The cosmological constant is a mathematical term (symbolized by the Greek letter lambda, &lgr;) that German-born physicist Albert Einstein placed in his general relativity equations in 1917. The theory of general relativity predicted that the universe must either expand or contract, but Einstein did not believe this was possible. Einstein attempted to fix his equations by adding the cosmological constant, which served to halt the expansion. It can therefore be thought of as a positive energy attributed to space that, if exactly balanced against the force of gravity, will make the universe static.
In the decade following the formulation of general relativity, scientists proved that the universe is not static, contrary to Einstein's belief. In the early 1920s, Russian meteorologist Alexander Friedmann proposed an expanding universe model, now the generally accepted the big bang theory. A few years later, astronomer Edwin Hubble calculated that the majority of the galaxies he observed recede from the Milky Way in speeds proportional to their distance from Earth, indicating that the universe expands. Einstein conceded that he had made a mistake and declared the cosmological constant to be "the biggest blunder of my life."
Many scientists continue to believe that the cosmological constant exists. Although the universe is expanding, and Einstein's original reason for inventing the constant is obsolete, there may be room for it elsewhere in cosmological theory.
Einstein made up the cosmological constant because he was attempting to reconcile a theory with the established view of what the universe actually looks like. When current scientists attempt to do the same thing, they experience problems. A cosmologist who thinks about the fate of the universe may ask, Will the universe continue to expand forever? Will the force of gravity slow it down enough so that it eventually collapses under its own weight? Will the universe slow down in another way? Knowing the density of the universe and its rate of expansion can help answer these questions.
Cosmologists and astronomers have predicted the age of the universe based on estimates of its density and observations of stars in distant galaxies. If the matter in the universe is very dense, as many cosmologists assert, then the universe is quite young. The predicted age of the universe (between 8 and 14 billion years) is younger than the calculated age of some stars. As this prediction appears impossible, it is attractive to scientists that there may exist a cosmological constant.
One explanation is that the universe is slowing in its expansion due to gravity. The rate that it slows depends upon its density and, as mentioned above, many scientists believe the density is such that the universe is younger than expected. The existence of a cosmological constant, a positive energy attributed to space, could allow the universe to slow down irregularly. In other words, a non-zero cosmological constant (i.e., one that exists) means the expansion of the universe happens at different rates. If this is true, cosmologists can justifiably add more time to the age of the universe. A non-zero cosmological constant could therefore reconcile theory and observation.
Some scientists believe that this kind of reconciliation is oversimplified. It is strange, they argue, that a number should have the exact value needed to explain the universe as it exists in current theories.
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