# Physical Cosmology

PHILLIP JAMES EDWIN PEEBLES
Pages: 298
https://www.jstor.org/stable/j.ctt13x0w7r

1. Front Matter
(pp. i-vi)
2. INTRODUCTION
(pp. vii-xii)

Physical science more often than not violates the rule that familiarity breeds contempt. We may be assured that we have a reasonable understanding of the atmosphere of Mars, the major constituents, the temperature and pressure, yet we know very well that it is hard to say whether it will rain tomorrow. This does not mean that we understand the Martian atmosphere better than our own. Quite the contrary, we have incomparably better observational material on the Earth’s atmosphere so we can and should ask much more detailed and deeper questions about it. Sometimes this order may be bent because greater...

(pp. xiii-2)
4. I. GOLDEN MOMENTS IN COSMOLOGY 1912-1950
(pp. 3-30)

The first observational basis of modern cosmology was the discovery of the nature of the extragalactic nebulae, or galaxies, and of their distribution and motion. Hubble gives an admirable account of this discovery inRealm of the Nebulae.¹

The apparent radial motion of the nebulae is indicated by the observed frequency shift in their spectra. In 1912 Slipher obtained the first positive measurement of this effect for an extragalactic nebula, the Andromeda Nebula, M 31. He found a velocity of approach of 300 km sec–1, consistent with the best modern value. Slipher continued painstaking accumulation of data on apparent...

5. II. THE HOMOGENEITY AND ISOTROPY OF THE UNIVERSE
(pp. 31-42)

A common assumption in cosmology is that the Universe is homogeneous and isotropic in the large scale, when minor irregularities like galaxies and clusters of galaxies are smoothed over. The major exceptions to this assumption have been the bounded world picture of the sort discussed by Klein and by Alfven in which it is assumed that the galaxies about us are a finite metagalactic system expanding into more or less empty space (ref. 1-37), and the hierarchical world picture, in which the observed sequence of structures ranging from star clusters to galaxies to pairs and groups of galaxies to great...

6. III. HUBBLE’S CONSTANT AND THE COSMIC TIME SCALE
(pp. 43-55)

The constant of proportionality in Hubble's law (eq. I-1) is important because it fixes the linear scale of the Universe, hence indirectly the time scale (eqs. I-21,22) and the critical mass density (eq. I-19). It is the first parameter in the basic cosmological equation (I-10). The purpose of this chapter is to give a brief review of the observational evidence on the value of the constant H, and to discuss how H–1compares with the cosmic time scales provided by stellar evolution ages and the radioactive decay ages of the elements.

To determine H one needs the distance of...

7. IV. THE MEAN MASS DENSITY OF THE UNIVERSE
(pp. 56-120)

The mean mass density of the Universe, ρ, is the second of the parameters in the fundamental cosmological equation (I-10). Discussion of ρ is particularly difficult and interesting because for every conceivable form of matter we have to find from observational and theoretical arguments an estimate of the amount or an upper limit. A convenient reference point in the discussion of ρ is the Einstein-deSitter density (eq. I-19), which in the notation of equation (II-3) is

${\rho _c} = 1.9 \times {10^{ - 29}}{h^2}gc{m^{ - 3}}$,

${n_c} = 1.1 \times {10^{ - 5}}{h^2}protonsc{m^{ - 3}}$. (1)

It should be emphasized that this is only a reference point. There is noa prioritheoretical prediction that the mean...

8. V. THE MICROWAVE BACKGROUND AND THE PRIMEVAL FIREBALL HYPOTHESIS
(pp. 121-158)

A Primeval Fireball is thermal blackbody radiation, a remnant from a presumed earlier epoch when the Universe was much denser and hotter than it is now. As will be described in this chapter there is strong evidence that a Primeval Fireball has been discovered. If this is substantiated by further measurements the great and profound significance will be that we will have direct evidence that the Universe did expand away from a “Big Bang,” and we will know something about the temperature history in the Big Bang.

It will be assumed here that the Universe is homogeneous and isotropic, and...

9. VI. A CHILD’S GARDEN OF COSMOLOGICAL MODELS
(pp. 159-189)

A central goal of cosmology has been to test cosmological models by looking for relativistic effects in the appearance of distant galaxies. The first major observational effort was Hubble’s counts of galaxies as a function of limiting apparent magnitude. More recently Sandage has considered in detail the relation between redshift and apparent magnitude.¹ In first approximation the energy flux f from a galaxy of fixed luminosity should vary as z–2($f\infty {\ell ^{ -2}}$by the inverse square law,$\ell \infty z$by Hubble’s law). In the next approximation the deviation from this law determines one parameter which (if general relativity with A =...

10. VII. HISTORY OF THE UNIVERSE – SCENARIOS
(pp. 190-239)

The previous chapters have been devoted mainly to what the Universe is like now or at modest redshift,$z \le 0.3$. A second great line of attack in cosmology has been the attempt to deduce from theoretical and philosophical considerations and whatever fossil evidence we may manage to find what the Universe was like in the distant past, and perhaps thereby to come to some understanding of why the Universe is now the way it is. Understandably progress here is even more uncertain than in the relatively more modest goal of exploring the present Universe, but we do have a few memorable...

11. VIII. PRIMEVAL HELIUM
(pp. 240-277)

This is an important subject because it is by far the most powerful probe we have on what might have been happening in the distant past when, according to the Big Bang Primeval Fireball picture, the Universe was dense and hot. The rather lengthy computation of primeval helium production may be summarized in three steps: (1) When t < 1 sec, time being measured from the singularity a = 0,$\rho = \infty$in the Big Bang, the temperature is greater than 10¹⁰ K, and matter and radiation are expected to have been very close to thermal equilibrium. The significant equilibrium constituents...

12. APPENDIX NOTATION, CONVENTIONS AND UNITS
(pp. 278-282)