How Old Is the Universe? The Big Bang

0:00How old is the universe, and how do we know?

0:04Space is filled with billions of galaxies, which are huge groups of stars.

0:10Astronomers see that distant galaxies are moving away from us.

0:15This tells us that space itself is stretching, so the universe is expanding.

0:21Now imagine running time backwards, like rewinding a video.

0:26As we go back, the galaxies get closer and closer together.

0:31Long ago, everything was packed together, and it was extremely hot and dense.

0:38Scientists call this hot, dense start the Big Bang.

0:42The Big Bang was not an explosion in space; it was space itself starting to stretch.

0:49By measuring how fast space is stretching, scientists work out when it began.

0:55The Big Bang happened about 13.8 billion years ago.

1:00A billion is a thousand million, so that is a very long time.

1:06Our Earth formed much later, about 4.5 billion years ago.

1:11So the universe is about three times as old as Earth.

1:16The universe began with the Big Bang, about 13.8 billion years ago.