| 1. |
- Capozziello, Salvatore, et al.
(författare)
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Hybrid Metric-Palatini Gravity
- 2015
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Ingår i: Universe. - : MDPI. - 2218-1997. ; 1:2, s. 199-238
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Forskningsöversikt (refereegranskat)abstract
- Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.
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| 2. |
- Akrami, Yashar, et al.
(författare)
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The nature of spacetime in bigravity : Two metrics or none?
- 2015
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Ingår i: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 47:1, s. 1838-
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.
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| 3. |
- Carloni, Sante, et al.
(författare)
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Dynamical system analysis of hybrid metric-Palatini cosmologies
- 2015
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 92:6
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Tidskriftsartikel (refereegranskat)abstract
- The so-called f(X) hybrid metric-Palatini gravity, where X = R + T, with T the stress-energy trace and R the Ricci scalar, presents a unique viable generalization of the f(R) theories within the metric-affine formalism. In this paper, the cosmology of the f(X) theories is studied using the dynamical system approach. The method consists of formulating the propagation equation in terms of suitable (expansion-normalized) variables as an autonomous system. The fixed points of the system then represent exact cosmological solutions described by power law or de Sitter expansion. The formalism is applied to two classes of f(X) models, revealing both standard cosmological fixed points and new accelerating solutions that can be attractors in the phase space. In addition, the fixed point with vanishing expansion rate is considered with special care in order to characterize the stability of Einstein static spaces and bouncing solutions.
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| 4. |
- Koivisto, Tomi S., et al.
(författare)
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Doubly-boosted vector cosmologies from disformal metrics
- 2015
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 90:9
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Tidskriftsartikel (refereegranskat)abstract
- A systematic dynamical system approach is applied to study the cosmology of anisotropic Bianchi I universes in which a vector field is assumed to operate on a disformal frame. This study yields a number of new fixed points, among which are anisotropic scaling solutions. Within the simplifying assumption of (nearly) constant-slope potentials these are either not stable attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, some solutions do have an appeal for cosmological applications in that isotropy is retained due to rapid oscillations of the vector field. Such cosmological scenarios could describe physics beyond standard model such as extra dimensional models that predict disformal couplings between vector and scalar fields.
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| 5. |
- Koivisto, Tomi S., et al.
(författare)
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Scalar-fluid theories : cosmological perturbations and large-scale structure
- 2015
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics (IOP). - 1475-7516. ; :9
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Tidskriftsartikel (refereegranskat)abstract
- Recently a new Lagrangian framework was introduced to describe interactions between scalar fields and relativistic perfect fluids. This allows two consistent generalizations of coupled quintessence models: non-vanishing pressures and a new type of derivative interaction. The implications of these to the formation of cosmological large-scale structure are uncovered here at the linear order. The full perturbation equations in the two cases are derived in a unified formalism and their Newtonian, quasi-static limit is studied analytically. Requiring the absence of an effective sound speed term in the coupled dark matter fluid restricts the Lagrangian to be a linear function of the matter number density. This leaves new potentially viable classes of both algebraically and derivatively interacting models wherein the coupling may impact the background expansion dynamics and imprint new signatures into the large-scale structure.
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