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.

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.