Table Of ContentPhysical Cosmology
Physical
Cosmology
BY P. J.E. PEEBLES
Princeton Series in Physics
Princeton University Press
Princeton, New Jersey · 1971
Copyrigh©t 1971, PrincetUonni versPirteys s
by
ALL RIGHTS RESERVED
L.C.C ard:7 4-181520
ISBN0- 691-0088-5( paperbaecdkn .)
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ISBN0 -691-0813(7h-a9r dcoveedrn .)
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INTRODUCTION
Physical science more often than not violates the rule that familiarity
breeds contempt. We may be assured that we have a reasonable under
standing 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 atmos
phere 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 physical simplicity can overweigh better data. For
all the wealth of evidence on the center of the Earth from seismic data, free
oscillations, heat flux, and gravitational anomalies, to say nothing of hard
rock geology, it could be that we understand the center of the Sun better
than the center of the Earth just because the ideal gas law is a good approxi
mation for the Sun.
In cosmology the reliance on physical simplicity, pure thought and re
vealed knowledge is carried well beyond the fringe because we have so
little else to go on. By this desperate course we have arrived at a few
simple pictures of what the Universe may be like. The great goal now is
to become more familiar with the Universe, to learn whether any of these
pictures may be a reasonable approximation, and if so how the approxima
tion may be improved. The great excitement in cosmology is that the pros
pects for doing this seem to be excellent. On one front we have some ex
plicit questions, like the shapes of the redshift-magnitude relation and
the redshift-angular size relation, and the values of some elementary
parameters like Hubble's constant and the mean mass density. Probably
we will only know for sure whether these questions are clever after we
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INTRODUCTION
understand the answers, but at least we can see how available technologi
cal developments can be turned to powerful new attacks on them. A second
exciting front is the search for new phenomena that may or may not inter
connect in a pleasant way with accepted ideas. Of course we have no plan
for finding these nuggets, only the expectation based on past form that as
"'
techniques of observation develop and extend our view we are liable to
stumble on them.
The goal of these notes is to aid this advance of cosmology by providing
a guide, mainly for the experimentalist or observer, to the popular questions
and answers in the subject. While there is no shortage of questions it is
true that I have restricted the list to the orthodox questions in an establish
ment cosmology. There is the point of view that in a science as primitive
as cosmology one ought to avoid orthodoxy, give equal time to all competing
cosmologies. The first problem is that one could never hope to think of all
reasonably possible views people might want to consider. Second there is
the danger that the approach will degenerate into long lists of alternative
possibilities, from which the reader can understand where we are only with
the help of an auxiliary guide. My own preference is to make a subjective
selection of a reasonably possible cosmology, and then study it in all the
detail one can muster. The end result of such an effort may be to reveal
that the cosmology is logically inconsistent or even in conflict with ob
servation, which is progress of a sort. The hope is that the development
of the observations may convince us of the rough validity of the chosen
cosmolog}', and guide us to how the cosmology should evolve.
Consistent with this view the Cosmological Constant is seldom
/\
mentioned in these notes because a non-zero can do only two essentially
/\
different things. If is less than zero, or positive but not too large, it
/\
alters somewhat the connection among expansion rate, mass density and
curvature in the present Universe, and it hardly at all affects the expansion
rate in the early Universe. We gain one more parameter, but we already
had more than we can fix. The second possibility is more interesting - if
were large enough it would eliminate the Big Bang, the singular infinite
/\
