Bouncing Cosmology in VCDM

TL;DR

This paper presents a bouncing cosmology model within VCDM, a modified gravity theory, that avoids instabilities and matches cosmological observations, offering an alternative to inflation with specific spectral properties.

Contribution

It constructs a stable asymmetric bouncing scenario in VCDM that generates observationally consistent cosmological perturbations without instabilities or ad-hoc conditions.

Findings

Stable bouncing scenario without ghost or gradient instabilities

Scalar spectral index can be tuned for near scale-invariance

Tensor spectrum is blue when matching CMB tensor-to-scalar ratio bounds

Abstract

We construct an asymmetric bouncing scenario within the VCDM model - also known as type-II minimally modified gravity -, a modified gravity theory with two local physical degrees of freedom. The scenario is exempt of any ghost or gradient instability, ad-hoc matching conditions or anisotropic stress issue (BKL instability). It moreover succeeds in generating the cosmological perturbations compatible with the observations. The scalar spectral index can be adapted by the choice of the equation of state of the matter sector and the form of the VCDM potential leading to an almost scale-invariant power spectrum. Satisfying the CMB bounds on the tensor-to-scalar ratio leads to a blue tensor spectrum.