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).

The big problems

The history recounted in the preceding four chapters represents quite extraordinary progress in understanding the astrophysical origins and evolution of our Universe. The contrast between the apparently insuperable problems of determining precise values of cosmological parameters up till the 1990s and the era of precision cosmology of the early years of the twenty-first century is startling.

Yet, despite the undoubted success of the concordance model, it raises as many problems as it solves. The picture is incomplete in the sense that, within the context of the standard world models, the initial conditions listed in Tables 15.2 and 15.3 have to be put in by hand in order to create the Universe as we observe it today. How did these initial conditions arise? As the quality of the observations improved, a number of fundamental issues for astrophysical cosmology became apparent. The resolution of these problems will undoubtedly provide insight into the laws of physics under physical conditions which at the moment can only be studied by cosmological observations.

The horizon problem

This problem, clearly recognised by Robert Dicke in 1961, can be restated, ‘Why is the Universe so isotropic?’ (Dicke, 1961). At earlier cosmological epochs, the particle horizon r ~ ct encompassed less and less mass and so the scale over which particles could be causally connected became smaller and smaller.

Overview of exoplanet properties and theory

In the past ten years, 170 exoplanets have been discovered orbiting 130 normal stars by using the Doppler technique to monitor the gravitational wobble induced by a planet, as summarized by Marcy et al. (2004) and Mayor et al. (2004). Multipleplanet systems have been detected around 17 of the 130 planet-bearing stars, found by superimposed multiple Doppler periodicities (Mayor et al., 2004; Vogt et al., 2005). Another five exoplanets have been found photometrically by the dimming of the star as the planet transits across the visible hemisphere of the star (Bouchy et al., 2005; Torres et al., 2005).

The Doppler surveys for planets have revealed several properties:

• Planet mass distribution: dN/dM α M−1.5 (Fig. 11.1).

• Planet occurrence increases with semimajor axis (Fig. 11.2).

• Hot Jupiters (a < 0.1 AU) exist around 0.8% of FGK stars.

• Eccentric orbits are common (Fig. 11.3).

• Planet occurrence rises rapidly with stellar metallicity (Fig. 11.4).

• Multiple planets are common, often in resonant orbits.

The transiting planets permit measurement of planet radius and mass, demonstrating that they are “gas giants”, as expected (Henry et al., 2000; Charbonneau et al., 2000). The properties of the 170 known exoplanets motivate theories of their formation and their dynamical interactions with both the protoplanetary disk and other planets (Bryden et al., 2000; Laughlin and Chambers, 2001; Rivera and Lissauer, 2001; Chiang and Murray, 2002; Lee and Peale, 2002; Ford et al., 2003; Ida and Lin, 2004).

With the words “Twas the night before Christmas…” does a good old story start. In December 2004, just a couple of days before Christmas, not three wise men but more than 60 wise men and women came to a castle in the Bavarian mountains. They traveled through a strong snow storm, but no-one turned back; all of them arrived. They had a noble goal in mind: to discuss the current understanding of the formation of planets. The meeting took place December 19–22, 2004 at the Ringberg castle of the Max-Planck-Society. Anyone who has had the chance to attend a meeting there knows what a friendly and stimulating atmosphere for a workshop it provides.

About a year beforehand we had called them, and now they came. The idea was to have a wonderful conference at the romantic Ringberg castle and to bring together theorists and observers, as well as meteoriticists and experimental astrophysicists. Only then, we thought, could we obtain a global picture of the ideas we have about how our planetary system came into life. We wanted to collect not only the accepted ideas, but also the speculative and competing ideas.

We were quickly convinced that this conference, unique in its composition, should generate a permanent record in the form of a proceedings book. But this book should not be just one more useless compendium of unrefereed papers, but should provide a concise and accurate picture of current planet formation theory, experiment, and observation.

Introduction

Directly after the discovery of the very first extrasolar planets around main-sequence stars it has become obvious that the new planetary systems differ substantially from our own Solar System. Amongst other properties one distinguishing feature is the close proximity of several planets to their host stars (hot Jupiters). As it is difficult to imagine scenarios to form planets so close to their parent star it is generally assumed that massive, Jupiter-like planets form further away, and then migrate inwards towards the star due to disk–planet tidal interactions. Hence, the mere existence of hot Jupiters can be taken as clear evidence of the occurence of migration. Interestingly, theoretically the possibility of migrating planets has long been predicted from the early 1980s.

Another observational indication that some migration of planets must have occured is the existence of planets in mean motion resonances. Due to converging differential migration of two planets both embedded in a protoplanetary disk they can be captured in a low-order mean motion resonance. The most prominent example is the system GJ 876 where the planets have orbital periods of roughly 30 and 60 days.

In this review we focus on the theoretical aspects of the disk–planet interaction which leads to a change in the orbital elements of the planet most notably its semimajor axis. We only treat systems with a single planet and do not consider planetery systems containing multiple planets.

Introduction

The count of extrasolar giant planets detected by radial velocity measurements is now well over a hundred, accounting for about 5% of F, G and K main-sequence stars in the Solar neighborhood; about 10% of the planets are in multiple systems. It thus seems an inescapable conclusion that giant planet formation is a ubiquitous and robust process. There is also strong observational evidence for a correlation between the occurrence rate of (detectable) planets and the metallicity of the parent star (Gonzalez, 1997; Fischer and Valenti, 2003). There are two possible explanations for this phenomenon: first, the planet formation process may tend to “pollute” the parent star with higher-metallicity material, as giant planets (Laughlin and Adams, 1997) or planetesimals (Murray et al., 2001) migrate in and are engulfed. If this is the case, higher-mass stars, which have thinner convective envelopes in which to preserve the pollution, ought to display a systematically higher metallicity. However, no such trend has been observed so far (Wilden et al., 2002; Dotter and Chaboyer, 2002; Quillen, 2002; Fischer and Valenti, 2003). Furthermore, Fischer and Valenti (2005) found no sign of various other potential accretion signatures, such as dilution of metallicity in subgiants with planets. The other explanation is that higher metallicity – and thus a higher fraction of solids in the protoplanetary disk – increases the chances of forming a giant planet.

Introduction: planet detection and studies in the historical context

Beyond Earth, only five planets have been known since historical times. These are the three “terrestrial planets,” Mercury, Venus and Mars, and the two “gas giants,” Jupiter and Saturn. The systematic studies of the skies starting in the seventeenth century marked the beginning of a new era. The ice giant Uranus, the next outer planet beyond Saturn, was first cataloged as 34 Tau by John Flamsteed in 1690, but not recognized as a “wanderer” in the skies. In 1781 William Herschel was the first to spatially resolve the disk of Uranus, initially classifying it as a comet. Uranus also played an important role in the discovery of Neptune. Neptune, the eighth planet in the Solar System, and the outermost of the ice giants, was first observed in 1612/1613 by Galileo Galilei. At that time, Neptune was in close conjunction with Jupiter. Galileo recognized the moving source in the vicinity of Jupiter (see Fig. 16.1), but decided not to get distracted from his studies of the orbital motions of Jupiter's four largest moons, which we now refer to as “Galilean Moons.” Another 230 years passed till precise measurements of Uranus' orbit indicated the presence of an additional outer planet. Based on predictions by Urbain Leverrier (which were in close agreement with independent calculations by John Couch Adams) in 1846, Johann Gottfried Galle finally identified and resolved Neptune.