formation. According to Weinberg, domains where the vacuum energy is compatible with life would be comparatively rare. Using this argument, Weinberg predicted that the cosmological constant would have a value of less than a hundred times the currently accepted value. In 1992, Weinberg refined this prediction of the cosmological constant to 5 to 10 times the matter density. This argument depends on the vacuum energy density being constant throughout spacetime, as would be expected if dark energy were the cosmological constant. There is no evidence that the vacuum energy does vary, but it may be the case if, for example, the vacuum energy is (even in part) the potential of a scalar field such as the residual inflaton (also see Quintessence). Another theoretical approach that deals with the issue is that of multiverse theories, which predict a large number of "parallel" universes with different laws of physics and/or values of fundamental constants. Again, the anthropic principle states that we can only live in one of the universes that is compatible with some form of intelligent life. Critics claim that these theories, when used as an explanation for fine-tuning, commit the inverse gambler's fallacy. In 1995, Weinberg's argument was refined by Alexander Vilenkin to predict a value for the cosmological constant that was only ten times the matter density, i.e. about three times the current value since determined. Failure to detect dark energy An attempt to directly observe dark energy in a laboratory failed to detect a new force. Inferring the presence of dark energy through its interaction with baryons in the cosmic microwave background has also led to a negative result, although the current analyses have been derived only at the linear perturbation regime. It is also possible that the difficulty in detecting dark energy is due to the fact that the cosmological constant describes an existing, known interaction (e.g. electromagnetic field). See also Big Rip Higgs mechanism Lambdavacuum solution Naturalness (physics) Quantum electrodynamics de Sitter invariant special relativity Unruh effect References Footnotes Bibliography Primary literature Secondary literature: news, popular science articles & books Secondary literature: review articles, monographs and textbooks External links Michael, E., University of Colorado, Department of Astrophysical and Planetary Sciences, "The Cosmological Constant" Carroll, Sean M., "The Cosmological Constant" (short), "The Cosmological Constant"(extended). News story: More evidence for dark energy being the cosmological constant Cosmological constant article from Scholarpedia Physical cosmology Big Bang General relativity Theories