Einstein first proposed the cosmological constant
(not to be confused with the Hubble Constant) usually symbolized by the greek
letter "lambda" (L),
as a mathematical fix to the theory of general relativity. In its simplest
form, general relativity 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, like balancing a pencil on its point, and
proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was in fact 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 on
theoretical grounds. Modern field theory associates this term with the energy
density of the vacuum. For this energy density to be comparable to other
forms of matter in the universe, 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 main attraction of the cosmological constant term is that it significantly
improves the agreement between theory and observation. The most spectacular
example of this is the recent effort to measure how much the expansion of
the universe has changed in the last few billion years. Generically, the
gravitational pull exerted by the matter in the universe slows the expansion
imparted by the Big Bang. Very recently it has become practical for astronomers
to observe very bright rare stars called supernova in an effort to measure
how much the universal expansion has slowed over the last few billion years.
Surprisingly, the results of these observations indicate that the universal
expansion is speeding up, or accelerating! While these results should be
considered preliminary, they raise the possibility that the universe contains
a bizarre form of matter or energy that is, in effect, gravitationally repulsive.
The cosmological constant is an example of this type of energy. Much work
remains to elucidate this mystery!
There are a number of other observations that are suggestive of the need
for a cosmological constant. For example, if the cosmological constant today
comprises most of the energy density of the universe, then the extrapolated
age of the universe
is much larger than it would be without such a term, which helps avoid the
dilemma that the extrapolated age of the universe is younger than some of
the oldest stars we observe! A cosmological constant term added to the inflationary model, an extension of the Big Bang theory, leads to a model that appears to be consistent with the observed largescale distribution of galaxies and clusters, with COBE's measurements of cosmic microwave background fluctuations, and with the observed properties of Xray clusters.
WMAP and the Cosmological Constant
By characterizing the detailed structure of the cosmic microwave background fluctuations, WMAP 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.
Last updated: Tuesday, 03012005
