Physical cosmology up to the time of Einstein

Gravity is the one long-range force which acts upon all matter. Soon after Isaac Newton had completed the unification of the laws of gravity and celestial physics through his discovery of the inverse square law of gravity, he appreciated that the unique form of this law has important consequences for the large-scale distribution of matter in the Universe. In 1692–1693, the cosmological problem was addressed in a remarkable exchange of letters between Newton and the young clergyman Richard Bentley (1662–1742), later to become master of Trinity College, Cambridge. The correspondence concerned the stability of a Universe uniformly filled with stars under Newton's law of gravity. The attractive nature of the force of gravity meant that matter tends to fall together, and Newton was well aware of this problem. His first solution was to suppose that the distribution of stars extends to infinity in all directions so that the net gravitational attraction on any star in the uniform distribution is zero. As he wrote,

The fixt Stars, everywhere promiscuously dispers'd in the heavens, by their contrary attractions destroy their mutual actions.

Newton made star counts to test the hypothesis that the stars are uniformly distributed in space and found that the numbers increased more or less as expected with increasing apparent magnitude. The problem, which was fully understood by Newton and Bentley, was that a uniform distribution of stars is dynamically unstable.

The red giant problem

While the understanding of main-sequence stars proceeded apace through the 1920s and 1930s, there remained the problem of accounting for the red giant stars, which are very much more luminous than main-sequence stars at the same effective temperatures. Russell adopted the position that matter existed in different states in the dwarf and giant stars, what he termed ‘giant stuff’ and ‘dwarf stuff’. Atkinson assumed that different nuclear processes were responsible for the luminosities of the giant stars.

The stellar models of Eddington are homogeneous, and it was assumed that homogeneity was maintained, probably by large-scale meridional circulation driven by the internal rotation of the star. It was only in the early 1950s, that a number of astrophysicists, Peter Sweet (1921–2005), Martin Schwarzschild, Ernst Öpik and Leon Mestel, showed that the mixing assumption was highly implausible.

The solution to the red giant problem was discovered in 1938 by the Estonian astrophysicist Ernst Öpik (1893–1985), then working at the University of Tartu (Öpik, 1938). Öpik realised that if the stars are not well mixed, it is inevitable that they become inhomogeneous. Within the central core of the star, nuclear burning of hydrogen into helium leads to the depletion of the nuclear fuel in the core. In Öpik's model it was assumed that the central core of the star was maintained in convective equilibrium, resulting in a uniform depletion of hydrogen in this region.

The galaxies

The Hubble sequence of galaxy types shown in Figure 5.5 gives some impression of the diversity of forms found among the galaxies. Hubble planned to publish an atlas of galaxies illustrating the different galaxy types but, although all the plates for this project were taken with the 60-inch and 100-inch telescopes by 1948, he died in 1953, before what became the Hubble Atlas of Galaxies was published. The project was completed by Allan Sandage, who was Hubble's last research assistant, and it was published in 1961 (Sandage, 1961b). The basic Hubble sequence was preserved, including the S0 galaxies, and the irregular galaxies were placed at the end of the sequence.

The morphological classification of large samples of galaxies was pursued by Antoinette (1921–1987) and Gérard de Vaucouleurs (1918–1995), who published a series of Reference Catalogues of Bright Galaxies, in which the Hubble classification was refined, the basic linear sequence being preserved (de Vaucouleurs et al., 1991). The distinction between the normal and barred spirals was maintained, but they showed that all intermediate types between pure barred spirals and normal spirals are also observed. What gave this morphological scheme physical significance was the fact that certain physical properties of galaxies are correlated with their position along the sequence.

The second part of our history concerns the understanding of the large-scale distribution of matter in the Universe. At the beginning of the period 1900 to 1939, little was known even about the structure of our own Galaxy; by the end of it, the Universe of galaxies was established, the system was known to be expanding and general relativity provided a theory capable of describing the distribution of matter in the Universe on the very largest scales.

‘Island universes’ and the cataloguing of the nebulae

The earliest cosmologies of the modern era were speculative conjectures. The ‘island universe’ model of René Descartes (1596–1650), published in The World of 1636, involved an interlocking jigsaw puzzle of solar systems. In 1750, Thomas Wright of Durham (1711–1786) published An Original Theory or New Hypothesis of the Universe, in which the Sun was one of many stars which orbit about the ‘Divine Centre’ of the star system. Immanuel Kant (1724–1804) in 1755 and Johann Lambert (1728–1777) in 1761 took these ideas further and developed the first hierarchical, or fractal, models of the Universe. Kant made the prescient suggestion that the flattening of these ‘island universes’ was due to their rotation. The problem with these early cosmologies was that they lacked observational validation, in particular because of the lack of information on the distances of astronomical objects.

Towards the end of the eighteenth century, William Herschel (1738–1822) was one of the first astronomers to attempt to define the distribution of stars in the Universe in some detail on the basis of careful observation. To determine the structure of the Milky Way, he counted the number of stars in different directions, assuming they all have the same intrinsic luminosities. In this way, he derived his famous picture for the structure of our Galaxy, consisting of a flattened disc of stars with diameter about five times its thickness, the Sun being located close to its centre (Figure 5.1) (Herschel, 1785).

How this book came about

The origin of this book was a request by Brian Pippard to contribute a survey of astrophysics and cosmology in the twentieth century to the three-volume work that he edited with Laurie Brown and the late Abraham Pais, Twentieth Century Physics (Bristol: Institute of Physics Publishing and New York: American Institute of Physics Press, 1995). This turned out to be a considerable undertaking, my first draft far exceeding the required page limit. By drastic editing, I reduced the text to about half its original length and the survey appeared in that form as Chapter 23 of the third volume.

I was reluctant to abandon all the important material which had to be excised from the published survey and was delighted that the Institute of Physics agreed to my approaching Cambridge University Press about publishing the full version. The Press were keen to take on the project, with some further expansion of the text and, in particular, with a number of explanatory supplements to chapters where a little simple mathematics can make the arguments more convincing for the enthusiast. I have also made liberal use of references to my other books, where I have already given treatments of topics covered in this book. The result has been a complete rethink of the whole project and an expansion of the text by a factor of five as compared with the original published version.

Sandage and the values of H0 and q0

In 1952, Walter Baade announced that the value of Hubble's constant, H0, had been overestimated because the distance to the Andromeda Nebula, M31, adopted by Hubble was about a factor of 2 too small (Baade, 1952). The cause of the discrepancy was that there is a difference in the period–luminosity relations for Cepheid variables of Populations I and II (see Section 12.2). By using the same type of Cepheid variable in our own Galaxy, in the Magellanic Clouds and in M31, the distance to M31 increased by a factor of 2. Consequently, Hubble's constant was reduced to 250 km s−1 Mpc−1 and H−10 increased to 4 × 109 years.

In 1956, Humason, Mayall and Sandage showed that the expected redshift–magnitude relation, m = 5 log10z + constant, is observed for galaxies selected at random, but there is a large scatter about the mean relation because of the breadth of the luminosity function of galaxies (Humason et al., 1956). It had been known since Hubble's pioneering studies of the 1930s, however, that the brightest galaxies in clusters of galaxies follow a very much tighter relation which follows precisely Hubble's law υ = H0r (Figure 13.1). Thus, in order to estimate the value of H0, it was only necessary to calibrate the observed relation by measuring the distance of the nearest rich cluster of galaxies, the Virgo cluster of galaxies, by techniques independent of its redshift.

Introduction

Until 1945, astronomy meant optical astronomy. The commissioning of the Palomar 200-inch telescope in 1949 highlighted the dominance of the USA in observational astrophysics in the period immediately after the Second World War. The need for greater light-gathering power to detect faint galaxies for cosmological studies led to George Ellery Hale's concept of the 200-inch telescope (Hale, 1928). Hale symbolised the entrepreneurial approach of US astronomers to the sponsorship of private US observatories, such as the Lick, Harvard, Yerkes and Mount Wilson Observatories, which began in the late nineteenth century. James Lick (1796–1876), for example, was a successful maker and seller of pianos and an enthusiast for astronomy who, on his death in 1876, left a bequest of $700 000 to build ‘a powerful telescope, superior to and more powerful than any telescope ever yet made … and also a suitable observatory connected therewith’. The observatory was constructed on Mount Hamilton and officially opened in 1888 with the completion of the 36-inch telescope, under which James Lick was buried, according to the terms of his bequest.

Hale's record of observatory and telescope construction is remarkable by any measure. He persuaded Charles T. Yerkes (1837–1905), the entrepreneur who built and electrified the Chicago street-train system and who was regularly on the verge of legal embarrassment, to provide the funds to build and equip the Yerkes Observatory as part of the University of Chicago.

Introduction

The great revolutions in physics of the early years of the twentieth century have their exact counterparts in the birth of astrophysics and astrophysical cosmology – these astronomical disciplines scarcely existed before 1900.

The history of the interaction between astronomy and fundamental physics is long and distinguished. From the birth of modern science, astronomy has provided scientific information on scales and under physical conditions which cannot be obtained in laboratory or terrestrial experiments. There is no better example than the history of the discovery of Newton's law of gravity, which provides a model for the process by which astronomical discovery is absorbed into the infrastructure of physics. The technological and managerial genius of the great Danish astronomer Tycho Brahe (1546–1601) and his magnificent achievements in positional astronomy during the period 1575 to 1595 provided the data which led to the discovery of the three laws of planetary motion of Johannes Kepler (1571– 1630) during the first two decades of the seventeenth century. The technical skill of Galileo Galilei (1564–1642) in telescope construction resulted in his discovery in 1610 of the satellites of Jupiter, which were recognised as a scale-model for the Copernican System of the World. Finally, in an extraordinary burst of scientific creativity, Isaac Newton (1643–1727) used Kepler's laws to discover the inverse square law of gravity and synthesised the laws of mechanics and dynamics into his three laws of motion.

The photoionisation of the interstellar gas

By 1939, the existence of various forms of interstellar matter had been established. From the study of interstellar absorption lines and the variation of interstellar extinction with distance, it was known that diffuse gas and dust are present in the interstellar medium (Plaskett and Pearce, 1933; Joy, 1939). Gaseous nebulae had been known to be constituents of the Galaxy since the time of Huggins’ pioneering observations in the 1860s. During the first two decades of the twentieth century, Edward Barnard (1857–1923) made extensive studies of the forms of the dark clouds apparent in photographs of the MilkyWay (Barnard, 1919). The nature of these clouds was studied by MaxWolf (1863–1932), who determined the amount of extinction they cause by making star counts in their vicinity (Wolf, 1923). He correctly attributed the extinction to dust grains rather than gas because in the latter case the strong dependence of Rayleigh scattering upon wavelength would have resulted in much greater reddening of background stars than was observed.

On the theoretical side, it was recognised in the early 1920s that both the central stars of planetary nebulae and the O stars are very hot and so radiate a great deal of energy in the ultraviolet waveband. Russell suggested that the excitation of the emission lines seen in gaseous nebulae and planetary nebulae were due to photoexcitation (Russell, 1921), and Eddington showed that, as a result, the gas would attain a temperature of about 10 000K (Eddington, 1926b).

This chapter concerns the development of astrophysical cosmology from 1945 to the early 1970s, by which time the success of the standard Big Bang models convinced the community at large that these provided the most satisfactory framework for the investigation of cosmological models. Then, in Chapter 13, we describe the endeavours to determine the values of the cosmological parameters and the problems which faced the observational cosmologists. It turned out that many of these endeavours encountered the problems of the evolution of the properties of the objects studied with cosmological epoch, and this is the subject of Chapter 14. In Chapter 15, we trace the development of ideas about the formation and evolution of galaxies and the large-scale structure of the Universe. These studies have provided many of the tools necessary to ask physical questions about the very early stages of the Universe, which is the subject of Chapter 16.

Many of the issues covered in this chapter on astrophysical cosmology up to the early 1970s are described in the book Cosmology and Controversy by Helge Kragh.

Gamow and the Big Bang

During the 1930s, there were two reasons why the synthesis of the chemical elements in the early stages of evolutionary world models was taken seriously. Firstly, the studies of Cecilia Payne and Henry Norris Russell had shown that the abundances of the elements in stars were remarkably uniform, suggesting a common origin for the elements (see Section 3.3).