Horizon of the Universe

How big is the universe? The question has been debated for centuries with no decision. Some theorists think that the universe is infinitely large, with galaxies extending forever into infinite space. Others believe that it is finite, curving back on itself in a fourth-dimensional way.

Cosmographer Thomas Gold of Britain's Royal Observatory was asked the "how big" question. His answer, given in Nature: the universe has no definitive size. Instead, it has a "horizon."

Gold is a principal proponent of "continuous creation." He does not believe that the universe came into being suddenly at some remote moment in the past. Instead, he thinks that matter is still being created. It "appears" continuously in the form of single hydrogen atoms out in the empty reaches between the galaxies. At first the lonely atoms form a very thin gas; they draw together by gravitational attraction. At last, after billions of years, the atoms gather into stars, and the stars into galaxies.

Because of some unknown property of large-scale space, the galaxies fly apart, as they can be seen to do. But since new galaxies are formed continuously in the ever-growing voids between them, the "population density" of space remains about the same. This process, says Gold, keeps the expanding universe in a steady state. It has no beginning, and will have no end.

As for the size of the universe, Gold believes that distant galaxies, which cannot be seen because they are moving away as fast or almost as fast as light, are not real in the ordinary sense. As man's instruments improve, he can catch more and more of these runaways, and this will widen the horizon of his observable universe just as the earthly horizon is widened by climbing to a hilltop. But it is not correct, says Gold, to assume that unseen and unseeable galaxies extend into space forever beyond the cosmic horizon. For the purposes of cosmological theory, a galaxy beyond the horizon is over the edge of reality.

"If we continued observations with a given apparatus for a very long time, what changes would we see?" asks Gold. As billions follow billions of years, the most distant galaxies slip over the edge of the universe as their light becomes too feeble to be observed. Faint nearby galaxies grow brighter as they collect more matter. As the space between them expands, new galaxies are born to glow faintly in the new space.

The galaxy population of the universe, Gold sums up, is like that of a human population, "which has a finite number of members at any instant, but which is in a steady state for all time."

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