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to hide intergalactic matter in ways which we could not then detect. Evidence for such missing mass has now begun to emerge. The High Energy Astronomical Observatories (HEAO) are a set of satellites orbiting the Earth and scanning the universe for particles and radiation that we cannot detect down here, under our thick blanket of air. Satellites of this sort have detected intense emission of X-rays from clusters of galaxies, from intergalactic spaces where there was hitherto no hint of any matter. Extremely hot gas between the galaxies would be invisible to other experimental methods and therefore missed in the inventory of cosmic matter made by Gott and his colleagues. What is more, ground-based radio astronomical studies with the Arecibo Observatory in Puerto Rico have shown that the matter in galaxies extends far beyond the optical light from the apparent edges of galaxies. When we look at a photograph of a galaxy, we see an edge or periphery beyond which there is no apparent luminous matter. But the Arecibo radio telescope has found that the matter fades off extremely slowly and that there is substantial dark matter in the peripheries and exteriors of galaxies, which had been missed by previous surveys.

The amount of missing matter required to make the universe ultimately collapse is substantial. It is thirty times the matter in standard inventories such as Gott’s. But it may be that the dark gas and dust in the galactic outskirts, and the astonishingly hot gas glowing in X-rays between the galaxies, together constitute just enough matter to close the universe, prevent an expansion forever—but condemn us to an irrevocable end in a cosmic fireball 50 billion or 100 billion years hence. The issue is still teetering. The deuterium evidence points the other way. Our inventories of mass are still far from complete. But as new observational techniques develop we will have the capability of detecting more and more of any missing mass, and so it would seem that the pendulum is swinging toward a closed universe.

It is a good idea not to make up our minds prematurely on this issue. It is probably best not to let our personal preferences influence the decision. Rather, in the long tradition of successful science, we should permit nature to reveal the truth to us. But the pace of discovery is quickening. The nature of the universe emerging from modern experimental cosmology is very different from that of the ancient Greeks who speculated on the universe and the gods. If we have avoided anthropocentrism, if we have truly and dispassionately considered all alternatives, it may be that in the next few decades we will, for the first time, rigorously determine the nature and fate of the universe. And then we shall see if Gott knows.

* But there is still a debate on how much deuterium can be made in the hot insides of stars and later spewed back into the interstellar gas. If this is substantial, the present deuterium abundance will have less impact on the density of the early universe.

CHAPTER 25

THE AMNIOTIC

UNIVERSE

It is as natural to man to die as to be born; and to a little infant, perhaps, the one is as painful as the other.

FRANCIS BACON,

Of Death (1612)

The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. He to whom this emotion is a stranger, who can no longer wonder and stand rapt in awe, is as good as dead: his eyes are closed.… To know that what is impenetrable to us really exists, manifesting itself as the highest wisdom and the most radiant beauty which our dull facilities can comprehend only in the most primitive forms—this knowledge, this feeling, is at the center of true religiousness. In this sense, and in this sense only, I belong to the ranks of the devoutly religious men.

ALBERT EINSTEIN,

What I Believe (1930)

WILLIAM WOLCOTT died and went to heaven. Or so it seemed. Before being wheeled to the operating table, he had been reminded that the surgical procedure would entail a certain risk. The operation was a success, but just as the