Broca's Brain - Carl Sagan [163]
What, then, would be the fate of the universe? Why, then an observer would see expansion eventually replaced by contraction, the galaxies slowly and then at an ever-increasing pace approaching one another, a careening, devastating smashing together of galaxies, worlds, life, civilizations and matter until every structure in the universe is utterly destroyed and all the matter in the cosmos converted into energy: instead of a universe ending in cold and tenuous desolation, a universe finishing in a hot and dense fireball. It is very likely that such a fireball would rebound, leading to a new expansion of the universe and, if the laws of nature remain the same, a new incarnation of matter, a new set of condensations of galaxies and stars and planets, a new evolution of life and intelligence. But information from our universe would not trickle into that next one and, from our vantage point, such an oscillating cosmology is as definitive and depressing an end as the expansion that never stops.
The distinction between a Big Bang with expansion forever and an Oscillating Universe clearly turns on the amount of matter there is. If the critical amount of matter is exceeded, we live in an Oscillating Universe. Otherwise we live in one that expands forever. The expansion times—measured in tens of billions of years—are so long that these cosmological issues do not affect any immediate human concerns. But they are of the most profound import for our view of the nature and fate of the universe and—only a little more remotely—of ourselves.
In a remarkable scientific paper published in the December 15, 1974 issue of the Astrophysical Journal, a wide range of observational evidence is brought to bear on the question of whether the universe will continue to expand forever (an “open” universe) or whether it will gradually slow down and recontract (a “closed” universe), perhaps as part of an infinite series of oscillations. The work is by J. Richard Gott III and James E. Gunn, then both of the California Institute of Technology, and David N. Schramm and Beatrice M. Tinsely, then of the University of Texas. In one of their arguments they review calculations of the amount of mass in and between galaxies in “nearby” well-observed regions of space and extrapolate to the rest of the universe: they find that there is not enough matter to slow the expansion down.
Ordinary hydrogen has a nucleus comprising a single proton. Heavy hydrogen, called deuterium, has a nucleus comprising one proton and one neutron. An astronomical telescope in Earth orbit called “Copernicus” has measured, for the first time, the amount of deuterium between the stars. Deuterium must have been made in the Big Bang in an amount that depends on the early density of the universe. The early density of the universe is connected with the present density of the universe. The amount of deuterium found by “Copernicus” implies a value to the early density of the universe and suggests that the present density is insufficient to prevent the universe from expanding forever.* And what is said to be the best value of the Hubbell constant—which specifies how much faster more distant galaxies are receding from us than nearby ones—is consistent with this whole story.
Gott and his colleagues stressed that there may be loopholes in their argument, that it may be possible