Cosmology and the Cosmological Constant

The First Model of the Universe

In February 1917 Einstein produced the first model of the universe based on general relativity. The paper, "Cosmological Considerations on the General Theory of Relativity," founded modern cosmology.

He ran into a problem at once. His equations would not allow a universe that simply sat still. Gravity would make it collapse. Like almost every astronomer of his day, Einstein assumed the universe was static and eternal, so he added a new term to the equations, the cosmological constant $\mathrm{\Lambda }$, acting as a gentle repulsion to balance gravity.

The Universe Refuses to Sit Still

The balance was unstable, like a pencil standing on its point. Meanwhile others took the equations at face value. Alexander Friedmann in 1922 and Georges Lemaître in 1927 found solutions in which the universe expands. Einstein at first thought Friedmann had made a mistake and, by Lemaître's later account, told him in 1927 that his physics was abominable.

Then the telescopes spoke. By 1929 Edwin Hubble's measurements showed that distant galaxies are receding, the more distant the faster. In 1931 Einstein accepted the expanding universe and dropped the constant. In 1932, with Willem de Sitter, he published a simple expanding model with no cosmological constant at all, which remained a standard reference for decades.

The "Biggest Blunder"

Einstein is said to have called the cosmological constant the biggest blunder of his life. The phrase does not appear anywhere in his own writing. It reached print through the physicist George Gamow in 1956, after Einstein's death, though two other physicists later reported similar remarks. Historians disagree: one detailed study concluded he probably never said it, a later one that he very likely said something of the kind. He certainly came to regard the term as a mistake. It is safest to say he reportedly called it that.

The Constant Returns

In 1998 two teams of astronomers measuring distant exploding stars discovered that the expansion of the universe is not slowing down, as everyone expected. It is speeding up. Saul Perlmutter, Brian Schmidt, and Adam Riess shared the 2011 Nobel Prize for the discovery.

The simplest description of whatever is driving the acceleration, known as dark energy, is a cosmological constant. In today's standard model it makes up roughly 68 percent of the energy content of the universe.

What People Get Wrong

• Adding the constant was not a mathematical error. The term is a perfectly legitimate part of the equations.
• Its return does not vindicate Einstein's reason for adding it. He wanted a static universe, and the universe is not static.
• Einstein did not discover that the universe is expanding. He resisted the idea until the evidence was overwhelming.