Oxford University Press, 1937

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THE OBSERVATIONAL APPROACH TO COSMOLOGY

By Edwin Hubble


Of the Mount Wilson Observatory
Carnegie Institution of Washington


Oxford
At the Clarendon Press
1937

PREFACE

This book contains the Rhodes Memorial Lectures delivered at Oxford in the Autumn of 1936, under the general title, `The Observational Approach to Cosmology'.

The observable region of space, the region that can be explored with existing instruments, is a sample of the universe. If the sample is fair, its observed characteristics should furnish important information concerning the universe at large. The lectures describe the general features now known, and discuss the nature of the inferences to which they lead.

The features, however, include the phenomena of red-shifts whose significance is still uncertain. Alternative interpretations are possible, and, while they introduce only minor differences in the picture of the observable region, they lead to totally different conceptions of the universe itself. One conception, at the moment, seems less plausible than the other, but this dubious world, the expanding universe of relativistic cosmology, is derived from the more likely of the two interpretations of red-shifts. Thus the discussion ends in a dilemma, and the resolution must await improved observations or improved theory or both.

However, the significance of the investigation lies not in the failure to reach a unique solution to the problem of the structure of the universe, but rather in the fact that the venture is now permissible. As late as fifteen years ago the observable region was restricted to our own system of stars, the system of the Milky Way. Since that time great reflectors have identified the nebulae as independent stellar systems, the true in- habitants of space. Explorations, using the nebulae as gigantic landmarks, have swept out beyond the Milky Way to the very limits of existing telescopes. The observable region, our sample of the universe, has been suddenly magnified a million million fold. Now, for the first time, the sample may be fair.

The break through into extra-galactic space and the preliminary reconnaissance of the observable region have been described in The Realm of the Nebulae, recently published by the Yale University Press. The Rhodes Memorial Lectures form a sequel to the story, for they present the results of accurate surveys which followed the reconnaissance and suggest their cosmological significance. Since the new results could not be discussed in complete isolation it has been necessary to include a considerable background derived from the earlier investigations. It is a pleasure to acknowledge the courtesy of the Yale University Press in permitting the generous use of material from The Realm of the Nebulae.

Although the subject is developed from the observers' point of view, it is necessarily permeated with cosmological theory. Fortunately, the writer has had the privilege of association with Richard C. Tolman of the California Institute of Technology, who has presented the theory in a manner especially adapted to the limitations of the observational technique. Any errors in the application of the theory must be attributed to the misuse of his friendly counsel.

The illustrations reproduce photographs made with the telescope chiefly responsible for the recent development of the field of nebular research, namely, the 100-inch reflector of the Mount Wilson Observatory of the Carnegie Institution of Washington.


Table of Contents

LIST OF ILLUSTRATIONS

I. THE OBSERVABLE REGION AS A SAMPLE OF THE UNIVERSE
The Observational Approach to Cosmology
The Copernican Revolution
The Theory of Island Universes
Nebulae as Island Universes
Family Characteristics of Nebulae
Criteria of Nebular Distances
The Average Nebula
Distribution of Nebulae over the Sky
Distribution of Nebulae in Depth
The Observable Region as a Sample of the Universe

II. THE ROLE OF THE RED-SHIFTS
Spectrum Analysis
Red-shifts
Possible Interpretations of Red-Shifts
Red-Shifts as Velocity-Shifts
Red-Shifts as Loss of Energy in Transit
The Critical Test between Alternative Interpretations
The Indirect Test
Effects of Red-Shifts on Apparent Luminosity
The Energy Effect
The Recession Factor
The Alternative Laws of Red-Shifts
The Dilemma

III. POSSIBLE WORLDS
Surveys of Nebulae
The Law of Nebular Distribution when Red-Shifts are not interpreted as Velocity-Shifts
The Problem of Distribution in an Expanding Universe
Expanding Universes of General Relativity
Comparison of Observations with Theory
Spatial Curvature
The Permissible Type of an Expanding Universe
Conclusion

SUMMARY OF OBSERVATIONAL RESULTS

INDEX

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