Annu. Rev. Astron. Astrophys. 1988. 26: 245-294 Copyright © 1988 by Annual Reviews. All rights reserved

4.3. Explanatory Hypotheses

Every hypothesis attempting to explain mass anomalies applies one or the other of the following two assumptions: (a) Galaxies and systems of galaxies contain enough dark matter in one or more forms that have so far escaped detection to resolve the mass anomaly (132, 201). (b) Newtonian/general relativity theory does not apply in extragalactic astronomy (cf. 72, 96a, 110, 119, 153, 168, 168a, 197). A theory is sought that is conceptually elegant and explains all of the mass anomalies without requiring any dark matter. A discussion by Bekenstein (26) describes major difficulties with both dark matter and new physics hypotheses toward explaining the mass anomalies (x_m > 0). It seems that the time-scale anomalies (x > 0), if they survive the detailed observational and theoretical scrutiny that they deserve, would create even more difficulties for both classes of hypotheses.

A plausibility argument in favor of dark-matter hypotheses is that the known physical laws allow many different ways in which matter could escape detection by modern detectors. A plausibility argument in favor of a new physical law is that the forces relevant to a given system tend to correlate with its mass density. For example, atomic nuclei and the solar system have vastly different mass densities. Measured on scales that are physically useful in other applications, the mass densities appropriate to a galaxy and a system of galaxies are much closer to the cosmological density than to the density of the solar system. Another plausibility argument is the following: Qualitative and quantitative descriptions of physical laws, including Newtonian/general relativistic dynamics, are specified from results of experiments in terrestrial laboratories. Our experience with verifying these laws in and beyond the solar system suggests that they are universal. If, in addition, our inventory of physical laws were complete, then these laws would provide intelligent creatures with the tools they need to understand the physical nature of the cosmos, and their role within it, to the maximum extent that the laws allow. But our ability to identify and quantify laws in the laboratory is limited by the range of physical parameters accessible to the laboratory. Therefore, to obtain a complete inventory of physical laws, it may be necessary to examine more fully the experiments performed by nature in the ultimate laboratory, the Universe.

ACKNOWLEDGMENTS

I thank Drs. G. Chincarini, T.L. Page, M. Postman, and M.F. Struble for critically reading an earlier draft of the manuscript and offering valuable suggestions. Significant editorial improvements to the manuscript have been made by Production Editor Keith Dodson and Associate Editor David Layzer. This article was written during a 1986-87 visit at the Institute for Advanced Study; the hospitality of the Faculty in the School of Natural Sciences is gratefully acknowledged. This work is supported in part by the National Science Foundation under grant AST-8513087.