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.
This theory of an expanding universe was consistent not only with the second law of thermodynamics but also with Einstein’s theory of relativity. Hubble found that the farther away a galaxy is from us, the faster it is receding from us. This is now called Hubble’s Law, and it fulfills a prediction that was made on the basis of Einstein’s theory. The expanding universe theory also solved an old conundrum that had been frustrating scientists for decades: why the galaxies continued to stay apart from each other. Why had the force of gravity not pulled them together? The reason was that they had been hurled apart in a primordial explosion whose force continued to thrust them farther and farther away from each other. Astronomer John Barrow calls Hubble’s finding “the greatest discovery of twentieth-century science.”
