‘Fate of the universe? You mean the universe is not immoral? But the sky seems so constant that nothing is going to change!’

‘Although the end of universe seems far away from us, many cosmologists concern about it, especially the curvature of space-time, which decides whether the universe is open or close, so as to the fate of universe: big crush or heat death. But there are so many uncertainties, like the presence of dark matter, that such discrimination between the two theories is not possible till now. Most importantly, the ingredient of dark matter is undefined, and the universe's total mass evaluated is so close to the critical value between open and closed universe that some scientists are now thinking of any possible mechanism of regulating universe's total mass so that its value can be what it is today, being so close to the critical.’

FATE OF THE UNIVERSE>THE FATE OF THE UNIVERSE>THE HUBBLE EXPANSION

A number of other arguments have been proposed that are equally inconclusive about the curvature of the universe. One example is the study of the Hubble expansion of the galaxies in the vicinity of our local supercluster. This system is centered on the Virgo cluster of galaxies. A glance at the distribution of galaxies on the sky reveals an immense concentration of galaxies toward the Virgo region. The local inhomogeneity is perhaps 100 million light-years in extent. The Virgo cluster of galaxies is itself a central peak in the galaxy distribution that spans about 5 million light-years. One effect of this local excess of galaxies is to decelerate slightly the overall Hubble rate of expansion. There is excess mass and therefore excess gravity, and galaxies accordingly recede at a lower velocity than they would in the absence of the local supercluster.

The amount of this deceleration turns out to be sensitive to cosmology. Consider a universe of critical density. Any small localized excess of mass will clearly affect the delicate balance between gravity and expansion and will tend to lower the recession velocities in the local region. However, in an open universe, the expansion energy would greatly exceed the gravitational energy. In this case, small enhancements in the local density would slightly increase the gravitational energy but would not appreciably slow the rate of expansion of any galaxies. Because galaxies would be expanding freely, they simply would not be affected by any gravitational force. Only a very large local inhomogeneity would produce enough force to decelerate the galaxies in a local region of an open universe.

Unfortunately, our calculations of redshifts and distances to many remote galaxies are so uncertain that we cannot yet pursue this test unambiguously. Tentative indications are that the local expansion flow of the galaxies around us is smooth and undistorted, which favors an open-universe model. However, this uniformity, quite surprisingly, has been found to break down over large scales, in excess of 15 megaparsecs, or the distance to the Virgo cluster. One clue was found in an infrared selected sample of galaxies, which has yielded a somewhat contrary view. The advantage of an infrared catalog of galaxies is that we can undertake an all-sky survey: optical samples are restricted to high galactic latitudes away from the obscuring dust clouds in the Milky Way.

Infared false-color image of the Orion Nebule, from IRAS satellite.

Preliminary studies based on a catalog of galaxies selected by the IRAS satellite indicate the presence of a weak density inhomogeneity that extends to about 200 million light-years from us. This inhomogeneity is in the same direction as the motion of the Local Group of galaxies relative to the microwave background. If mass traces light on such large scales, then the magnitude of this motion can best be explained if the universe is at critical density. Without a complete set of redshifts, however, it is difficult to ascertain the precise depth of the survey, and therefore of the inhomogeneity.

Studies of a sample of some 400 elliptical galaxies with measured redshifts that extend to a similar distance have provided a more detailed view of the velocity field on such large scales. By using an apparently universal correlation between luminosity (the determination of which depends on distance) and intrinsic velocity dispersion of the galaxy (which is independent of distance), distance to groups and clusters could be obtained to an accuracy of a few percent. Comparison of these actual distances with those inferred from the redshifts, assuming Hubble’s law, led a group of astronomers (nicknamed the Seven Samurai for their challenge to conventional wisdom) in California, Arizona, Massachusetts, and Cambridge, England, to infer that large-scale flows as large as 600 kilometers per second were occurring. In particular, our local group, the Virgo supercluster, and the more remote Centaurus cluster, all appear to be rushing toward a point in space that is obscured by the Milky Way but inferred to be the center of a great mass concentration. However, these results will be confirmed only when the studied samples of galaxies are shown to provide an unbiased probe of the large-scale matter distribution in the universe.

We should be reluctant to accept such far-reaching conclusions about gross deviations from the Hubble flow, with their corresponding implications for cosmology, without unimpeachable evidence. The bulk flows, if gravitationally driven, require considerable large-scale power in the primordial fluctuation spectrum. Yet the cosmic microwave background tells us from its impressive uniformity that on very large scales (1 billion light-years or more) the universe was and is exceedingly quiescent and expanding uniformly.Whether there are large deviations at intermediate scales of 100 million light-years is still a controversial issue. For the present, we have to conclude that the samples of space studied have not been deep enough to let us bring in a definite verdict. Whether the local Hubble rate is significantly decelerated by gravity—whether the universe is open or closed—remains uncertain.

Another argument depends on a precise determination of the age of the universe. If the age can be shown to equal H₀^-1 rather than, say, (⅔) H₀^-1 or a smaller time scale, this evidence would confirm an open universe. The best determination of the age applies stellar-evolution theory to the Hertzprung-Russell diagram for stars in globular star clusters. As in the Hertzprung-Russell diagram, the age can be inferred from the main-sequence turn-off. The best value is about 14 billion years: presumably, if our galaxy took about 1 billion years to form, the age of the universe is about 15 billion years. Unfortunately, because we cannot refine the Hubble constant to better than a factor of 2, we can only compare the stellar evolution age with H₀^-1 equal to between 10 and 20 billion years. Consequently, no conclusion can be drawn from this comparison until a more reliable value of H₀ is forthcoming.