Cosmology with scalar–Euler form coupling

Toloza, Adolfo; Zanelli, Jorge

doi:10.1088/0264-9381/30/13/135003

Cited by 32 publications

(55 citation statements)

References 52 publications

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“…From the definition of torsion we see that this degree of freedom in both quantities is the same. This degree of freedom propagates to the curvature (a set of 2-forms dependent on 2 tetrad indices) in the form of the Weyl tensor components (24) and (25). It is absent in the metric, and also in the matter content, since the stressenergy τ a forms a set of 3-forms with a single tetrad index.…”

Section: Discussionmentioning

confidence: 99%

Parity violating Friedmann universes

Magueijo

Złośnik

2019

Phys. Rev. D

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We revisit extensions of the Einstein-Cartan theory where the cosmological constant Λ is promoted to a variable, at the cost of allowing for torsion even in the absence of spinors. We remark that some standard notions about FRW Universes collapse in these theories, most notably spatial homogeneity and isotropy may now coexist with violations of parity invariance. The parity violating solutions have non-vanishing Weyl curvature even within FRW models. The presence of parity violating torsion opens up the space of possible such theories with relevant FRW modifications: in particular the Pontryagin term can play an important role even in the absence of spinorial matter. We present a number of parity violating solutions with and without matter. The former are the non-self dual vacuum solutions long suspected to exist. The latter lead to tracking and non-tracking solutions with a number of observational problems, unless we invoke the Pontryagin term. An examination of the Hamiltonian structure of the theory reveals that the parity even and the parity violating solutions belong to two distinct branches of the theory, with different gauge symmetries (constraints) and different numbers of degrees of freedom. The parity even branch is nothing but standard relativity with a cosmological constant which has become pure gauge under conformal invariance if matter is absent, or a slave of matter (and so not an independent degree of freedom) if non-conformally invariant matter is present. In contrast, the parity violating branch contains a genuinely new degree of freedom.

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Section: Discussionmentioning

confidence: 99%

Parity violating Friedmann universes

Magueijo

Złośnik

2019

Phys. Rev. D

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“…When V = const., this last term becomes a topological invariant proportional to the Euler characteristic and does not contribute to the field dynamics in the bulk, although it becomes relevant in the regularization of Noether charges for asymptotically locally anti-de Sitter spacetimes [82,83]. In the general case, namely V = const., this term contributes to the field equations acting as a source of torsion [51,[58][59][60][61][62][63][64]66]. The particular nonminimal coupling with the GB term we use is but one choice; the results regarding the speed of GWs are still valid even if the 1/ (Λ + κ 4 V ) coupling is replaced by an arbitrary function of (the magnitude of) the scalar field, f (|φ|).…”

Section: Scalar-tensor Model With Gauss-bonnet Couplingmentioning

confidence: 99%

“…When scalar fields are coupled to the Nieh-Yan topological invariant [52], a regularization procedure of the axial anomaly in RC spacetimes can be prescribed [53][54][55][56][57], and a torsion-descendent axion that might solve the strong CP problem in a gravitational fashion is predicted [58][59][60]. The nonminimal coupling to the Gauss-Bonnet invariant, on the other hand, can be motivated from dimensional reduction of stringgenerated gravity models [61], and it could drive the latetime acceleration of the Universe in the absence of the cosmological constant [62][63][64]. The first-order formulation of Chern-Simons modified gravity produces interesting phenomenology when coupled with fermions [65], and it has been shown that the different nonminimal couplings support four-dimensional black string configurations in vacuum, possessing locally AdS 3 × R geometries with nontrivial torsion [66].…”

Section: Introductionmentioning

confidence: 99%

Luminal propagation of gravitational waves in scalar-tensor theories: The case for torsion

et al. 2019

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Scalar-tensor gravity theories with a nonminimal Gauss-Bonnet coupling typically lead to an anomalous propagation speed for gravitational waves, and have therefore been tightly constrained by multimessenger observations such as GW170817/GRB170817A. In this paper we show that this is not a general feature of scalar-tensor theories, but rather a consequence of assuming that spacetime torsion vanishes identically. At least for the case of a nonminimal Gauss-Bonnet coupling, removing the torsionless condition restores the canonical dispersion relation and therefore the correct propagation speed for gravitational waves. To achieve this result we develop a new approach, based on the first-order formulation of gravity, to deal with perturbations on these Riemann-Cartan geometries.topological invariants, e.g., the Pontryagin or Gauss-Bonnet (GB) terms, motivated by effective field theories, string theory, and particle physics [15]. From a phenomenological viewpoint, the scalar-Pontryagin modification to GR-also known as Chern-Simons modified gravity-is an interesting extension that might explain the flat galaxy rotation curves dispensing with dark matter [16], while leaving the propagation speed of GWs unaffected [17]. This interaction generates nontrivial effects when rotation is included [18][19][20][21][22][23], providing a smoking gun in future GW detectors [24][25][26][27]. The couplings between scalar fields and the GB term, on the other hand, have been studied in different setups and several solutions that exhibit spontaneous scalarization have been reported [28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Their stability, however, depends on the choice of the coupling between the scalar field and the GB term [42][43][44]. In spite of this, the theory is experimentally disadvantaged from an astrophysical viewpoint, since it develops an anomalous propagation speed for GWs [45].Scalar-tensor theories have been formulated in geometries that depart from the pseudo-Riemannian framework several times in the past. In particular, the gravitational role of Riemann-Cartan (RC) geometries, characterized by curvature and torsion, was first discussed by Cartan and Einstein themselves [46], and later on in the framework of gauge theories of gravitation [47][48][49][50]. Within its simplest formulation-the Einstein-Cartan-Sciama-Kibble (ECSK) theory-torsion is a nonpropagating field sourced only by the spin density of matter. The nonminimal coupling of scalar fields to geometry dramati-arXiv:1910.00148v3 [gr-qc]

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“…[38]). More recently, this term has been show to lead to non-trivial torsional dynamics, and even more, it seems to have interesting consequences in cosmology [39,40].…”

Section: Discussionmentioning

confidence: 99%