Modern Cosmology

A Science of the Whole

Before 1917 the structure and history of the universe as a whole were matters for philosophy. General relativity changed that. Because the theory describes how matter shapes space and time, it can be applied to everything there is, and it yields equations for how the universe evolves. Einstein took the first step himself. See Cosmology and the Cosmological Constant.

The Expanding Universe

The equations do not want the universe to stand still. Alexander Friedmann in Russia and Georges Lemaître in Belgium found the expanding solutions in the 1920s, and Lemaître connected them to the observed motions of galaxies. Edwin Hubble's measurements in 1929 made the case convincing.

Run the expansion backward, and everything was once packed together in a hot, dense state. That idea, the Big Bang, was confirmed in 1965 by the discovery of the faint microwave radiation left over from the early universe, which fills the sky to this day.

The Standard Model of Cosmology

Today's picture, built from measurements of that radiation, of the distribution of galaxies, and of distant exploding stars, is remarkably specific.

• The universe is about 13.8 billion years old.
• Ordinary matter, the kind that makes stars, planets, and people, is about 5 percent of its contents.
• Dark matter, detected only by its gravity, is about 27 percent.
• Dark energy, which is driving the expansion to speed up, is about 68 percent.

The mathematical framework holding all of this together is Einstein's field equations, including the cosmological constant he introduced in 1917 and later renounced.

How Direct Is the Link?

General relativity is the framework of all modern cosmology; without it there is no way even to pose the questions. But the great conclusions were reached by others, and often against Einstein's instincts. He did not predict the expansion, resisted it until 1931, and did not live to see the Big Bang confirmed. What he built was the language in which the universe's history could be written.