I started writing Modern Cosmology in 1969, just four years after the discovery of the 3 K cosmic microwave background. The significance of that remarkable discovery was rapidly appreciated by cosmologists, and it naturally dominated a large part of my book. Now, nearly a quarter of a century later, a new topic has come to dominate cosmology, namely, the dark matter problem. This problem, however, is not at all well understood. According to modern estimates some of the dark matter is in the form of ordinary particles — protons, neutrons and electrons — while some of it has a more exotic character. We do not know what form the ordinary dark matter takes, and we do not even know the identity of the exotic dark matter. Yet together they are a pervasive and indeed dominating constituent of the universe, in galaxies, groups and clusters of galaxies and intergalactic space. I therefore thought it desirable to update my book by writing a connected account of what has now become the single most important problem in astronomy and cosmology.

I must confess that I have a second reason for writing this book. In 1990 I proposed the idea that most of the widespread ionization of hydrogen observed in our Galaxy is produced by photons emitted by decaying dark matter neutrinos of non-zero rest-mass. The original motivation for this proposal was that the observed ionisation was puzzling astronomers because it seemed difficult to account for in terms of known sources of ionisation.

This theory, once proposed, rapidly took on a life of its own.

Introduction

The detection of dark matter in astronomy has a long history. In past years it was called “the astronomy of the invisible”. The story begins in 1844 when, by chance, two different dark matter problems were identified. In that year it was noted that the planet Uranus had moved away from its calculated position by as much as two minutes of arc. In the same year F. W. Bessell drew attention to the sinuous motion of the star Sirius, the brightest star in the sky.

The subsequent development of the Uranus problem led to one of the most famous stories in the history of astronomy. In 1845 J. C. Adams, who had just ceased to be an undergraduate at Cambridge University, succeeded in calculating fairly accurately the position of a hypothetical planet whose gravitational effect on Uranus might be responsible for its disturbed motion. He attempted unsuccessfully to interest the Astronomer Royal G. B. Airy in this prediction. Apparently Airy had attributed the discrepancy to a departure from Newton's law of gravity. Perhaps also he was unimpressed by the student's youth.

Independently of Adams, in 1846 the Frenchman Le Verrier calculated the position of the hypothetical planet with a precision of 1 degree. (For a much shortened version of the needed calculations see Lyttleton 1958). Le Verrier contacted a German astronomer, Galle, at the Berlin Observatory, who rapidly succeeded in observing a new planet (Neptune) within 1 degree of the predicted position. The discovery of this planet (no longer “dark”) is widely considered to be a triumph of nineteenth century science, and naturally became the subject of chauvinistic controversy.

Introduction

We saw in the last chapter that the Milky Way contains diffuse ionised gas (DIG) with a large scale height. We also saw that there is strong, but not decisive, evidence that conventional sources in the Galaxy are not adequate to account for the observed ionisation. What seem to be needed are sources which are smoothly distributed, so that the opacity of the neutral hydrogen can be overcome, and which possess a large enough scale height to account for the large scale height of the DIG. Dark matter neutrinos in the Galaxy would be expected to possess both these properties, as we discuss in detail in the next part of this book. If the radiative decay of these neutrinos is to be a serious candidate for the ionization source of the DIG in our Galaxy, we would expect to find the same ionisation problems in nearby galaxies whose structure is similar to ours. This is the subject of the present chapter.

There is one advantage and one disadvantage in studying the ionisation in other galaxies. The advantage is that by observing from a point outside the galaxies it is easier to discover the global properties of the ionisation. The disadvantage is that pulsars are not observable in other galaxies (except the Magellanic Clouds), so that we cannot use the pulsar dispersion measure to determine the distribution of the electron density, and have to rely on measurements of Hα and other emission lines. As we shall see, it has been possible by these means to observe the DIG in nearby galaxies and to discover that conventional ionisation sources in these galaxies again seem to be inadequate.

Introduction

In this chapter we study the implications of the neutrino decay theory for the reionisation of the universe and the consequent suppression of fluctuations in the microwave background. We saw on page 48 that we expect the early high temperature universe to have become neutral at a red shift ∼ 1000, when it had cooled down to a temperature ∼ 3000 K. On the other hand we know from considerations of the Gunn-Peterson effect that the intergalactic medium is highly ionised at redshifts between zero and 4.9. The questions then arise, at what red shift between 4.9 and 1000 did the reionisation occur, and by what process?

These questions, and the general thermal history of the universe, have been much discussed. They are obviously relevant to our understanding of the processes of galaxy formation. In addition it has long been realised that they play a crucial role in determining the present anisotropy ΔT/T of the microwave background on small angular scales. As has often been discussed (e.g. Efstathiou 1988 and references cited therein), if the postrecombination universe had been reionised so early that its optical depth for Thomson scattering exceeded unity, then the ΔT/T induced by fluctuations associated with galaxy formation after recombination at z ∼ 1000 would have been severely attenuated by z = 0. This is an important question because the present stringent observational limits on ΔT/T at small angular scales would impose severe constraints on several theories of galaxy formation in the absence of a scattering screen.

Introduction

The evidence which has been accumulated in this book relating to our neutrino decay hypothesis is strong but circumstantial. It is crucially important to test the validity of the hypothesis by attempting to make a direct detection of the postulated radiation. Fortunately the kinematics of the decay imply that the emitted photons are monochromatic, so that the radiation from a given source, if strong enough to be detected, would show up as an unidentified line broadened by the velocity dispersion of the neutrinos in the source. Had the emission possessed a continuous spectrum it would have been much more difficult to distinguish it convincingly from radiation of a conventional origin.

Since the line is predicted to have an energy Eγ ∼ 15 eV, the problem of detectability is tied up with the high opacity of the interstellar medium for radiation of this energy. This problem is a natural one since the opacity is mainly due to the photoionisation of neutral hydrogen, the very process which originally led to the postulate that the decay radiation lies in this energy region. It does mean that care must be taken to choose a suitable observing target.

For example, a number of attempts were made to detect decay photons from dark matter in the Virgo and Coma clusters under the stimulus of the earlier neutrino decay theories of Cowsik (1977) and de Rujula and Glashow (1980). These attempts were made by Shipman and Cowsik (1981), Henry and Feldman (1981) and Holberg and Barber (1985).

Introduction

We saw in chapter 6 that some nearby spiral galaxies contain diffuse ionised gas (DIG) reminiscent of the Reynolds layer in our Galaxy. This DIG has been studied in particular detail in NGC 891. It was found difficult to account for the DIG observed in that galaxy several kiloparsecs from its plane in terms of known sources of ionisation. The observers concerned therefore concluded that a new galactic source is required, a conclusion which is reminiscent of the situation prevailing for the Reynolds layer in our Galaxy. In this chapter we examine the hypothesis (Sciama and Salucci 1990) that the new source required is decaying dark matter neutrinos with the same properties as we have already invoked in discussing the Reynolds layer in the previous chapter.

This hypothesis has been criticised by Dettmar and Schulz (1992) on the grounds that the decay photons would not heat the gas to the temperature required to account for the emission line ratios [NII]/ Hα and [SII]/ Hα which they observed. This criticism suffers from the defect that in their calculation they assume that the only heat source for the gas is the decay photons themselves. Since in the decaying neutrino theory Eγ is close to 13.6 eV, it is true that there is not much heat input associated with each ionisation. Indeed this point is relevant to our discussion of the temperature of Lyman α clouds in chapter 11. However, in the present case one would expect that other heating processes should be important.

Introduction

The idea that there may be significant quantities of dark matter distributed smoothly throughout the universe as a whole developed gradually. In its modern formulation the idea is based on a number of considerations. The first concerns the role played by the mean density of the universe in the cosmological models of general relativity. These models have a fundamental status in discussions of cosmological dark matter, and so we devote much of this chapter to an account of them.

The second consideration concerns the mean density of ordinary matter in the universe. By “ordinary matter” I mean atoms, neutral or ionised, which are collectively referred to as baryonic. Estimates have been made of the contribution of visible baryons to the mean density of the universe using direct astronomical measurements. An estimate has also been made, using indirect arguments, of the total contribution of baryons, visible and invisible, to the mean density. This estimate is based on a comparison of the measured abundances of certain light elements (D, He3, He4 and Li7) with the calculated output of thermonuclear reactions occurring in the “first three minutes” after the hot big bang origin of the universe. These arguments will also be described in this chapter. We shall find that, according to modern estimates, the mean density in visible baryons is significantly less than the total mean density in baryons. If these estimates are correct, an appreciable number of baryons must be dark.

The third consideration concerns the contribution of nonbaryonic matter to the mean density of the universe. Various forms of more or less “exotic” matter have been proposed under this heading.

INTRODUCTION

My first impressions of Dennis Sciama came from a short introductory astrophysics course he gave to undergraduates in 1964. Then in 1966-7 I took his Cambridge Part III course in relativity, in which he charitably ignored my inadvertent use of Euclidean signature in the examination (an error I spotted just at the very end of the allowed time) and gave me a good mark. In both these courses he showed the qualities of enthusiasm and encouragement of students with which I was to become more familiar later in 1967 when I began as a research student. A project on stellar structure had taught me that I did not want to work on that, and I began under Dennis with the idea of looking at galaxy formation. However, by sharing an office with John Stewart I came to read John's paper with George Ellis (Stewart and Ellis, 1968) and its antecedent (Ellis, 1967) and developed an interest in relativistic cosmological models, which led to George becoming my second supervisor.

I was still in Sciama's group, and I learnt a lot from the tea-table conversations, which seemed to cover all of general relativity and astrophysics. Dennis taught us by example that the field should not be sub—divided into mathematics and physics, or cosmological and galactic and stellar, but that one needed to know about all those things to do really good work.