Cosmological constant problem
|Beyond the Standard Model|
In cosmology, the cosmological constant problem is the disagreement between measured values of the vacuum energy density (the small value of the cosmological constant) and the theoretical zero-point energy suggested by quantum field theory.
Depending on the assumptions, the discrepancy ranges from 40 to more than 100 orders of magnitude, a state of affairs described by Hobson and Efstathiou (2006) as "the worst theoretical prediction in the history of physics." The magnitude of this discrepancy is such that the statement "the observable universe consists of exactly one elementary particle" is at least ten orders of magnitude more accurate.
The basic problem of a vacuum energy producing a gravitational effect was identified as early as 1916 by Walther Nernst. After the development of quantum field theory in the 1940s, the first to address contributions of quantum fluctuations to the cosmological constant was Zel’dovich (1967, 1968).  The value of the cosmological constant was first measured in 1998, but it was known before this that it was either zero or very small, so that the theoretical problem was already apparent, and began to be actively discussed in the 1970s. With the development of inflationary cosmology in the 1980s, the problem became much more important: as cosmic inflation is driven by vacuum energy, differences in modelling vacuum energy leads to huge differences in the resulting cosmologies. Therefore, the problem became increasingly central as an obstacle to theoretical progress during the later 1980s and the 1990s, and was variously dubbed an "unexplained puzzle", a "veritable crisis" and "the most striking problem in contemporary fundamental physics". On the other hand, there was the view that there is no real problem, as the vacuum energy in QFT can be set to any value by renormalization. This view treats the cosmological constant as simply another fundamental physical constant not predicted by theory. 
- MP Hobson, GP Efstathiou & AN Lasenby (2006). General Relativity: An introduction for physicists (Reprint ed.). Cambridge University Press. p. 187. ISBN 978-0-521-82951-9.
- W Nernst (1916). "Über einen Versuch von quantentheoretischen Betrachtungen zur Annahme stetiger Energieänderungen zurückzukehren". Verhandlungen der Deutschen Physikalischen Gesellschaft (in German) 18: 83.
- Zel’dovich, Y.B., ‘Cosmological Constant and Elementary Particles’ JETP letters 6 (1967), 316-317 and ‘The Cosmological Constant and the Theory of Elementary Particles’ Soviet Physics Uspekhi 11 (1968), 381-393.
- S. Weinberg “The cosmological constant problem”, Review of Modern Physics 61 (1989), 1-23.
- Rugh and Zinkernagel (2002), 36f.
- SE Rugh, H Zinkernagel; Zinkernagel (2002). "The quantum vacuum and the cosmological constant problem". Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 33 (4): 663–705. doi:10.1016/S1355-2198(02)00033-3.