In the early twentieth century, Albert Einstein published his equations of general relativity and a Dutch astronomer, Willem de Sitter, found a solution to them that predicted an expanding universe. This, too, was a highly significant prediction because if the universe has been expanding and if galaxies are moving farther apart, this implies that in the past they once were closer together. If the universe has been “blowing up” for the duration of its existence, that means that it must have had an actual beginning.

Einstein, who didn’t realize that his equations suggested an expanding universe, was distressed to hear about this implication of his famous theory. When Russian mathematician Alexander Friedmann tried to persuade him, Einstein sought to prove Friedmann wrong. Actually Einstein was wrong. The great physicist was, by his own account, “irritated” by the idea of an expanding universe. He went so far as to invent a new force, the “antigravity” force, as well as a number called the “cosmological constant:’ to try to disprove the notion of a beginning. Later Einstein admitted his errors and called his cosmological constant the biggest mistake of his life.In the late 1920s, astronomer Edwin Hubble, peering through the hundred-inch telescope at Mount Wilson Observatory in California, observed through the “red shift” of distant nebulae that galaxies were moving rapidly away from each other. The number of stars involved in this galactic dispersal suggested an astoundingly vast universe, much bigger than anyone had thought. Some galaxies were millions of light years away. The impression that many people had long held of the stillness and changelessness of space was an illusion. Hubble noticed that planets and entire galaxies were hurtling away from one another at fantastic speeds. Moreover, space itself seemed to be getting bigger. The universe wasn’t, expanding into background space, because the universe already contains all the space there is. Incredibly, space itself was expanding along with the universe. Hubble’s findings, subsequently confirmed by numerous others, generated great excitement in the scientific community.

Scientists realized right away that the galaxies were not flying apart because of some mysterious force thrusting them away from each other. Rather, they were moving apart because they were once flung apart by a primeval explosion. Extrapolating backward in time, all the galaxies seem to have had a common point of origin approxi- mately fifteen billion years ago. Scientists projected a moment in which all the mass in the universe was compressed into a point of infinite density. The entire universe was smaller than a single atom.

Then in a single cosmic explosion—the Big Bang—the universe we now inhabit came into existence. “The universe was filled with light,” Steven Weinberg writes. In fact, “it was light that then formed the dominant constituent of the universe.” The temperature was about a hundred trillion degrees Centigrade. Then, in a process vividly described by Weinberg in The First Three Minutes, the first protons and neutrons began to form into atoms. Once matter was formed, gravitational forces began to draw it into galaxies and then into stars. Eventually heavier elements like oxygen and iron were formed and, over billions of years, gave birth to our solar system and our planet. Crazy though it may seem, our terrestrial existence, indeed the very matter of which we are made, owes itself to a “creation event” that occurred around fifteen billion years ago.