Cosmological constant is a mathematical term that represents the repulsion (pushing away) of every point in space by the surrounding points. Determining the value of the cosmological constant ranks among the most important problems in cosmology (the study of the universe’s structure and origin). The German-born physicist Albert Einstein first proposed the idea of a cosmological constant in 1917 as an addition to the general theory of relativity, his theory of gravitation. Einstein later abandoned the idea. Today, cosmologists study the cosmological constant in an attempt to explain evidence that the universe is expanding at an increasing rate.
General relativity.
In relativity theory, time and space are not absolutely separate. Instead, physicists often refer to them as a single entity (individual thing) called space-time. Einstein’s general theory of relativity states that gravity results from the curvature (bending) of space-time.
Soon after announcing the theory in 1915, Einstein began applying it to the problems of cosmology. Astronomers at the time thought that the universe was static—that is, it was neither expanding nor contracting. Using general relativity, Einstein could only create models of the universe in which it was either expanding or contracting. For this reason, he suggested changing the original theory to include the cosmological constant. The constant‘s repulsion helped balance the tendency of the universe to contract under the influence of gravity. This balance enabled Einstein to construct a model in which the universe was static.
In 1929, the American astronomer Edwin P. Hubble announced his discovery that the universe is expanding. Because astronomers no longer considered the universe to be static, general relativity did not require the existence of the cosmological constant. Einstein promptly abandoned the idea and later expressed regret at having proposed it.
The energy of empty space.
Physicists later realized that the cosmological constant could be related to the idea of vacuum energy. In the context of general relativity, empty space can contain energy even though it contains no matter. If this vacuum energy existed, it would work against the influence of gravity, in effect producing a cosmological constant.
In the late 1990’s, astronomers discovered that the expansion of the universe is accelerating under the influence of a little-understood form of energy known as dark energy. Cosmologists realized that dark energy could represent the existence of a small cosmological constant produced by vacuum energy. This observation and other measurements led them to develop a model in which 70 percent of the universe’s energy is vacuum energy and the other 30 percent consists of matter and radiation. Cosmologists include matter in models of the universe’s energy because matter and energy are equivalent in relativity theory.
The model indicates that the total energy in matter and radiation currently is roughly comparable to the amount of vacuum energy. As the universe expands, however, the density of matter and radiation diminishes dramatically, while the density of vacuum energy remains constant. The universe has been expanding throughout its history, so in the past, the density of matter and radiation must have far exceeded the density of vacuum energy. As the universe continues to expand, the density of vacuum energy will become much greater than the density of matter and radiation.
This model of the universe matches a wide variety of observations, but it suggests two unresolved puzzles. First, the model indicates that the density of vacuum energy and that of matter and radiation should be roughly equal for only a relatively short time. If the model is correct, then, we happen to be living during that brief, unique period. Cosmologists are not sure how to interpret this unlikely fact in relation to the broader history of the universe. They question whether it is just a meaningless coincidence or whether it indicates that our understanding of vacuum energy is incomplete in some important way.
The second mystery involves the extremely small value of the cosmological constant that the model indicates. When cosmologists try to use the known laws of physics to predict the amount of vacuum energy in the universe, they arrive at either no cosmological constant or a cosmological constant much larger than that produced by the model. Scientists are developing new ideas and gathering new data to help solve these problems.
See also Cosmology ; Dark energy ; Gravitation (Expansion of the universe) ; Relativity .
