Section 2: Before the Beginning: A Static Universe
Our understanding of the universe has changed dramatically in the last century. One hundred years ago, astronomers thought the stars of our galaxy made up the whole universe, that the galaxy was nearly motionless, and that the universe was unchanging through time. Physicists trying to think of a model for the universe had to match these "facts."
Figure 2: In Einstein's general relativity, mass warps the fabric of space.
Source: © NASA/STScI. More info
As Albert Einstein struggled to invent general relativity between 1914 and 1917, he was mindful of the possibilities for testing its predictions. Because gravity is so weak on the Earth, no laboratory measurements could test whether his ideas were right or wrong. So, Einstein looked to astronomers to help him find places to test his predictions, either because gravity was stronger (for example, near the edge of the Sun) or because the distances involved were large enough to show the cumulative effect of subtle differences from Newton's gravity. He posed three astronomical tests for the theory: the orbit of Mercury, the path of light near the limb of the Sun, and the effect of gravity on light from dense stars—tests that general relativity eventually passed summa cum laude.
Einstein's ideas about gravity were deeply original. He imagined that mass (and energy) would warp the fabric of space and time. Light or massive particles would then travel through this curved space. Einstein applied his equations to the universe as a whole, using the astronomical understanding of the day. In 1917, astronomers thought that our Milky Way galaxy, of which the Sun is an inconspicuous member, was, in fact, the whole universe.
Figure 3: Edge-on view of the Milky Way.
Source: © NASA Goddard Space Flight Center. More info
As far as astronomers knew at that time, the stars in the Milky Way were not moving in any systematic way. So when he wrote down an expression for the way gravity acts in the universe, Einstein added in an extra term to keep the universe static. This cosmological constant acted as a repulsive force that would balance out gravity and ensure that the universe would endure indefinitely without clumping together. Einstein found he could choose the value of the cosmological constant to produce just the right amount of curvature to make the universe "closed." This meant it behaved like the two-dimensional surface of a sphere, which has a finite surface area and has no edge, but Einstein was thinking of four dimensions of space and time. As Einstein apologized at the time, "...we admittedly had to introduce an extension of the field equations which is not justified by our actual knowledge of gravitation.... That [cosmological constant] term is necessary only for the purpose of making possible a quasi-static distribution of matter, as required by the fact of the small velocities of the stars."
Within a decade, astronomical observations showed that Einstein's picture of a static universe did not match the rapidly improving observational evidence of cosmic expansion. By 1931, Einstein considered the cosmological constant an unfortunate mistake. Today, however, careful measurements of distances and velocities from exploding stars observed halfway back to the Big Bang show that we need something very much like the cosmological constant to understand why the universe is speeding up.