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Since about 1970 the broadly accepted theory of the universe has been the standard hot big-bang model. However, there is and has always been alternative theories which challenge one or more features of the standard model or, more radically, question the scientific nature of cosmology. Is the universe governed by Einstein’s field equations? Is it really in a state of expansion? Did it begin with a big bang? The chapter discusses various alternative or heterodox theories in the period from about 1930 to 1980, among them the idea of a static universe and the conception that our universe evolves cyclically in infinite cosmic time. While some of these theories have been abandoned long ago, others still live on and are cultivated by a minority of cosmologists and other scientists.

Observations of the cosmic microwave background (CMB) form the basis for modern ‘precision cosmology’. This chapter treats the discovery of a ≈3 K microwave background and the demonstration of its cosmic origin. Key observational results, up to and including the results from the COBE mission, follow. The major impact of the CMB comes from measurements of the power spectra of fluctuations in the temperature and polarization. The chapter ends with results derived from the power spectra obtained by the Planck mission, including values for the baryon, dark matter, and dark energy densities; the curvature of space; and the expansion rate of the Universe.

Although modern cosmology is essentially a twentieth-century science, its birth can reasonably be traced back to discussions about the universe in the previous century. With the emergence of astrophysics in the 1860s astronomy was substantially changed and the material content of the universe became an issue of science. At about the same time thermodynamics was applied to the universe at large, with the result that the beginning and end of the universe entered astronomical thought. Moreover, it became slowly realized that space can be described as curved rather than flat. In that case it would be possible to speak about a finite and yet unbounded universe and in this way to solve some of the problems associated with the traditional view of an infinite number of stars. These and other problems were only fully understood in the twentieth century, but they were discussed before Einstein revolutionized cosmology.

The success of the Λ CDM model has raised a number of challenging problems for the origin of structure in the universe and the initial state from which it evolved. The origins of these basic cosmological problems are described. The dark matter must be non-baryonic, but its nature has not been established. Likewise, the nature of the dark energy is not understood. The inflationary model for the very early universe has had some undoubted successes in accounting for the initial power-spectrum of fluctuations from which large-scale structures formed but there is no physical realization of the inflaton field. Defects formed during phase transitions in the early universe cannot account for the initial power spectrum of fluctuations, but may have some part to play in structure formation. The origin of the baryon-antibaryon asymmetry in the early universe is not understood in terms of theories of particle physics.

Following the pioneering studies of galaxies and the universe during the pre-Second World War years, the period 1940-1980 saw the consolidation of the observational and theoretical basis of geometrical and astrophysical cosmology. By the early 1950s, the cosmological time-scale problem had been resolved by Baade’s recalibration of the Cepheid distance scale, but new disputes arose about the best estimate of Hubble’s constant, the value of the deceleration parameter and the presence or otherwise of the cosmological constant in the cosmological field equations. The evolution of the contents of the universe was established by radio astronomical observations of active galaxies and, most spectacularly, by the discovery of the cosmic microwave background radiation. The latter enabled the problems of the origin of galaxies and large scale structures in the universe to be placed on a secure physical basis, but many issues remained unresolved, including the dark matter problem.

Since 1980, our empirical knowledge of the universe has advanced tremendously and precision cosmology has become a reality. These developments have been largely technology-driven, the result of increased computer power, new generations of telescopes for all wavebands, new types of semiconductor detectors, such as CCDs, and major investments by many nations in superb observing facilities. The discipline also benefitted from the influx of experimental and theoretical physicists into the cosmological arena. The accuracy and reliability of the values of the cosmological parameters has improved dramatically, many of them now being known to about 1%. The Λ CDM model provides a remarkable fit to all the observational data, demonstrating that the cosmological constant is non-zero and that the global geometry of the universe is flat. The underlying physics of galaxy and large-scale structure formation has advanced dramatically and demonstrated the key roles played by dark matter and dark energy.

The first detailed and extensive studies of nebulae were made by William and Caroline Herschel in the late eighteenth century. These researches led to wide-ranging debates on the nature of these objects: are they truly clouds of nebulous material or are they perhaps distant star systems? By the end of the nineteenth century, astronomers generally agreed that nebulae are either within or closely linked to our own stellar system, and that no galaxies beyond our own Galaxy had been sighted, even in the largest telescopes. But early in the twentieth century, astronomers managed to fashion novel ways to determine the distances to a class of nebulae known as spiral nebulae. With the aid of these distance indicators, the spiral nebulae were transformed into galaxies of stars. Modern extragalactic cosmology thereby came into being in the first few decades of the twentieth century.

The development of cosmological theories has been accompanied by philosophical debates inspired by the contrasts between cosmology and other areas of physics. This chapter reviews aspects of these debates from a historical perspective, beginning with debates about whether the uniqueness of the universe implies that cosmology needs a distinctive methodology. Underdetermination of theory by the evidence is particularly challenging due to horizons and inaccessible physics. Theories of the ‘origin’ of the universe do not have the same structure as other physical theories. Recent debates have focused on how to evaluate theories that predict a multiverse, in particular regarding the significance of fine-tuning and how to conduct anthropic reasoning.

Although relativistic astrophysics began in the 1930s with study of supernovae and neutron stars, it was only three decades later that the discovery of extragalactic radio sources, quasars and pulsars marked the emergence of special and general relativity as essential tools of the high energy astrophysicist. X-ray and γ -ray astronomy provided many new insights, culminating in the discovery of γ -ray bursts at cosmological distances in 1997. Supermassive black holes in active galactic nuclei provided major new challenges for theorists and observers alike, revealing many remarkable relativistic phenomena, such as superluminal motions observed in some of the most active galaxies. Einstein’s prediction of gravitational waves of 1916 was substantiated exactly 100 years later with their discovery in coalescing binary black hole systems by the LIGO project. These remarkable discoveries, mostly in the non-optical wavebands, brought a wide range of physicists into the astronomical and cosmological communities.

The origin and evolution of the universe constitutes one of the most fascinating and challenging questions in the scientific investigation of nature. The general theory of relativity has made it possible to properly address this question. Einstein transformed cosmology when he formulated, in 1917, a relativistic model that could describe the universe in its entirety. The incorporation of the observational evidence of extragalactic recession into relativistic world models culminated in 1930 with the recognition of the expanding universe, which was a breakthrough in the scientific understanding of the universe as a whole. This chapter traces the history of the early phase of modern cosmology, from the formulation of the first cosmological models based on general relativity to the acceptance of the expanding universe and the early systematization of relativistic cosmology as a new scientific discipline.

This chapter deals with the development of modern cosmology as a consequence of relativity theory testing and the space race during the Cold War. The chapter describes the dynamics of competition and collaboration among the two blocs with emphasis on the sectors of Soviet radio astronomy and space science. Developments of both fields are analysed in relationship with the military context and the technological development taking place in the USA. The chapter also takes into account the relationship between the two blocs and other Countries, such as France and Australia in order to show how the transfer of knowledge and know-how played a role in the extraordinary impulse that cosmology received during the Cold War.

The presently accepted big-bang model of the universe emerged during the period 1930-1970, following a road that was anything but smooth. By 1950 the essential features of the big-bang theory were established by George Gamow and his collaborators, and yet the theory failed to win recognition. A major reason was that the big-bang picture of the evolving universe was challenged by the radically different picture of a steady-state universe favoured by Fred Hoyle and others. By the late 1950s there was no convincing reason to adopt one theory over the other. Out of the epic controversy between the two incompatible world models arose our modern view of the universe. Although the classical steady-state model was abandoned in the mid-1960s, attempts to modify it can be followed up to the present.

The period (roughly) 1990-today is characterized by a big watershed and branching of cosmology into multiple and hitherto unexpected directions. On one side, the generic chaotic/eternal inflation has provided physical grounds for rather wild speculative ideas about the multiverse: the possibly infinite set of cosmological domains (‘universes’). In order to determine how observed features of our universe are (im)probable in the multiverse context requires application of anthropic reasoning which is still controversial in many circles. On the other side, we encounter applications of other speculative physical theories, like the string/M-theory to cosmology, resulting in unusual hypotheses like those of the pre-Big Bang cosmologies. In this period we have also witnessed the birth of physical eschatology as the true ‘cosmology of the future’. This chapter will attempt a survey of these and related developments, with necessary qualifications which accompany any ongoing, evolving research activity.