Let there be Light

摘要

In the beginning there was light—the light created by the fireball of the big bang. However, as the fireball expanded and cooled, the light shifted out of the visible region of the spectrum, plunging the Universe into a dark age. For millions of years, the Universe grew and grew—a hundred times in size—but still the cosmic darkness persisted. What brought it to an end was the birth of the first stars and galaxies. "Suddenly light was everywhere," says Abraham Loeb of Harvard's Center for Astrophysics. "The Universe lit up like a Christmas tree."