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This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. Whereas philosophers and astronomers have always been interested in the universe at large, cosmology as a physical science is of relatively recent origin. Two roots of so-called modern cosmology can be identified, one observational and the other theoretical. On the observational side, spectroscopic study of the distant nebulae in the late 19th century was one of the roots. The other root, purely theoretical in nature, was Einstein’s cosmological model of 1917 based on general relativity. For a long time, the two traditions developed independently, but with the recognition of the expanding universe at about 1930 they merged into one. Physical cosmology in more or less the modern sense of the term relied on quantum mechanics and advances in nuclear physics, which provided the basis for the first theories of the very early universe established in the late 1940s. During the following decade the new theory of the hot big bang met stiff competition from the rival steady state theory. However in 1965, and largely as a result of the discovery of the cosmic microwave background, the latter theory was abandoned or at least marginalized. Five years later, the standard hot big bang theory of the universe had obtained an almost paradigmatic status in the small but growing community of cosmologists. Modern cosmology had come into being. This was only a beginning and over the next decades the paradigm (if so it was) continued to be refined and on occasions even questioned.

The pioneering works of E. Rutherford related to radioactivity and nuclear science were contributions to physics, but they also had important implications for chemistry. Thus, in his early study of...

According to quantum mechanics, an oscillator at zero absolute temperature will continue to vibrate with a zero-point energy. This non-classical phenomenon was predicted by Max Planck in 1911, but for a long time it was contested and remained hypothetical. Only in experiments with molecular spectra from 1924 was the zero-point energy confirmed experimentally, and the following year it emerged as a consequence of quantum mechanics. Although Walther Nernst claimed that also vacuum was filled with zero-point energy, specialists in quantum physics denied that this was the case. Nonetheless, with the discovery of the Casimir effect predicted in 1948 Nernst’s idea was vindicated. Later still, the zero-point energy of free space turned up in the vacuum energy or so-called dark energy associated with Einstein’s cosmological constant. The chapter offers a history of the concept of zero-point energy with a focus on the period from 1911 to about 1928.

Quantum mechanics, a theory developed during the first three decades of the twentieth century, constituted a revolutionary change in our understanding of nature. Since the late 1960s a growing numb...

This paper offers background and perspective on a little-known memoir by Ludvig Lorenz on light scattering by spheres, which was published in Danish in 1890. It is a companion to an English translation of the memoir appearing separately. Apart from introducing Lorenz and some of his contributions to optics and electrodynamics, the paper focuses on the emergence, content and reception of the 1890 memoir and its role in what is often called the Lorenz-Mie theory. In addition to the historical analysis, the paper illuminates aspects of modern Lorenz-Mie theory and its many applications, with an eye to Lorenz’s original work.

Twentieth-century cosmology and cosmogony only appeared sporadically in Karl Popper’s writings, and yet he was deeply interested in the subjects, which he commented on in lectures and articles. He followed the developments which during his lifetime resulted in a new picture of the universe, the hot big-bang theory, but remained sceptical with regard to the scientific status of cosmology. From the early 1950s to the present, Popper’s falsifiability criterion and related methodological rules have influenced physicists’ and astronomers’ discussion of cosmological theories, sometimes explicitly and at other times implicitly. Examples are the steady-state theory, inflation models, the multiverse hypothesis, and the controversy over cold dark matter. This chapter covers both aspects, Popper’s own views about cosmology, including his dislike of the big-bang model, as well as the continual impact of his ideas on cosmological thinking from about 1950 to 2020.

The Danish chemist Julius Thomsen is probably best known as one of the fathers of classical thermochemistry and perhaps also for his more theoretical work related to the periodic system of the elements. According to Thomsen’s basic assumption, the evolution of heat in a chemical reaction was a measure of the involved affinities. It followed that thermal measurements would yield information of, for example, equilibrium and dissolution processes, and also elucidate the structure of organic molecules. Thomsen’s work on chemical elements resulted in an innovative version of the periodic system and speculations about the constitution of atoms. Moreover, shortly after the discovery of argon he predicted the group of inert elements. The article offers a summary account of Thomsen’s life and influence, nationally as well as internationally, and refers to his interest in applying science for technical and social purposes. His most important work in this area was of an industrial method to produce soda from the mineral cryolite located in Greenland.

Maxwell’s celebrated electromagnetic theory of light dates from 1865. Two years later, without appealing to the ether as a carrier of light waves, the Danish physicist Ludvig Lorenz (1829–1891) independently published another electrical theory of light based on optical equations and the novel idea of retarded potentials. In spite of resting on a very different conceptual foundation, Lorenz’s theory led to almost the same results as Maxwell’s. But whereas Maxwell’s field theory heralded a revolution in physics, Lorenz’s alternative was largely forgotten and soon relegated to a footnote in the history of physics. In part based on archival material and other sources in Danish, this paper offers a detailed contextual account of Lorentz’s theory and its reception in the physics community. Moreover, it includes a brief introduction to other of Lorenz’s scientific contributions and discusses the reasons why his electrical theory of light failed to attract serious interest.

No other scientist may have had a greater impact on modern cosmology than the Belgian physicist, astronomer and priest Georges Lemaitre. In 1927 he predicted the expansion of the universe on the basis of the cosmological field equations; and four years later he proposed what he called the primeval-atom hypothesis, the first version of the later big bang universe. In all his work on cosmology the cosmological constant Λ played a significant role. A recognized expert in the theory of general relativity, Lemaitre also contributed significantly to the theoretical clarification of local and global singularity problems. Still, when he died in 1968, at a time when the standard big bang model celebrated its first victories, he was largely forgotten or recalled only as a somewhat shadowy figure of the past. This essay reviews in a historical context the scientific work of Lemaitre with particular attention to his seminal contributions in the decade between 1925 and 1934.

There is no Nobel Prize in astronomy, and perhaps for this reason, historians of science have rarely examined the astronomical sciences from the perspective of the Nobel Prize system. And yet, since the establishment of this system many astronomers and astrophysicists have been nominated for the physics prize and these nominations provide the historian with valuable sources for understanding the dynamical relationship between astronomy and physics during the twentieth century. Apart from giving a general account of the role of astronomy in a Nobel context, this paper investigates a select number of scientists who contributed to astronomy and its allied sciences and were nominated for a Nobel Prize. The focus is on the period from about 1950 to 1966, but earlier nominations of, for example, M. Saha, H. N. Russell, and A. Eddington are also considered.

The concept of constants of nature originated in the late-nineteenth century and has since then increasingly occupied the minds of physicists. But are the constants truly constant? Inspired by Paul Dirac’s suggestion that the gravitational constant varies slowly in time, the question was addressed not only by physicists but also by astronomers, geologists, and paleontologists. Pascual Jordan in Germany and Robert Dicke in the United States formulated theories of gravitation that went beyond general relativity by incorporating a varying gravitational constant. These theories had cosmological consequences and also implications for the earth sciences. During the period 1955–1975, theories of varying gravity played a significant role in the process that led to the plate-tectonics revolution. Although the theories turned out to be wrong, this chapter in the history of interdisciplinary science deserves attention. For one thing, it changed the landscape of both the cosmological and geological sciences. For another thing, the question of varying natural constants is still unsettled and the subject of scientific investigation. The article focuses on the period from about 1930–1975, but also includes some comments of a more general nature.