What is a Cosmological Constant?

Einstein first proposed the cosmological constant, usually symbolized by the greek letter "lambda", as a mathematical fix to the theory of general relativity. In its simplest form, the theory predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. Friedmann, a Russian mathematician, realized that this was an unstable fix and proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake".

Many cosmologists advocate reviving the cosmological constant term. Modern field theory associates this term with the energy density of the vacuum. For this term to be cosmologically interesting, it would require new physics: the addition of a cosmological constant term has profound implications for particle physics and our understanding of the fundamental forces of nature.

The attraction of the cosmological constant term is that it significantly improves the agreement between theory and observation. If the cosmological constant today comprises most of the energy density of the universe, then the extrapolated age of the universe is much larger. Adding a cosmological constant term to the inflationary model, an extension of the Big Bang theory, leads to a model that appears to be consistent with the observed large-scale distribution of galaxies and clusters, with COBE's measurements of cosmic microwave background fluctuations, and with the observed properties of X-ray clusters.

MAP and the Cosmological Constant

By characterizing the detailed structure of the cosmic microwave background fluctuations, MAP should be able to accurately determine the basic cosmological parameters, including the cosmological constant, to better than 5%.

Further Reading:

• Donald Goldsmith, "Einstein's Greatest Blunder? The Cosmological Constant and Other Fudge Factors in the Physics of the Universe", (Harvard University Press: Cambridge, Mass.) A well written, popular account of the cosmological constant and the current state of cosmology.

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Last updated: Friday, 05-21-1999