| 61. |
|
|
| 62. |
- Jimenez, Jose Beltran, et al.
(författare)
-
Extended Gauss-Bonnet gravities in Weyl geometry
- 2014
-
Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 31:13, s. 135002-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we consider an extended Gauss-Bonnet gravity theory in arbitrary dimensions and in a space provided with a Weyl connection, which is torsion-free but non-metric-compatible, the non-metricity tensor being determined by a vector field. The action considered consists of the usual Einstein-Hilbert action plus all the terms quadratic in the curvature that reduce to the usual Gauss-Bonnet term for vanishing Weyl connection, i.e., when only the Levi-Civita part of the connection is present. We expand the action in terms of Riemannian quantities and obtain vector-tensor theories. We find that all the free parameters only appear in the kinetic term of the vector field, so two branches are possible: one with a propagating vector field and another one where the vector field does not propagate. We focus on the propagating case. We find that in four dimensions, the theory is equivalent to Einstein's gravity plus a Proca field. This field is naturally decoupled from matter, so it represents a natural dark matter candidate. Also for d = 4, we discuss a non-trivial cubic term in the curvature that can be constructed without spoiling the second-order nature of the field equations, because it leads to the vector-tensor Horndeski interaction. In arbitrary dimensions, the theory becomes more involved. We show that, even though the vector field presents kinetic interactions which do not have U(1) symmetry, there are no additional propagating degrees of freedom with respect to the usual massive case. We show that, interestingly, this relies on the fact that the corresponding Stuckelberg field belongs to a specific class within the general Horndeski theories. Finally, since Weyl geometries provide the natural ground on which to build scale invariant theories, we apply the usual Weyl gauging in order to make the Horndeski action locally scale invariant, and discuss new terms that can be added.
|
|
| 63. |
- Jimenez, Jose Beltran, et al.
(författare)
-
Modified Gravity with Vector Distortion and Cosmological Applications
- 2017
-
Ingår i: Universe. - : MDPI AG. - 2218-1997. ; 3:2
-
Tidskriftsartikel (refereegranskat)abstract
- We briefly review the basics of Weyl geometry and its natural extension by a general linear "distortion" of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl's semi-metricity condition. We present ghost-free gravitational theories in this geometrical setup and highlight their possible cosmological applications, such as new self-tuning solutions and new bouncing solutions found in the quadratic-curvature theories. The vector distortion can mimic the cosmological effects of dark matter.
|
|
| 64. |
- Jimenez, Jose Beltran, et al.
(författare)
-
Spacetimes with vector distortion : Inflation from generalised Weyl geometry
- 2016
-
Ingår i: Physics Letters B. - : Elsevier. - 0370-2693 .- 1873-2445. ; 756, s. 400-404
-
Tidskriftsartikel (refereegranskat)abstract
- Spacetime with general linear vector distortion is introduced. Thus, the torsion and the nonmetricity of the affine connection are assumed to be proportional to a vector field (and not its derivatives). The resulting two-parameter family of non-Riemannian geometries generalises the conformal Weyl geometry and some other interesting special cases. Taking into account the leading nonlinear correction to the Einstein-Hilbert action results uniquely in the one-parameter extension of the Starobinsky inflation known as the alpha-attractor. The most general quadratic curvature action introduces, in addition to the canonical vector kinetic term, novel ghost-free vector-tensor interactions.
|
|
| 65. |
|
|
| 66. |
|
|
| 67. |
|
|
| 68. |
|
|
| 69. |
|
|
| 70. |
|
|
