Cosmological constant problem
 | This article needs attention from an expert in Physics. The specific problem is: to address the various problem tags in the text. (April 2017) |
 | This article needs additional citations for verification. (April 2017) |
| Beyond the Standard Model |
|---|
 |
| Standard Model |
In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between measured values of the vacuum energy density (the small value of the cosmological constant) and the zero-point energy suggested by quantum field theory.
Depending on the assumptions[which?], the discrepancy ranges from 40 to more than 100 orders of magnitude, a state of affairs described by Hobson et al. (2006) as "the worst theoretical prediction in the history of physics."[1]
Overview
Gravitational description
The basic problem of a vacuum energy producing a gravitational effect was identified as early as 1916 by Walther Nernst.[2][further explanation needed]
The value was predicted[according to whom?] to be either zero or very small[why?], so that the theoretical problem was already apparent, and began to be actively discussed in the 1970s.
Importance in cosmology
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 modeling vacuum energy leads to huge differences in the resulting cosmologies.[3][further explanation needed]
The problem became increasingly central as an obstacle[why?] to theoretical progress during the later 1980s and the 1990s, and was variously dubbed an "unexplained puzzle"[citation needed], a "veritable crisis"[citation needed] and "the most striking problem in contemporary fundamental physics"[citation needed].
Quantum description
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).[4][further explanation needed]
Renormalization
The vacuum energy in quantum field theory can be set to any value by renormalization. This view treats the cosmological constant as simply another fundamental physical constant not predicted by theory.[5]
Quantum field theory predictions based on Light front quantization, a possible solution.
Light front quantization is a rigorous alternative due to Paul Dirac to the usual second quantization method (instant-form method). Causality and frame-independence (Poincaré invariance) are explicit, contrary to quantization in the instant-form method. The light-front vacuum state is defined as the eigenstate of lowest invariant mass.
Vacuum fluctuations do not appear in the Light-Front vacuum since all particles have positive momenta $p+= p0+p3$. Since $P+$ is conserved, particles cannot couple to the light front vacuum since it has $p+=0.$
These features make the quantum field theory vacuum essentially trivial, with no vacuum dynamics such as condensate (i.e. vacuum expectation value). In contrast, vacuum fluctuations appear in the vacuum of the ordinary instant-form (the lowest energy eigenstate of the instant-form Hamiltonian), but the physical effects depend on the arbitrary choice of Lorentz frame. This fact and the violation of causality indicate that the instant-form vacuum cannot represent of the physical vacuum.
While the features of the LF vacuum have been known for a long time[6],[7], in 2011, Stanley Brodsky and Richard Shrock showed[8] that the absence of condensates implies that in the Standard Model of Particle Physics, there is no contribution from QED, Weak interactions and QCD to the cosmological constant. It is thus predicted to be zero in a flat space-time. This was later validated and developed[9],[10] by other prominent QCD theorists.
In the case of the Higgs mechanism, the usual Higgs vacuum expectation value in the instant-form vacuum is replaced by a constant scalar background field - a "zero mode" with kμ=0. The phenomenological predictions are unchanged using the LF formalism. Since the Higgs zero mode has no energy or momentum density, it does not contribute to the cosmological constant.
The small non-zero value of the cosmological constant must then be attributed to other mechanisms; for example a slight curvature of the shape of the universe (which is not excluded within 0.4% (as of 2017)[11][12][13]) modifies the Higgs field zero-mode, thereby possibly producing a non-zero contribution to the cosmological constant.
Measurement
The value of the cosmological constant was first measured in 1998.[according to whom?]
With the ability to measure the speed of gravity[clarification needed], its relation to the speed of light may soon provide confirmation of which theories[further explanation needed] and models best fit the cosmological constant.[14][15]
See also
References
- ^ 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.
- ^ S. Weinberg “The cosmological constant problem”, Review of Modern Physics 61 (1989), 1-23.
- ^ 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.
- ^ Rugh and Zinkernagel (2002), 36ff.
- ^ Quantum field theory on lightlike slabs H. Leutwyler, J.R. Klauder, L. Streit. Nuovo Cim. A66 (1970) 536 DOI: 10.1007/BF02826338
- ^ A. Casher and L. Susskind Chiral magnetism (or magnetohadrochironics) Phys. Rev. D9 (1974) 436 DOI: 10.1103/PhysRevD.9.436
- ^ S. J. Brodsky and R. Shrock. "Condensates in Quantum Chromodynamics and the Cosmological Constant". Proc.Nat.Acad.Sci. 108 (2011) 45-50 [arXiv:0905.1151].
- ^ S. J. Brodsky, C. D. Roberts, R. Shrock and P. C. Tandy. "Essence of the vacuum quark condensate". Phys.Rev. C82 (2010) 022201 [arXiv:1005.4610].
- ^ S. J. Brodsky, C. D. Roberts, R. Shrock and P. C. Tandy. "Confinement contains condensates". Phys.Rev. C85 (2012) 065202 [arXiv:1202.2376]
- ^ "Will the Universe expand forever?". NASA. 24 January 2014. Retrieved 16 March 2015.
- ^ "Our universe is Flat". FermiLab/SLAC. 7 April 2015.
- ^ Marcus Y. Yoo (2011). "Unexpected connections". Engineering & Science. LXXIV1. Caltech: 30.
- ^ "Quest to settle riddle over Einstein's theory may soon be over". phys.org. 2017-02-10. Retrieved 2017-02-10.
{{cite news}}: Cite has empty unknown parameter:|dead-url=(help) - ^ Lombriser, Lucas; Lima, Nelson A. (2017-02-10). "Challenges to self-acceleration in modified gravity from gravitational waves and large-scale structure". Physics Letters B. 765: 382–385. doi:10.1016/j.physletb.2016.12.048.
- 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.