Abstract
It is of interest to understand whether or not one can construct a classical field theory description of early cosmology which would be free of the initial singularity and stable throughout the whole evolution. One of the known possibilities is genesis within the Horndeski theory, which is thought to be an alternative to or a possible completion of the inflationary scenario. In this model, the strong coupling energy scale tends to zero in the asymptotic past, $t \to - \infty$, making the model potentially intractable. We point out that despite the latter property, the classical setup may be trustworthy since the energy scale of the classical evolution (the inverse of its timescale) also vanishes as $t \to -\infty$. In the framework of a concrete model belonging to the Horndeski class, we show that the strong coupling energy scale of the cubic interactions vastly exceeds the classical energy scale in a certain range of parameters, indicating that the classical description is possible.
Highlights
The price to pay is that F T, GT, F S, and GS vanish in the asymptotic past, which may signalize the strong coupling problem coming from either a scalar, tensor, or mixed scalar-tensor sector
We have studied the nonsingular genesis scenario in the framework of the Horndeski theory, which is capable of avoiding the gradient instability at the expense of potential strong coupling problem
We have seen that with the additional restrictions on α and δ, the classical genesis solution is away from the strong coupling regime inferred from the study of all cubic interactions of metric perturbations: scalar, mixed and tensor
Summary
Considerable attention is attracted to cosmological scenarios without an initial classical singularity. The latter property enables one to construct completely healthy genesis models in beyond Horndeski theories [23,26] [29]) that this is the case: the strong coupling energy scale vanishes in the asymptotic past.1 At first sight, the latter property makes the whole construction intractable. We consider genesis in a restricted class of Horndeski theories These genesis constructions were suggested in Ref. While in the main text we work exclusively in the Jordan frame, some new insight is obtained by going to the Einstein frame; this aspect has to do with the peculiarities of strong coupling in the case of a singular metric
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