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89
Quantization of Four-form Fluxes and Dynamical Neutralization Of The Cosmological Constant
, 2000
"... A four-form gauge flux makes a variable contribution to the cosmological constant. This has often been assumed to take continuous values, but we argue that it has a generalized Dirac quantization condition. For a single flux the steps are much larger than the observational limit, but we show that wi ..."
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Cited by 274 (21 self)
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A four-form gauge flux makes a variable contribution to the cosmological constant. This has often been assumed to take continuous values, but we argue that it has a generalized Dirac quantization condition. For a single flux the steps are much larger than the observational limit, but we show that with multiple fluxes the allowed values can form a sufficiently dense ‘discretuum’. Multiple fluxes generally arise in M theory compactifications on manifolds with non-trivial threecycles. In theories with large extra dimensions a few four-forms suffice; otherwise of order 100 are needed. Starting from generic initial conditions, the repeated nucleation of membranes dynamically generates regions with λ in the observational range. Entropy and density perturbations can be produced.
The acceleration of the universe, a challenge for string theory
- JHEP
, 2001
"... Preprint typeset in JHEP style.- PAPER VERSION hep-th/0104181 ..."
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Cited by 50 (2 self)
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Preprint typeset in JHEP style.- PAPER VERSION hep-th/0104181
Categorizing different approaches to the cosmological constant problem
"... Abstract. We have found that proposals addressing the old cosmological constant problem come in various categories. The aim of this paper is to identify as many different, credible mechanisms as possible and to provide them with a code for future reference. We find that they all can be classified in ..."
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Cited by 22 (1 self)
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Abstract. We have found that proposals addressing the old cosmological constant problem come in various categories. The aim of this paper is to identify as many different, credible mechanisms as possible and to provide them with a code for future reference. We find that they all can be classified into five different schemes of which we indicate the advantages and drawbacks. Besides, we add a new approach based on a symmetry principle mapping real to
The Cosmological Constant Problem from a Brane-World Perspective
, 2000
"... We point out several subtleties arising in brane-world scenarios of cosmological constant cancellation. We show that solutions with curvature singularities are inconsistent, unless the contribution to the effective four-dimentional cosmological constant of the physics that resolves the singularities ..."
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Cited by 20 (5 self)
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We point out several subtleties arising in brane-world scenarios of cosmological constant cancellation. We show that solutions with curvature singularities are inconsistent, unless the contribution to the effective four-dimentional cosmological constant of the physics that resolves the singularities is fine-tuned. This holds for both flat and curved branes. Irrespective of this problem, we then study an isolated class of flat solutions in models where a bulk scalar field with a vanishing potential couples to a 3-brane. We give an example where the introduction of a bulk scalar potential results in a nonzero cosmological constant. Finally we comment on the With the discovery of string dualities it became clear that extended objects (branes) are built into any string theory. In the form of D-branes [1] they first served as tools in addressing conceptual questions of string theory (e.g. dualities, black hole entropy, holography). An interesting property of D-branes is that they provide a natural picture for having some fields (typically
The Quantum Vacuum and the Cosmological Constant Problem
, 2000
"... The cosmological constant problem arises at the intersection between general relativity and quantum field theory, and is regarded as a fundamental problem in modern physics. In this paper we describe the historical and conceptual origin of the cosmological constant problem which is intimately conne ..."
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Cited by 12 (1 self)
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The cosmological constant problem arises at the intersection between general relativity and quantum field theory, and is regarded as a fundamental problem in modern physics. In this paper we describe the historical and conceptual origin of the cosmological constant problem which is intimately connected to the vacuum concept in quantum field theory. We critically discuss how the problem rests on the notion of physical real vacuum energy, and which relations between general relativity and quantum field theory are assumed in order to make the problem well-defined.
Supersymmetry at Large Distance Scales
"... We propose that the UV/IR relation that underlies the AdS/CFT duality may provide a natural mechanism by which high energy supersymmetry can have large distance consequences. We motivate this idea via (a string realization of) the Randall-Sundrum scenario, in which the observable matter is localized ..."
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Cited by 12 (0 self)
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We propose that the UV/IR relation that underlies the AdS/CFT duality may provide a natural mechanism by which high energy supersymmetry can have large distance consequences. We motivate this idea via (a string realization of) the Randall-Sundrum scenario, in which the observable matter is localized on a matter brane separate from the Planck brane. As suggested via the holographic interpretation of this scenario, we argue that the local dynamics of the Planck brane – which determines the large scale 4-d geometry – is protected by the high energy supersymmetry of the dual 4-d theory. With this assumption, we show that the total vacuum energy naturally cancels in the effective 4-d Einstein equation. This cancellation is robust against changes in the low energy dynamics on the matter The observed smallness of the cosmological constant requires a remarkable cancellation of all vacuum energy contributions [1]. Supersymmetry seems at present the only known symmetry that could naturally explain this cancellation, but thus far no mechanism for supersymmetry breaking is known that would not destroy this property. Nonetheless, in
Dark energy in perturbative string cosmology
- JHEP 0110
"... The apparent observation of dark energy poses problems for string theory. In de Sitter space, or in quintessence models, one cannot define a gauge-invariant S-matrix. We argue that eternal quintessence does not arise in weakly coupled string theory, but point out that it is difficult to define an S- ..."
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Cited by 11 (6 self)
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The apparent observation of dark energy poses problems for string theory. In de Sitter space, or in quintessence models, one cannot define a gauge-invariant S-matrix. We argue that eternal quintessence does not arise in weakly coupled string theory, but point out that it is difficult to define an S-matrix even in the presence of perturbative potentials for the moduli. The solutions of the Fischler-Susskind equations all have Big Bang or Big Crunch Singularities. We believe that an S-matrix (or S-vector) exists in this context but cannot be calculated by purely perturbative methods. We study the possibility of metastable de Sitter vacua in such weakly coupled scenarios, and conclude that the S-matrix of the extreme weak coupling region cannot probe de Sitter physics. We also consider proposed explanations of the dark energy from the perspective of string theory, and find that most are implausible. We note that it is possible that the axion constitutes both the dark matter and the dark energy. 1
The cosmological constant problem: a user’s guide
, 2009
"... We discuss the validity of general relativity at low-energy and relate the threshold below which the theory breaks down with the observed value of the cosmological constant. This suggests the existence of a mass scale of O(10 −3) eV and a putative violation of the equivalence principle at about 10 − ..."
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Cited by 6 (2 self)
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We discuss the validity of general relativity at low-energy and relate the threshold below which the theory breaks down with the observed value of the cosmological constant. This suggests the existence of a mass scale of O(10 −3) eV and a putative violation of the equivalence principle at about 10 −14 level.
Can we live in a self-tuning universe
, 2001
"... The self-tuning brane scenario is an attempt to solve the cosmological constant problem in the context of extra dimensions. Rather than making the vacuum energy small, this approach proceeds by removing the gravitational effect of vacuum energy on the expansion of the universe. Such behavior is only ..."
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Cited by 5 (0 self)
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The self-tuning brane scenario is an attempt to solve the cosmological constant problem in the context of extra dimensions. Rather than making the vacuum energy small, this approach proceeds by removing the gravitational effect of vacuum energy on the expansion of the universe. Such behavior is only possible through changing the Friedmann equation of conventional cosmology, and we discuss difficulties in obtaining cosmological evolution compatible with observation in this context. Specific models considered include a bulk scalar field coupling to the brane via a conformal transformation of the brane metric, and via a rescaling of the brane volume element. 1 1
