Fully viable DHOST bounce with extra scalar

TL;DR

This paper develops a two-field DHOST scalar-tensor theory that provides a stable, non-superluminal bouncing cosmology consistent with observational data, including nearly scale-invariant perturbations and small non-Gaussianities.

Contribution

It introduces the first fully viable two-field DHOST bouncing model that avoids instabilities and superluminality, aligning with observational constraints.

Findings

Stable bouncing solutions without instabilities

No superluminal propagation in the model

Compatibility with observational data on perturbations

Abstract

In this paper we construct a class of Degenerate Higher-Order Scalar-Tensor (DHOST) theories with an extra scalar field, which admits viable solutions of bouncing universe satisfying the following requirements: (i) absence of Belinski-Khalatnikov-Lifshitz (BKL) instability, ghost and gradient instability, (ii) absence of superluminality, (iii) generation of nearly scale-invariant curvature perturbations and very small tensor-to-scalar ratio, and (iv) conventional asymptotics in the distant past and future, where gravity sector is described by General Relativity and the DHOST scalar has a canonical form of Lagrangian. We also expect our models to have sufficiently small non-Gaussianities of primordial curvature perturbations to be compatible with observations. As such, this work exemplifies for the first time the fully viable two-field DHOST bouncing cosmology, which is free of instability and superluminality problems as well as compatible with observations.