DHOST bounce

Abstract

We present a new class of nonsingular bounce cosmology free from instabilities, using a single scalar field coupled to gravity within the framework of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. In this type of scenarios, the gradient instability that widely exists in nonsingular bounce cosmologies in the framework of scalar-tensor and Horndeski/Galileon theories is removed by the effects of new operators introduced by the DHOST, due to the modification that they later bring about to the dispersion relation of perturbations. Hence, our results demonstrate that there is indeed a loophole for this type of bounce scenarios to be free from pathologies when primordial perturbations evolve through the bounce phase, and thus the theoretical no-go theorem for nonsingular bounce cosmology of Horndeski/Galileon theories can be delicately evaded in DHOST extensions.

Emergent universe and Genesis from the DHOST cosmology

“…It has been pointed out that this issue is a generic property for cosmological applications of Galilean/Horndeski theories [67][68][69][70]. We argued that for the DHOST cosmology, this issue is quite model-dependent [45]. Thus, we expect careful model construction of DHOST Genesis may avoid the superluminality issue.…”

“…Now ζ represents the propagating scalar degree of freedom. We have already acquired the form of quadratic action for linear perturbations in our previous work [45] as…”

Cosmology at the top of the α′ tower

“…These solutions are particularly relevant for the early universe as they could potentially explain how the spectrum of cosmological fluctuations observed today were produced (e.g., [54,[82][83][84][85][86][87][88][89][90][91]), thus representing alternatives to inflation. It is known that bouncing solutions in GR are generically singular in a flat FLRW background, so the existence of new physics is typically assumed (e.g., [92][93][94][95][96][97][98][99][100][101] and more references therein) to account for the non-singular behavior of the universe. In our context, the hope is that this new physics is given by the infinite tower of α corrections as motivated by string theory.…”

Notes on the post-bounce background dynamics in bouncing cosmologies

“…In the single bounce models the transition to the standard cosmology is implemented by allowing the equation of state of the bounce matter responsible for the bounce to be larger than 1/3 after the bounce, so that it dilutes faster than regular matter or radiation. Typical examples of this can be found in the matter/ekpyrotic bounce scenarios studied in [10][11][12][13][14][15][16]. In these scenarios the bounce is driven by a scalar field φ with a potential V (φ) of the ekpyrotic form, i.e.…”

“…Bouncing cosmologies can fall into two classes of scenarios (cf. [6]): single bounce models [7][8][9][10][11][12][13][14][15][16] and Cyclic scenarios [17][18][19] (see also [20]). One universal and very important assumption/requirement of all single bounce models is that the late time behavior of the post-bounce evolution must be the same as the evolution of the standard cosmology, in the same vein as the phase after the cosmic reheating in the inflationary cosmology must be the same as the standard big bang phase.…”