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2016, viXra
n Riemannian geometry there is a unique combination of the Riemann-Christoffel curvature tensor, Ricci tensor and Ricci scalar that defines a fourth-order Lagrangian for conformal gravity theory. This Lagrangian can be greatly simplified by eliminating the curvature tensor term, leaving a unique combination of just the Ricci tensor and scalar. The resulting formalism and the associated equations of motion provide a tantalizing alternative to Einstein-Hilbert gravity that may have application to the problems of dark matter and dark energy without the imposition of the cosmological constant or extraneous scalar, vector and spinor terms typically employed in attempts to generalize the Einstein-Hilbert formalism. Gauss-Bonnet gravity specifies that the full Lagrangian hides an ordinary divergence (or surface term) that can be used to eliminate the curvature tensor term. In this paper we show that the overall formalism, outside of surface terms necessary for integration by parts, does no...
The European Physical Journal Plus, 2014
This paper reports a study on the cosmological application of interacting Ricci Dark Energy (RDE) density in the scalar Gauss-Bonnet framework. The interacting holographic RDE model has been employed to obtain the equation of state (EoS) in a spatially flat universe. The main results of this paper are that the reconstructed potential of scalar Gauss-Bonnet gravity for the interacting RDE model decays with the evolution of the universe. However, it is an increasing function of the scalar field φ. Both the strong and weak energy conditions are violated. A phantom-like behavior of the EoS parameter has been obtained. The effective EoS parameter ω eff stays below −1 but tends to −1 with the evolution of the universe. However, it cannot cross the phantom boundary. Finally, the interacting RDE model in Gauss-Bonnet gravity gives accelerated expansion of the universe.
International Journal of Theoretical Physics, 2014
In this paper, we studied the cosmological application of the interacting Ricci Dark Energy (RDE) model in the framework of the scalar Gauss-Bonnet modified gravity model. We studied the properties of the reconstructed potential V (t), the Strong Energy Condition (SEC), the Weak Energy Condition (WEC) and the deceleration parameter q for three different models of scale factor, i.e. the emergent, the intermediate and the logamediate one. We obtained that V (t), for the emergent scenario, has a decreasing behavior, while, for the logamediate scenario, the potential start with an increasing behavior then, for later times, it shows a slowly decreasing behavior. Finally, for the intermediate scenario, the potential has an initial increasing behavior, then for a time of t ≈ 1.2, it starts to decrease. We also found that both SEC and WEC are violated for all the three scale factors considered. Finally, studying the plots of q, we derived that an accelerated universe can be achieved for the three models of scale factor considered.
General Relativity and Gravitation, 2008
We give all exact solutions of the Einstein-Gauss-Bonnet Field Equations coupled with a scalar field in four dimensions under certain assumptions. The main assumption we make in this work is to take the second covariant derivative of the coupling function proportional to the spacetime metric tensor. Although this assumption simplifies the field equations considerably, to obtain exact solutions we assume also that the spacetime metric is conformally flat. Then we obtain a class of exact solutions. Keywords Gauss-Bonnet gravity • Conformally flat spacetimes • Exact solutions of field equations • Scalar fields • Gauss-Bonnet term Recently there is an increasing interest in the Gauss-Bonnet theory with a scalar field to look for possible theoretical explanation to some cosmological problems such as acceleration of the universe [1]. Accelerated cosmological solutions were first suggested in [2,3] and also discussed in [4,5]. It is also expected that this theory or its modifications may have some contributions to some astrophysical phenomena. For this purpose, spherically symmetric solutions of this theory were first studied in [6,7]. It has been observed that the Post-Newtonian approximation does not give any new contribution in addition to the post-Newtonian parameters of the general relativity [8]. Black hole solutions in the framework of the GB gravity are investigated recently in [9] (see also [10,11]). There are also attempts to find exact solutions and to study the stability of the Gauss-Bonnet theory in various dimensions with actions containing higher derivative scalar field couplings [12-14].
Physical Review D - PHYS REV D, 2009
The modified gravity with F(R,G) Lagrangian, G is the Gauss-Bonnet invariant, is considered. It is shown that the phantom-divide-line crossing and the deceleration to acceleration transition generally occur in these models. Our results coincide with the known results of f(R)-gravity and f(G)-gravity models. The contribution of quantum effects to these transitions is calculated, and it is shown that in some special cases where there are no transitions in classical level, quantum contributions can induce transitions. The quantum effects are described via the account of conformal anomaly.
2018
We propose a new model of D=4 Gauss-Bonnet gravity. To avoid the usual property of the integral over the standard D=4 Gauss-Bonnet scalar becoming a total derivative term, we employ the formalism of metric-independent non-Riemannian spacetime volume elements which makes the D=4 Gauss-Bonnet action term non-trivial without the need to couple it to matter fields unlike the case of ordinary D=4 Gauss-Bonnet gravity models. The non-Riemannian volume element dynamically triggers the Gauss-Bonnet scalar to be an arbitrary integration constant M on-shell, which in turn has several interesting cosmological implications: (i) It yields specific solutions for the Hubble parameter and the Friedmann scale factor as functions of time, which are completely independent of the matter dynamics, i.e., there is no back reaction by matter on the cosmological metric; (ii) For M>0 it predicts a "coasting"-like evolution immediately after the Big Bang, and it yields a late universe with dynami...
Journal of Cosmology and Astroparticle Physics - JCAP, 2006
We consider a physically viable cosmological model that has a field dependent Gauss–Bonnet coupling in its effective action, in addition to a standard scalar field potential. The presence of such terms in the four dimensional effective action gives rise to several novel effects, such as a four dimensional flat Friedmann–Robertson–Walker universe undergoing a cosmic inflation at the early epoch, as well as a cosmic acceleration at late times. The model predicts, during inflation, spectra of both density perturbations and gravitational waves that may fall well within the experimental bounds. Furthermore, this model provides a mechanism for reheating of the early universe, which is similar to a model with some friction terms added to the equation of motion of the scalar field, which can imitate energy transfer from the scalar field to matter.
International Journal of Geometric Methods in Modern Physics
In a [Formula: see text]-dimensional Friedmann–Robertson–Walker metric, it is rigorously shown that any analytical theory of gravity [Formula: see text], where [Formula: see text] is the curvature scalar and [Formula: see text] is the Gauss–Bonnet topological invariant, can be associated to a perfect-fluid stress–energy tensor. In this perspective, dark components of the cosmological Hubble flow can be geometrically interpreted.
Classical and Quantum Gravity, 2015
In this paper, we consider the generalized Gauss-Bonnet action in 4-dimensional Weyl-Cartan space-time. In this space-time, the presence of torsion tensor and Weyl vector implies that the generalized Gauss-Bonnet action will not be a total derivative in four dimension space-time. It will be shown that the higher than two time derivatives can be removed from the action by choosing suitable set of parameters. In the special case where only the trace part of the torsion remains, the model reduces to GR plus two vector fields. One of which is massless and the other is massive. We will then obtain the healthy region of the 5-dimensional parameter space of the theory in some special cases.
Physical Review D
We use a description based on differential forms to systematically explore the space of scalar-tensor theories of gravity. Within this formalism, we propose a basis for the scalar sector at the lowest order in derivatives of the field and in any number of dimensions. This minimal basis is used to construct a finite and closed set of Lagrangians describing general scalar-tensor theories invariant under Local Lorentz Transformations in a pseudo-Riemannian manifold, which contains ten physically distinct elements in four spacetime dimensions. Subsequently, we compute their corresponding equations of motion and find which combinations are at most second order in derivatives in four as well as arbitrary number of dimensions. By studying the possible exact forms (total derivatives) and algebraic relations between the basis components, we discover that there are only four Lagrangian combinations producing second order equations, which can be associated with Horndeski's theory. In this process, we identify a new second order Lagrangian, named kinetic Gauss-Bonnet, that was not previously considered in the literature. However, we show that its dynamics is already contained in Horndeski's theory. Finally, we provide a full classification of the relations between different second order theories. This allows us to clarify, for instance, the connection between different covariantizations of Galileons theory. In conclusion, our formulation affords great computational simplicity with a systematic structure. As a first step we focus on theories with second order equations of motion. However, this new formalism aims to facilitate advances towards unveiling the most general scalar-tensor theories.
Physical Review D, 2020
Cosmic history can be traced considering further curvature contributions inside the gravitational action. Assuming that standard General Relativity can be extended by other curvature invariants, we discuss the possibility that an action containing higher-order curvature terms can fit, in principle, the whole universe evolution. In particular , a theory like F (R, G), with R the Ricci scalar and G the Gauss-Bonnet topological term, contains all the curvature invariants that, depending on the energy regime, can address inflation, matter dominated and dark energy regimes. In this paper, we investigate this possibility considering how F (R, G) models can lead gravity from ultraviolet to infrared scales. Specifically, we will take into account a cosmographic approach for this purpose. PACS numbers: 98.80.-k, 95.35.+d, 95.36.+x
The European Physical Journal Plus, 2016
A modified Einstein-Gauss-Bonnet gravity in four dimensions where the quadratic Gauss-Bonnet term is coupled to a scalar field is considered. The field equations of the model are obtained by variational methods by making use of the constrained-first order formalism covering both pseudo-Riemannian and non-Riemannian cases. In the pseudo-Riemannian case, the Lagrange multiplier forms, which impose the vanishing torsion constraint, are eliminated in favor of the remaining fields and the resulting metric field equations are expressed in terms of the double-dual curvature 2-form. In the non-Riemannian case with torsion, the field equations are expressed in terms of the pseudo-Riemannian quantities by a perturbative scheme valid for a weak coupling constant. It is shown that, for both cases, the model admits a maximally symmetric de-Sitter solution with nontrivial scalar field. Minimal coupling of a Dirac spinor to the Gauss-Bonnet modified gravity is also discussed briefly.
Journal of Cosmology and Astroparticle Physics
A 'novel' pure theory of Einstein-Gauss-Bonnet gravity in four-spacetime dimensions can be constructed by rescaling the Gauss-Bonnet coupling constant, seemingly eluding Lovelock's theorem. Recently, however, the well-posedness of this model has been called into question. Here we apply a 'dimensional regularization' technique, first used by Mann and Ross to write down a D → 2 limit of general relativity, to the case of pure Einstein-Gauss-Bonnet gravity. The resulting four-dimensional action is a particular Horndeski theory of gravity matching the result found via a Kaluza-Klein reduction over a flat internal space. Some cosmological solutions of this four-dimensional theory are examined. We further adapt the technique to higher curvature Lovelock theories of gravity, as well as a low-energy effective string action with an α ′ correction. With respect to the D → 4 limit of the α ′-corrected string action, we find we must also rescale the dilaton to have a non-singular action in four dimensions. Interestingly, when the conformal rescaling Φ is interpreted as another dilaton, the regularized string action appears to be a special case of a covariant multi-Galileon theory of gravity.
Journal of Cosmology and Astroparticle Physics, 2006
Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear naturally from Kaluza-Klein compactifications of pure higher-dimensional gravity. We study linear, cosmological perturbations in the limits of weak coupling and slow-roll, and derive simple expressions for the main observable sub-horizon quantities: the anisotropic stress factor, the time-dependent gravitational constant, and the matter perturbation growth factor. Using present observational data, and assuming slow-roll for the dark energy field, we find that the fraction of energy density associated with the coupled Gauss-Bonnet term cannot exceed 15%. The bound should be treated with caution, as there are significant uncertainies in the data used to obtain it. Even so, it indicates that the future prospects for constraining the coupled Gauss-Bonnet term with cosmological observations are encouraging.
m-hikari.com
The divergence of the space-matter tensor has been studied in detail and the perfect-fluid spacetimes with divergence-free space-matter tensor are considered. It is seen that such spacetimes either satisfy the vacuum-like equation of state or represent a ...
2018
We discuss a new extended gravity model in ordinary $D=4$ spacetime dimensions, where an additional term in the action involving Gauss-Bonnet topological density is included without the need to couple it to matter fields unlike the case of ordinary D=4 Gauss-Bonnet gravity models. Avoiding the Gauss-Bonnet density becoming a total derivative is achieved by employing the formalism of metric-independent non-Riemannian spacetime volume-forms. The non-Riemannian volume element triggers dynamically the Gauss-Bonnet scalar to become an arbitrary integration constant on-shell. We describe in some detail the class of static spherically symmetric solutions of the above modified D=4 Gauss-Bonnet gravity including solutions with deformed (anti)-de Sitter geometries, black holes, domain walls and Kantowski-Sachs-type universes. Some solutions exhibit physical spacetime singular surfaces not hidden behind horizons and bordering whole forbidden regions of space. Singularities can be avoided by pa...
Physics of the Dark Universe, 2020
We study the possibility that thee Gauss-Bonnet term proposed in [1] gives rise to cosmic acceleration in four-dimensional (D = 4) FLRW space-time. Inserting a Gauss-Bonnet term multiplied by 1/(D − 4) in the action produces non-trivial terms in the dynamical equations and changes the cosmic evolution. We consider a model consisting of this term (playing the role of dark energy) and a barotropic fluid (e.g. dark matter) to describe a Universe dominated by dark matter and dark energy, then the evolution of this Universe is investigated.
Physical Review D, 2006
Dark energy cosmology is considered in a modified Gauss-Bonnet (GB) model of gravity where an arbitrary function of the GB invariant, f (G), is added to the General Relativity action. We show that a theory of this kind is endowed with a quite rich cosmological structure: it may naturally lead to an effective cosmological constant, quintessence or phantom cosmic acceleration, with a possibility for the transition from deceleration to acceleration. It is demonstrated in the paper that this theory is perfectly viable, since it is compliant with the Solar System constraints. Specific properties of f (G) gravity in a de Sitter universe, such as dS and SdS solutions, their entropy and its explicit one-loop quantization are studied. The issue of a possible solution of the hierarchy problem in modified gravities is addressed too.
International Journal of Modern Physics D, 2007
We consider dark energy cosmology in a de Sitter universe filled with quantum conformal matter. Our model represents a Gauss-Bonnet model of gravity with contributions from quantum effects. To the General Relativity action an arbitrary function of the GB invariant, f (G), is added, and taking into account quantum effects from matter the cosmological constant is studied. For the considered model the conditions for a vanishing cosmological constant are considered. Creation of a de Sitter universe by quantum effects in a GB modified gravity is discussed.
Physical Review D, 2011
For the lagrangian L = G ln G where G is the Gauss-Bonnet curvature scalar we deduce the field equation and solve it in closed form for 3-flat Friedman models using a statefinder parametrization.
Proceedings of Corfu Summer Institute 2018 "School and Workshops on Elementary Particle Physics and Gravity" — PoS(CORFU2018), 2019
We consider a generalised gravitational theory that contains the Ricci scalar curvature and a scalar field coupled to the higher-derivative, quadratic Gauss-Bonnet gravitational term through an arbitrary coupling function f (ϕ). We review both of the existing no-hair theorems, the old and the novel, and show that these are easily evaded; this opens the way for black holes to emerge in the context of this theory. Indeed, we demonstrate that, under mild only assumptions for f (ϕ), we may construct asymptotic solutions that describe either a regular black-hole horizon or an asymptotically-flat solution. We then demonstrate, through numerical integration, that these asymptotic solutions may be smoothly connected and that novel, regular black-hole solutions with non-trivial scalar hair emerge for any form of the coupling function f (ϕ). We present and discuss the physical characteristics of a large number of such solutions for a plethora of coupling functions f (ϕ). Finally, we consider the pure scalar-Gauss-Bonnet theory, under the assumption that the Ricci scalar may be ignored, and we investigate whether novel black-hole solutions may arise in this case.
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