In an expanding universe, astronomers realized, it should be possible to determine how and when the expansion began. Hubble's initial calculations indicated that the universe was only two billion years old, younger than the age of the Earth as determined by geologists. Once discrepancies in Hubble's calculations had been adjusted, a more geologically agreeable age of six billion years emerged.
Astronomers now believe the universe probably began 10 billion to 15 billion years ago in an immense explosion of matter and energy known as the Big Bang. Since then, the expansion of space itself has carried the galaxies away from each other, in accord with Hubble's Law of expansion.