The DN, already introduced in Sections 1.2 and 1.3 with their classification scheme described in Section 2.1, are arguably the most valuable of objects for the study of accretion discs. Among them examples may be found of optically thin discs and optically thick discs, of face-on discs and edge-on discs, of non-steady discs and of nearly steady state discs and of transitions between them. Furthermore, the brightest DN at maxima reach apparent magnitudes of 8–10, at which time the entire flux is conveniently of almost pure accretion origin.

Well-Observed DN

It is inevitable that a few relatively bright DN, especially the eclipsing systems, have been preferentially observed. Although over 200 DN have been classified by their light curves, only a small fraction have been studied sufficiently to establish their orbital periods. It will be seen in this chapter that POrb plays an important rôle in the systematics of DN. Among the DN in general, 12 Z Cam stars, 29 definite U Gem stars (including, slightly unconventionally, the three systems BV Cen, GK Per and V1017 Sgr with large POrb), 34 SU UMa stars and 22 objects suspected of belonging in the DN class have known orbital periods. The SU UMa stars may be overrepresented because their orbital periods are easy to estimate, independent of inclination, from photometric observations made during super outbursts. Orbital periods for the U Gem and Z Cam class have come predominantly from spectroscopic observations, with the addition of a few found from photometric orbital variations (eclipses, bright spot modulation, IR ellipsoidal modulation).

The many measurements of the spectrum of the CBR discussed in the preceding chapter are consistent with a Planck spectrum with T0 = 2.73 ± 0.02 K over a wavelength range 0.1 cm ≲ λ≲75 cm. Only the submillimeter observations reviewed in Section 4.8.4 provided any evidence for a significant deviation from a thermal spectrum, and these appear to be erroneous. What conclusions may we draw from the essentially thermal spectrum of the CBR? This chapter provides some answers to that question. It is presented as an introduction to, not an exhaustive treatment of, the processes that determine the CBR spectrum. There are a number of reviews, which treat these topics in more detail, such as Danese and De Zotti (1977), Sunyaev and Zel'dovich (1980) and Bond (1988).

We begin by noting the conditions under which we would expect an exactly thermal spectrum to have been produced early in the Hot Big Bang, then consider a number of physical processes that could have distorted an initially thermal spectrum. We also consider the possibility that one or more additional ‘cosmic’ backgrounds may be present, adding to the CBR at wavelengths below about 1 mm. Finally, we investigate the constraints that the spectral measurements of Chapter 4 place on these processes.

A characteristic feature of the CBR, noted at the time of its discovery by Penzias and Wilson (1965), is its approximate isotropy (see Appendix A). Approximately equal intensity in all directions is expected if the radiation is a relic of the Hot Big Bang. On the other hand, there are a variety of mechanisms that can induce small amplitude variations in intensity, or anisotropies, into an initially uniform CBR; some of these were outlined in Chapter 2 and will be discussed in detail in Chapter 8.

Careful measurements of the angular distribution of the CBR have therefore been pursued both to confirm the cosmic, Hot Big Bang, origin of the CBR and to search for small amplitude anisotropies imprinted in it. In this chapter we deal with observations of the angular distribution of the CBR on the largest angular scales, θ ≳ 10°, and in particular with the dipole and quadrupole moments of the CBR. The value of about 10° for the boundary between ‘large’ scale anisotropies (discussed in this chapter) and smaller scale anisotropies (Chapter 7) is obviously rather artificial. When we turn in Chapter 8 to the implications of the measurements of and upper limits on CBR anisotropies, we will be drawing on the results of both Chapters 6 and 7.

One of the basic problems of cosmology is the singularity characteristic of the familiar cosmological solutions of Einstein's field equations. Also puzzling is the presence of matter in excess over antimatter in the universe, for baryons and leptons are thought to be conserved. Thus, in the framework of conventional theory we cannot understand the origin of matter or of the universe. We can distinguish three main attempts to deal with these problems.

1. The assumption of continuous creation (Bondi and Gold 1948; Hoyle 1948), which avoids the singularity by postulating a universe expanding for all time and a continuous but slow creation of new matter in the universe.

2. The assumption (Wheeler 1964) that the creation of new matter is intimately related to the existence of the singularity, and that the resolution of both paradoxes may be found in a proper quantum mechanical treatment of Einstein's field equations.

3. The assumption that the singularity results from a mathematical over-idealization, the requirement of strict isotropy or uniformity, and that it would not occur in the real world (Wheeler 1958; Lifshitz and Khalatnikov 1963).

If this third premise is accepted tentatively as a working hypothesis, it carries with it a possible resolution of the second paradox, for the matter we see about us now may represent the same baryon content of the previous expansion of a closed universe, oscillating for all time.

The science that treats the properties and evolution of the Universe as a whole is cosmology. Among the sciences, it is unique in having only a single object of study – there are no other Universes for us to use as controls, nor can we readily run the whole experiment over again. As a consequence, much of the effort in modern cosmology has been to determine the best mathematical description, or ‘model’, of the Universe we inhabit. As we shall see, that task is not yet complete, despite the rapid advances of the past few decades. The range of possible models is presented later in this chapter. First, though, we need to look at the observational bases of modern cosmology, a set of astronomical observations which have established the Hot Big Bang theory and restricted the range of models we need to consider.

Astronomical constituents of the Universe

Since cosmology is the study of the Universe as a whole and as a single system, it is only indirectly concerned with subsystems within the Universe. Here, I will mention only two: galaxies and clusters of galaxies. The galaxies are assemblies of 108–1012 stars; many galaxies also contain appreciable amounts of interstellar gas and dust.

The very first astronomical signal at radio wavelengths, by happy coincidence, was also detected at the Bell Telephone Laboratories; in 1932, Karl Jansky detected at 15 m wavelength radio emission, which he correctly identified as coming from the Galactic plane. Astronomers paid little attention. Observational radio astronomy did not really come into its own until after World War II (see, e.g., Hey, 1973, and Sullivan, 1984). It is now recognized as a powerful adjunct to optical astronomy, particularly in the study of low density cosmic matter and of energetic objects and phenomena such as quasars and the collimated jets seen in radio galaxies. We will look very briefly at radio sources later in this chapter, but most of it will be devoted to the tools and techniques of observational radio astronomy. Chapter 3 is designed to introduce the more specialized radio astronomical techniques used in studying the CBR; it is not intended to be a complete introduction to radio astronomy. For further details, readers may want to consult one or more of the following texts: Kraus (1986); Rohlfs (1986); and Christiansen and Högbom (1985). Interferometry is very fully treated by Thompson, Moran and Swenson (1986), and radio sources by Pacholczyk (1970) and Verschuur and Kellermann (1988), among others. The treatment of radio astronomy in this book is closest to the work of Rohlfs.

Humankind has made stories about the origin of the world since prehistoric times. These creation stories often have a grand beauty and are sometimes richly detailed. It is only in the present century that such myths and images have been supplanted by a well-established scientific description of the origin of the world. ‘World’ is now understood to mean the Universe as a whole, not just the Earth or the solar system, and the modern picture of its origin and evolution is the Hot Big Bang model. This book describes one crucial piece of astronomical evidence supporting the Big Bang model, namely the cosmic microwave background radiation, heat radiation left over from a hot and dense phase early in the history of the Universe.

The cosmic background radiation (CBR) was discovered, by accident as it happens, a quarter of a century ago. Within a few years, the basic properties of the radiation had been established. Those properties, especially the thermal 3 K spectrum and the very uniform distribution of the CBR across the sky, have convinced virtually all astrophysicists that the radiation is a relic of the Hot Big Bang, and that it comes to us from a very early time in the history of the Universe. It thus provides information about the early history of the Universe obtainable in no other way.

The publishers have kindly agreed to allow me to include a brief appendix in order to update some of the observational results contained in Chapters 4, 6 and 7. As noted in the preface, references in the main body of the text are complete to early 1992; here I provide summaries of some of the results reported in 1992, 1993 and early 1994. In the space of a few pages, I cannot hope to cover all the work in this field, in which hundreds of papers were published in that time interval. Instead, I will emphasize the most crucial observational results, and provide a few references to theoretical papers which establish new results or provide good reviews of the field.

Spectrum

The COBE team (Mather et al., Fixsen et al., and Wright et al., all 1994) have reported new and more complete results on the short-wavelength spectrum of the CBR. From a direct measurement of the spectrum using the FIRAS instrument (Section 4.9), Mather et al. (1994) determine T0 = 2.726±0.010 K. Their observations allow them to place limits on distortions of the spectrum; in the notation introduced in Chapter 5, these limits are |y| <2.5×10-5 and |µ| <3.3×10-4. These values, like the error in T0, are given at the 95% confidence level.

Soon after the discovery of the CBR it was recognized that measurements of, or upper limits on, its anisotropy on scales of degrees or less would provide unique information about the origin and development of structure within the Universe. Such observations are of particular value because they probe cosmic times well before the appearance of any luminous objects in the Universe such as galaxies or stars. The surface of last scattering from which the CBR photons reach us is more distant than any QSO or galaxy yet detected (these lie at z < 5), and the CBR thus carries information about the state of the Universe at earlier times. It is quite likely, in fact, that the CBR photons we study reach us from the first few hundred thousand years of the history of the Universe; and, as we shall see in Chapter 8, the CBR may encode information from the epoch of inflation nearly 50 orders of magnitude earlier still.

With so much to learn, astronomers have worked hard to detect fluctuations in the temperature of the CBR. Indeed, there have been as many searches for structure in the background on scales ≤ 10° as measurements of the dipole and of the spectrum combined.

It is fair to claim that the cosmic background, announced in 1965, was discovered by telephone. As we will see, this radiation had been both observed and predicted before a fateful 1964 telephone call from Arno Penzias and Robert Wilson of the Bell Telephone Laboratories to Robert Dicke at Princeton, a few miles away. Nevertheless, that telephone conversation led to the recognition that the source of the puzzling ‘excess noise’ observed by Penzias and Wilson was in fact a relic of the Hot Big Bang.

In this chapter, I will start with a brief review of the state of cosmology in the early 1960s, look at earlier predictions of the radiation, particularly the prescient work of Gamow and his colleagues, and then attempt to unravel the complicated knot of circumstances that led to the telephone call of late 1964. Later in the chapter, I will describe the important early experiments which established the cosmological nature of the radiation first detected by Penzias and Wilson, and then sketch the subsequent history of the field.

Cosmology in the early 1960s

Thirty years ago, cosmology played a far less central role in physics and astronomy than it does today. For instance, cosmology is nowhere mentioned in the text I was assigned in my first college astronomy course more than thirty years ago; the word ‘Universe’ appears only twice, where it is used, inappropriately, to refer to the Milky Way.

Measurements of the effective zenith noise temperature of the 20 foot horn-reflector antenna (Crawford, Hogg, and Hunt 1961) at the Crawford Hill Laboratory, Holmdel, New Jersey, at 4080 Mc/s have yielded a value about 3.5 K higher than expected. This excess temperature is, within the limits of our observations, isotropic, unpolarized, and free from seasonal variations (July, 1964–April, 1965). A possible explanation for the observed excess noise temperature is the one given by Dicke, Peebles, Roll, and Wilkinson (1965) in a companion letter in this issue.

The total antenna temperature measured at the zenith is 6.7 K of which 2.3 K is due to atmospheric absorption. The calculated contribution due to Ohmic losses in the antenna and back-lobe response is 0.9 K.

The radiometer used in this investigation has been described elsewhere (Penzias and Wilson 1965). It employs a traveling-wave maser, a low-loss (0.027 db) comparison switch, and a liquid helium-cooled reference termination (Penzias 1965). Measurements were made by switching manually between the antenna input and the reference termination. The antenna, reference termination, and radiometer were well matched so that a round-trip return loss of more than 55 db existed throughout the measurement; thus errors in the measurement of the effective temperature due to impedance mismatch can be neglected.

In the standard Big Bang model (introduced in Chapter 1), the Universe is assumed to be exactly homogeneous and isotropic, and only gravitational forces between particles are considered. Under these conditions, all particles in the Universe are at rest in comoving coordinates and the CBR is completly isotropic.

The actual Universe we observe clearly differs from this idealized model in a number of obvious ways; for instance, the Universe is visibly inhomogeneous on all length scales up to many megaparsecs. Any departure from the idealized conditions assumed in the Big Bang model can introduce anisotropy into the CBR.

Sources of anisotropy in the CBR

In this section, we list briefly some cosmological processes which can introduce anisotropy into the CBR on various angular scales, starting with the largest. Suppose, for instance, that we allow large-scale magnetic fields, and not just gravity, into our cosmological model. Then the CBR will become anisotropic (Thorne, 1967). Likewise, long wavelength gravitational waves added to an otherwise homogeneous Universe will induce a quadrupole moment (Dautcourt, 1969; Burke, 1975).

A more fundamental source of anisotropy in the CBR is the anisotropic expansion of the Universe as a whole (see, e.g., Barrow et al., 1983). The cosmological equations derived from General Relativity permit a wide variety of anisotropic solutions as well as the isotropic Robertson–Walker models discussed in Chapter 1.

If the microwave background radiation discovered by Penzias and Wilson is a relic of the Hot Big Bang origin of the Universe, it ought to have a thermal spectrum. The first few measurements of the spectrum of the radiation were consistent with a Planck curve with T0 = 3 K, hence strengthening our belief that the microwave background was indeed cosmic in origin. In the more than 25 years that have followed the early measurements of Penzias and Wilson (1965, 1967), Roll and Wilkinson (1966), Howell and Shakeshaft (1966), Field and Hitchcock (1966) and Thaddeus and Clauser (1966), a number of increasingly precise measurements of the spectrum have been made. The range of wave-lengths has been extended to 75 cm ≳ λ ≳0.5 mm. An early aim of these observational programs was to check the Big Bang model for the origin of the microwave radiation; another was to look for small perturbations in the spectrum of the radiation, which might have been produced by energetic processes occurring well after the Big Bang. These processes, the nature of the spectral perturbations they produce, and the limits the observations place on them, will all be discussed in Chapter 5. In this chapter, we will look at the observational techniques and the resulting values of T0 found at different wavelengths.

This element was discovered in 1803 by W. Wollaston in London. The name alludes to the asteroid Pallas discovered in 1802 by Olbers. Pallas Athene is the name of the goddess of wisdom in Greek mythology.

lonization energy

PdI 8.3 eV, PdII 19.4 eV, PdIII 32.9 eV.

Absorption lines of PdI

The equivalent width of PdI 3405(2) in the sun is 0.036.

Behavior in non-normal stars

Pd was first discovered by Bidelman (1966) in one Ap star. PdII was found by Adelman (1974) in one Bp star of the Si-4200 subgroup and by Adelman et al. (1979) in another Ap star of the Cr-Eu subgroup.

Isotopes

Pd occurs in the form of six stable isotopes, namely Pd 102 104, 105, 106, 108 and 110, which occur in the solar system with frequencies 1%, 11%, 22%, 27%, 27% and 12% respectively. There exist also 15 short-lived isotopes and isomers. The longest lived is Pd107 with a half life of 7 × 106 years, which can be used for radioactive dating.

Origin

Pd 105, 106 and 108 are produced by both the r process and the s process. Pd110 is a pure r process product, Pd104 a pure s process product and Pd102 are pure p process product.

This element was discovered in 1669 by H. Brandt in Hamburg, Germany. The name phosphorus in Greek means bringer of light, which was also the name given to the planet Venus.

lonization energies

PI 10.5 eV, PII 19.7 eV, PIII 30.2 eV.

Absorption lines of PI

The equivalent width of PI 9796(2) in the sun is 0.012. According to Underhill (1977) it is present in an A 2Ia star.