# Viable tensor-to-scalar ratio in a symmetric matter bounce

## TL;DR

This paper presents a symmetric matter bounce model with two scalar fields that produces a small tensor-to-scalar ratio compatible with current cosmological observations, addressing previous challenges in such scenarios.

## Contribution

The authors construct a symmetric matter bounce model using two scalar fields and demonstrate it yields a low tensor-to-scalar ratio consistent with observational data.

## Key findings

- Scalar and tensor spectra are scale invariant.
- Tensor-to-scalar ratio is much smaller than observational upper bounds.
- Model depends on a single parameter related to the bounce scale.

## Abstract

Matter bounces refer to scenarios wherein the universe contracts at early times as in a matter dominated epoch until the scale factor reaches a minimum, after which it starts expanding. While such scenarios are known to lead to scale invariant spectra of primordial perturbations after the bounce, the challenge has been to construct completely symmetric bounces that lead to a tensor-to-scalar ratio which is small enough to be consistent with the recent cosmological data. In this work, we construct a model involving two scalar fields (a canonical field and a non-canonical ghost field) to drive the symmetric matter bounce and study the evolution of the scalar perturbations in the model. If we consider the scale associated with the bounce to be of the order of the Planck scale, the model is completely described in terms of only one parameter, viz the value of the scale factor at the bounce. We evolve the scalar perturbations numerically across the bounce and evaluate the scalar power spectra after the bounce. We show that, while the scalar and tensor perturbation spectra are scale invariant over scales of cosmological interest, the tensor-to-scalar ratio proves to be much smaller than the current upper bound from the observations of the cosmic microwave background anisotropies by the Planck mission. We also support our numerical analysis with analytical arguments.