The Last Three Minutes: Conjectures About the Ultimate Fate of the Universe by Paul Davies

Author:Paul Davies [Davies, Paul]
Language: eng
Format: epub
Tags: Science, Cosmology
ISBN: 9780786725045
Publisher: Basic Books
Published: 1997-01-09T00:00:00+00:00

CHAPTER 7

FOREVER IS A LONG TIME

The important thing about infinity is that it is not just a very big number. Infinity is qualitatively different from something that is merely stupendously, unimaginably huge. Suppose the universe were to continue expanding forever so that is has no end. For it to endure for all eternity means that it would have an infinite lifetime. If this were the case, any physical process, however slow or improbable, would have to happen sometime, just as a monkey forever tinkering on a typewriter would eventually type the works of William Shakespeare.

A good example is provided by the phenomenon of gravitational-wave emission, which I discussed in chapter 5. Only in the case of the most violent astronomical processes will energy loss in the form of gravitational radiation produce conspicuous changes. The emission of about a milliwatt caused by the Earth orbiting the sun has an infinitesimal effect on the Earth’s motion. Yet even a milliwatt power drain, extended over trillions upon trillions of years, would eventually cause the Earth to spiral into the sun. Of course, it is likely to be engulfed by the sun long before this, but the point is that processes that are negligible on a human time scale, but are nevertheless persistent, may eventually come to predominate and thus serve to determine the ultimate fate of physical systems.

Let us imagine the state of the universe a very, very long time in the future—say, in a trillion trillion years. The stars have long since burned out; the universe is dark. But it is not empty. Amid the black vastness of space lurk spinning black holes, stray neutron stars, and black dwarfs—even a few planetary bodies. At this epoch, the density of such objects is exceedingly low: the universe has expanded to ten thousand trillion times its present size.

Gravity would play out a strange battle. The expanding universe attempts to pull every object farther apart from its neighbors, but the mutual gravitational attractions oppose this and try to bring bodies together. As a result, certain collections of bodies—for example, clusters of galaxies, or what pass for galaxies after eons of structural degeneration—remain gravitationally bound, but these collections drift ever farther from neighboring collections. The ultimate outcome of this tug-of-war depends on how fast the rate of expansion decelerates. The lower the density of matter in the universe, the more “encouragement” these collections of bodies get to disengage from their neighbors and move apart freely and independently.

Within a gravitationally bound system, the slow but inexorable processes of gravity exert their dominance. Gravitational-wave emission, feeble though it is, insidiously saps the system’s energy, causing a slow spiral of death. Ever so gradually, dead stars creep closer to other dead stars or black holes, and coalesce in an extended orgy of cannibalism. It takes a trillion trillion years for gravitational waves to completely degrade the orbit of the sun, a black dwarf cinder silently gliding toward the galactic center, where a gigantic black hole waits to swallow it.

It

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