The Big Bang: Space Itself Is Expanding

0:00How did the universe begin, and what has it been doing ever since?

0:05About 13.8 billion years ago, the universe was extremely hot and dense, which means everything was packed tightly together.

0:15Since then, space itself has been expanding, which means it stretches, so the distances between places grow.

0:24As space expands, the universe cools down.

0:28This idea is called the big bang theory.

0:31Over time, matter gathered into galaxies, which are huge groups of stars held together by gravity.

0:39As space keeps stretching, galaxies that are far apart move away from each other.

0:44It is like raisins in rising bread dough: as the dough rises, every raisin moves away from every other raisin.

0:53But galaxies themselves do not stretch, because gravity holds their stars together.

0:59In 1927, the scientist Georges Lemaître proposed that the universe is expanding.

1:06In 1929, Edwin Hubble showed that distant galaxies are moving away from us.

1:13Nearer galaxies move away more slowly, and the farther away a galaxy is, the faster it moves away.

1:21Seen from any other galaxy, distant galaxies move away too, so the expansion has no centre.

1:30So the big bang was not an explosion at one point into empty space. Space itself stretches everywhere at once.

1:38The big bang theory: for about 13.8 billion years, space itself has been expanding and cooling.