a combination of vector and axial currents, V–A. The beautiful Goldhaber experiment on the helicity of the neutrino.

The coupling of all leptons is universal, but not that of the quarks. To obtain universality, Cabibbo introduced the concept of mixing of the hadronic currents, namely of quarks. Then the Glashow–Iliopoulos–Maiani mechanism solved a problem introducing the hypothesis that a fourth quark would exist, the charm, completing a doublet with the strange one. With the discovery of two more quarks, the quark mixing matrix contains a phase factor that is the origin of CP violation in the Standard Model.

The weak neutral currents were discovered with the Gargamelle bubble chamber at CERN in 1973. This showed a close similarity between weak and electromagnetic interactions and opened the way to their unification.

This chapter briefly discusses two recent, apparently visually tractable anomalies that challenge the Copernican principle whereby our location in the universe is not special. The radial grouping of galaxy clusters (and alleged distortion of the redshifts due to expansion), the “fingers of god,” indirectly challenges the orthodox interpretation of the CMB. A more recent anomaly, the “axis of evil,” points to the visually observable “conspiring” of interchanging hot and cold regions in the CMB to form an axis of anisotropy. The chapter discusses various responses and notes the epistemic standing of the challenges in comparison to the worked-out alternatives to the orthodox explanation of the CMB.

As this chapter explains, the first systematic predictions of the cosmic microwave background radiation were put forward by three independent groups of early proponents of the Big Bang Model: Gamow and his students, Doroshkevich and Novikov, and Dicke and his collaborators. The theoretical inferences that a uniform background radiation should be present came to fruition in 1965 with a serendipitous discovery by Penzias and Wilson. The prediction was that after the period of electrons and nuclei finally combining into atoms was finished some 400,000 years ago, the omnipresent radiation (cosmological photons) would scatter off the atoms and continue traveling until they reach us, while cooling down to the tens of kelvin from the initial 3000 kelvin. Various early predictions differed and were off from the measured 3 kelvin, due to some theoretical unknowns at the time. Yet the first measurement likely occurred before World War II with the measuring of the excitation of cyanogen molecules in a distant nebula, a fact pointed out by Fred Hoyle, the main critic of the Big Bang model (who also coined the phrase Big Bang, albeit pejoratively). The early predictions were only the first segment of a long convergence to the Hot Big Bang model as a standard model of cosmology.