The Expanding Universe

What happened

Applying Einstein's general relativity to the cosmos, physicists found the universe could not be static. In 1929 the astronomer Edwin Hubble showed that distant galaxies are all rushing away from us, and the farther they are the faster they recede — evidence that space itself is expanding. Run backward, the expansion implied the universe began in a hot, dense state, the idea that became the Big Bang.

Why it matters

The discovery that the universe is expanding transformed cosmology from speculation into a physical science. It gave physics a history of the cosmos itself — later confirmed by the cosmic microwave background — and is one of the great achievements of 20th-century physics.

Sources

• NASA Science. The Big Bang · Reputable source