The Expanding Universe

Abstract

Nowadays, anyone wishing to learn what the structure of the Galaxy is can find out from a variety of sources—including this chapter. But, from the late 19th century until 1924, the thenunknown size of the Galaxy was at the heart of a controversy. The controversy concerned the existence of galaxies other than our own: were the so-called spiral and elliptical nebulae^a in fact other galaxies—“island universes,” as the German philosopher Immanuel Kant presciently suggested in 1755—or was the Milky Way so large that it encompassed everything? The American astronomer Edwin Hubble is credited not only with settling the controversy but also with providing the first observational evidence that the Universe is expanding. Expansion means that the vast majority of the galaxies are rushing away—receding—from one another. That the Universe is expanding is not simply one of its salient features; it is among the most momentous cosmological findings of the 20th century. The discovery and the interpretation of the expansion of the Universe occurred along a pair of linked paths, one observational, the other theoretical. It is a marvelous story to relate, containing not only controversy, but speculation, error, leaps of the imagination, overlooked results, triumphs, and paradigm shifts; in short, a very human tale, not what is sometimes thought of as the linear progress of science.

Nebula is the Latin word for “cloud” and was used to refer to the cloudy patches in the night sky visible to the unaided eye. That some of them can be spiral or elliptical in shape, and indeed are other galaxies, is part of the story told in the present chapter.