What is The Cosmological constant?

Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant) usually symbolized by the greek letter "lambda" (Λ), 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".

Since then increasingly accurate probes have confirmed to high precision the need for dark energy, but the nature of the dark energy is now the issue being investigated. As of 2010 the measured properties of dark energy remain consistent with those of a cosmological constant. However, massive observing efforts are underway to test whether this is the correct explanation for the acceleration or whether some other sort of dark energy, perhaps one that changes with time or one that is motivated by some form of quantum gravity, is needed to explain the acceleration we see.

Article links and sources: http://map.gsfc.nasa.gov/universe/uni_accel.html

Scholarpedia: http://www.scholarpedia.org/article/Cosmological_constant

UCLA Article: http://www.astro.ucla.edu/~wright/cosmo_constant.html

Space.com - what is dark energy? : http://www.space.com/6619-dark-energy.html

Image source: NASA, Dark Energy by ~JerX88 on deviantART

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