Cosmological consequences of a scalar field with oscillating equation of state. III. Unifying inflation with dark energy and small tensor-to-scalar ratio

TL;DR

This paper explores a scalar field model unifying inflation and dark energy, predicting an extremely small tensor-to-scalar ratio and suggesting gravitational particle creation as a reheating mechanism, consistent with current observations.

Contribution

It introduces a generalized oscillating dark energy model that naturally predicts a very small tensor-to-scalar ratio, aligning with observational constraints and providing a new perspective on inflation and dark energy unification.

Findings

Predicts tensor-to-scalar ratio r~10^{-7} consistent with Planck data.

Proves r<0.01 in a generalized model, supporting the smallness of primordial gravitational waves.

Suggests gravitational particle creation can reheat the universe after inflation.

Abstract

We investigate the inflationary consequences of the oscillating dark energy model proposed by Ti\'an [\href{https://doi.org/10.1103/PhysRevD.101.063531}{Phys. Rev. D {\bf 101}, 063531 (2020)}], which aims to solve the cosmological coincidence problem with multi-accelerating Universe (MAU). We point out that the inflationary dynamics belong to slow-roll inflation. The spectral index of scalar perturbations and the tensor-to-scalar ratio $r$ are shown to be consistent with current \textit{Planck} measurements. Especially, this model predicts $r\sim10^{-7}$, which is far below the observation limits. This result motivates us to explore the smallness of $r$ in the general MAU. We propose a quintessential generalization of the original model and prove $r<0.01$ in general. The null detection to date of primordial gravitational waves provides a circumstantial evidence for the MAU. After the end of inflation, the scalar field rolls toward infinity instead of a local minimum, and meanwhile its equation of state is oscillating with an average value larger than $1/3$. In this framework, we show that gravitational particle creation at the end of inflation is capable of reheating the Universe.