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In theories of gravity and cosmology a cosmological constant refers to an energy contribution associated with the vacuum itself.
This is called a cosmological constant because the simplest way to see this effect in theory is by a summand to the Einstein-Hilbert action which is proportional to the volume of spacetime, with proportionality factor some “constant” $\lambda$, as in (1) below.
See below
More generally, almost-constant contributions to matter fields may effectively have the same kind of effect.
Specifically in perturbative quantum field theory on curved spacetimes the vacuum expectation value of the stress-energy tensor of the various matter-fields receives constant or essentially constant contributions, notably from renormalization freedom (e.g. Hack 15, section 3.2.1).
See below
In perturbative string theory the string perturbation series associated with a 2d SCFT is (supposedly) UV-finite and hence has “chosen its own renormalization” already, hence here the cosmological constant may in principle be read off from a choice of perturbative string theory vacuum. See below
Observation shows that the effective cosmological constant of the observable universe is comparatively small but positive, see
In an action functional on a space of pseudo-Riemannian manifolds – such as the Einstein-Hilbert action functional for gravity – a cosmological constant is a term proportional to the volume
where $\lambda \in \mathbb{R}$ is the cosmological constant .
For instance, pure Einstein-Hilbert gravity with cosmological constant (and no other fields) is given by the functional
Generically it happens that one considers action functionals where $\lambda$ is in fact not a constant, but a function of other fields $\phi$ on $X$.
In this context those solutions to the Euler-Lagrange equations are of interest in which $\lambda(\phi)$ happens to be exactly or approximately constant. Many such models of not-really-constant-but-effectively-constant terms proportional to the volume are being proposed and considered in attempts to explain observed or speculated dynamics of the cosmos.
See in particular at FRW model for the role of the cosmological constant in homogeneous and isotropic models as in the standard model of cosmology. In that context the cosmological constant is also called the dark energy (density), which makes up about 70% of the energy density of the observable universe (the rest being dark matter) and a comparatively little bit of baryonic matter.
In perturbative quantum field theory on curved spacetimes the cosmological constant receives contributions from the vacuum expectation value of the stress-energy tensor of the matter fields. There is renormalization-freedom in this contribution (Wald 78, Tichy-Flanagan 98 Moretti 01).
Explicitly for FRW models this is discussed in (Dappiaggi-Fredenhagen-Pinamonti 08, Dappiagi-Hack-Moeller-Pinamonti 10). Specifically for the standard model of cosmology see (Hack 13, around (4)).
A useful review is in (Hack 15, section 3.2.1).
This means that apart from the freedom of choosing a classical comsological constant in the Einstein-Hilbert action as above, its perturbative quantization (perturbative quantum gravity) introduces renormalization freedom to the value of the cosmological constant.
The folklore discussion of the “cosmological constant problem” (see the references below) tends not to take this freedom in the theory into account.
(e.g. Einasto 09, fig 17)
Discussion of the cosmological constant in the rigorous formulation of perturbative AQFT on curved spacetimes includes the following.
The freedom of renormalization of the vacuum expectation value of any stress-energy tensor, hence of the cosmological constant, was discussed in
Notice that (Wald 78) is based on
which claimed no freedom of renormalization, but Wald 78 explains that this was due to a mistake inherited from a citation.
Further development of this includes
Wolfgang Tichy, Eanna E. Flanagan, How unique is the expected stress-energy tensor of a massive scalar field?, Phys.Rev. D58 (1998) 124007 (arXiv:gr-qc/9807015)
Valter Moretti, Comments on the Stress-Energy Tensor Operator in Curved Spacetime, Commun. Math. Phys. 232 (2003) 189-221 (arXiv:gr-qc/0109048)
A useful review is in
Realization of renormalization of stress-energy/cosmological constant in concrete FRW models is discussed in
Claudio Dappiaggi, Klaus Fredenhagen, Nicola Pinamonti, Stable cosmological models driven by a free quantum scalar field, Phys. Rev. D77:104015, 2008 (arXiv:0801.2850)
Claudio Dappiaggi, Thomas-Paul Hack, Jan Möller, Nicola Pinamonti, Dark Energy from Quantum Matter (arXiv:1007.5009)
Discussion specifically for the standard model of cosmology is in
Discussion of the experimentally observed tiny cosmological constant and the folklore theoretical problem with that includes the following
Subir Sarkar, New results in cosmology (arXiv:hep-ph/0201140)
Stefanus Nobbenhuis, The cosmological constant problem – an inspiration for new physics PhD thesis (2006) (web pdf)
Joan Sola, Cosmological constant and vacuum energy: old and new ideas, J.Phys.Conf.Ser. 453 (2013) 012015 (arXiv:1306.1527)
Jaan Einasto, Dark matter (arXiv:0901.0632) 2009
Discussion from the point of view of perturbative string theory, where the cosmological constant is fixed by the choice of perturbative string theory vacuum includes
See also
for discussion in terms of the M-theory/type IIA relation KK-compactified to a 4d/3d scenario, where the 3d physics is weakly coupled and the 4d physics strongly coupled. (Recall the super 2-brane in 4d.)
This discussion was later supplemented by
But a) there might be a large space of perturbative string theory vacua and b) the de Sitter vacua that seem to correspond to observation tend to exist (only) as metastable vacua:
This observation led to the discussion of the “landscape of string theory vacua”.
For review see
Last revised on April 8, 2018 at 04:14:56. See the history of this page for a list of all contributions to it.