The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

“While in a limited sense gravitation is of no great importance to a physicist, this is much too naive an interpretation.” In the first paragraph of the “Cosmology” section Peebles expanded on that philosophy. For physicists, he wrote, cosmology doesn’t satisfy just “the obvious interest” in the origins of the universe; “we need cosmology as a basis of any complete theory of the galaxies, or for that matter, of the solar system.” If you wanted to understand specific problems concerning the evolution and structure of the universe—the clustering of galaxies, for instance—then you had to abandon any residual “island universe” assumptions. You had to learn to think about the universe not only as a collection of individual galaxies but as the sum of its galaxies—a single unit, a whole. You had to keep in mind that while the whole—the universe—was expanding, its parts—the galaxies—were evolving. “The moral of this section,” Peebles offered in conclusion, twelve pages later, “is the unity of the universe.”