Horndeski Genesis: strong coupling and absence thereof

Genesis within the Horndeski theory is one of possible scenarios for the start of the Universe. In this model, the absence of instabilities is obtained at the expense of the property that coefficients, serving as effective Planck masses, vanish in the asymptotics t → −∞, which signalizes the danger of strong coupling and inconsistency of the classical treatment. We investigate this problem in a specific model and extend the analysis of cubic action for perturbations (arXiv:2003.01202) to arbitrary order. Our study is based on power counting and dimensional analysis of the higher order terms. We derive the latter, find characteristic strong coupling energy scales and obtain the conditions for the validity of the classical description. Curiously, we find that the strongest condition is the same as that obtained in already examined cubic case.

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“…1 In refs. [29,30], the strong coupling problem in the model of ref. [19] has been addressed at the level of cubic action for perturbations.…”

Section: Introductionmentioning

confidence: 99%

Horndeski genesis: consistency of classical theory

Ageeva

Petrov

Рубаков

2020

J. High Energ. Phys.

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“…[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.…”

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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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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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“…This completely determines Kðπ; XÞ through (16b). Finally, still undetermined functions f 1 ðtÞ, f 2 ðtÞ, f 3 ðtÞ in (16a) are chosen in such a way that the background Einstein equations (18) and (19) are satisfied, and the remaining stability condition G S ≥ F S holds (recall that F S > 0 by the above construction). Einstein equations (18) and (19) enable us to express f 1 ðtÞ and f 2 ðtÞ in terms of already defined functions g 40 , g 41 , f 40 , k 1 and the unknown f 3 ðtÞ as follows:…”

Section: Stable Subluminal Genesis Model: An Examplementioning

confidence: 99%

“…One of the main reasons for going beyond Horndeski theories, at least in the context of early cosmology, is to construct examples of complete, spatially flat, nonsingular cosmological scenarios like the Genesis model. Modulo options that are dangerous from the viewpoint of geodesic completeness and/or strong coupling [16][17][18] (see, however, [19]), Horndeski theories are not suitable for this purpose because of the inevitable development of gradient or ghost instabilities at some stage of the evolution [16,17,20,21]. However, this no-go theorem does not apply to DHOST theories, as demonstrated in Refs.…”

Section: Introductionmentioning

confidence: 99%