A covariant Lagrangian for stable nonsingular bounce

Cai, Yong; Piao, Yun-Song

doi:10.1007/jhep09(2017)027

Cited by 100 publications

(83 citation statements)

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“…[24,25,3] for a subclass usually referred to as "beyond Horndeski" (aka GLVP [9]). Indeed, this subclass has been used for constructing non-singular cosmological models of the bouncing Universe and Genesis, which are stable at the linearised level during entire evolution [3,26,27].Previous constructions of complete bouncing and Genesis models in beyond Horndeski theories were limited by overestimating the danger of a phenomenon called γ-crossing (or Θ-crossing). The discussion of this phenomenon is fairly technical, and we postpone it to Section 2.…”

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confidence: 99%

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Bounce beyond Horndeski with GR asymptotics and γ-crossing

Mironov

Rubakov

Volkova

2018

J. Cosmol. Astropart. Phys.

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We suggest a novel version of a cosmological Genesis model within beyond Horndeski theory. It combines the initial Genesis behavior of Creminelli et al. [1,2] with complete stability property of the previous beyond Horndeski construction [3]. The specific features of the model are that space-time rapidly tends to Minkowski in asymptotic past and that both asymptotic past and future are described by General Relativity (GR). 1 sa.mironov 1@physics.msu.ru 2 rubakov@inr.ac.ru 3 volkova.viktoriya@physics.msu.ru 1 arXiv:1905.06249v1 [hep-th] 15 May 2019Horndeski theories are general scalar-tensor gravities with second order equations of motion. These have been further generalised to theories with higher order equations of motion, dubbed DHOST theories [8,9,10,11,12,13]. The constraint structure of the DHOST theories is such that they propagate only three dynamical degrees of freedom, just like Horndeski theories. Horndeski theories and their generalizations are interesting playground for studying stable NEC/NCC-violating cosmologies (for a review see, e.g., Ref. [14]), and Genesis in particular [15,16,17].One of the main reasons for going beyond Horndeski, at least in the context of the early cosmology, is to construct examples of complete spatially flat, non-singular cosmological scenarios like Genesis. Modulo options that are dangerous from the viewpoint of geodesic completeness and/or strong coupling [18,19,20] (see, however [21]), Horndeski theories are not suitable for this purpose because of inevitable development of gradient or ghost instabilities at some stage of the evolution [18,19,22,23]. However, this no-go theorem does not apply to DHOST theories, as demonstrated in Refs. [24,25,3] for a subclass usually referred to as "beyond Horndeski" (aka GLVP [9]). Indeed, this subclass has been used for constructing non-singular cosmological models of the bouncing Universe and Genesis, which are stable at the linearised level during entire evolution [3,26,27].Previous constructions of complete bouncing and Genesis models in beyond Horndeski theories were limited by overestimating the danger of a phenomenon called γ-crossing (or Θ-crossing). The discussion of this phenomenon is fairly technical, and we postpone it to Section 2. It suffices to point out here that insisting on the absence of γ-crossing prevents one from constructing bounce and Genesis models where linearized gravity agrees with GR both in asymptotic future and in asymptotic past, and, in Genesis case, whose space-time rapidly tends to Minkowski in asymptotic past. An example is a Genesis-like model of Ref. [3] where the scale factor behaves as a(t) ∝ |t| −1/3 as t → −∞.It has been shown, however, that γ-crossing is, in fact, an innocent phenomenon. Originally, this fact has been established in Newtonian gauge [28], and then confirmed in unitary gauge [27]. It opens up a possibility to construct new bouncing and Genesis models 4 . Indeed, an example of fully stable spatially flat bouncing model has been constructed in beyond Horndeski theory [27], whose ...

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confidence: 99%

Bounce beyond Horndeski with GR asymptotics and γ-crossing

Mironov

Rubakov

Volkova

2018

J. Cosmol. Astropart. Phys.

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“…Let us demonstrate explicitly that our solution satisfies our subset of stability conditions. We have arranged the Lagrangian functions so that H = G = F = 1, which automatically ensures that the parity odd modes obey (26)- (28) and (30), (31). As for the parity even modes, it follows from eqs.…”

Section: Wormhole Beyond Horndeski: An Examplementioning

confidence: 99%

More about stable wormholes in beyond Horndeski theory

Mironov

Рубаков

Volkova

2019

Class. Quantum Grav.

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It is known that Horndeski theories, like many other scalar-tensor gravities, do not support static, spherically symmetric wormholes: they always have either ghosts or gradient instabilities among parity-even linearized perturbations. Here we address the issue of whether or not this no-go theorem is valid in "beyond Horndeski" theories. We derive, in the latter class of theories, the conditions for the absence of ghost and gradient instabilities for non-spherical parity even perturbations propagating in radial direction. We find, in agreement with existing arguments, that the proof of the above no-go theorem does not go through beyond Horndeski. We also obtain conditions ensuring the absence of ghosts and gradient instabilities for all parity odd modes. We give an example of beyond Horndeski Lagrangian which admits a wormhole solution obeying our (incomplete set of) stability conditions. Even though our stability analysis is incomplete, as we do not consider spherically symmetric parity even modes and parity even perturbations propagating in angular directions, as well as "slow" tachyonic instabilities, our findings indicate that beyond Horndeski theories may be viable candidates to support traversable wormholes. 1 sa.mironov 1@physics.msu.ru 2 rubakov@inr.ac.ru 3 volkova.viktoriya@physics.msu.ru 1 arXiv:1812.07022v2 [hep-th] 5 Jun 2019 Bπ XK Xwith

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“…Motivated by this, alternative scenarios which do not suffer from this problem have also been explored (see, e.g., [5] for a review). Non-singular cosmology has its own difficulty regarding gradient instabilities when constructed within second-order scalar-tensor theories [6][7][8][9][10], but its resolution has been proposed in the context of higher-order scalar-tensor theories [8,9,[11][12][13][14][15]. It is also important to discuss the validity of non-singular alternatives from the viewpoint of cosmological observations.…”

Section: Introductionmentioning

confidence: 99%

Primordial non-Gaussianities of scalar and tensor perturbations in general bounce cosmology: Evading the no-go theorem

2020

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It has been pointed out that matter bounce cosmology driven by a k-essence field cannot satisfy simultaneously the observational bounds on the tensor-to-scalar ratio and non-Gaussianity of the curvature perturbation. In this paper, we show that this is not the case in more general scalartensor theories. To do so, we evaluate the power spectra and the bispectra of scalar and tensor perturbations on a general contracting background in the Horndeski theory. We then discuss how one can discriminate contracting models from inflation based on non-Gaussian signatures of tensor perturbations.

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A covariant Lagrangian for stable nonsingular bounce

Cited by 100 publications

References 87 publications

Bounce beyond Horndeski with GR asymptotics and γ-crossing

Bounce beyond Horndeski with GR asymptotics and γ-crossing

More about stable wormholes in beyond Horndeski theory

Primordial non-Gaussianities of scalar and tensor perturbations in general bounce cosmology: Evading the no-go theorem

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