Cascading dust inflation in Born-Infeld gravity

TL;DR

This paper explores a novel inflationary scenario within Born-Infeld gravity where dust components induce accelerated expansion, leading to a predictive, bounce-capable model with distinctive superinflationary features and no tensor mode excitation.

Contribution

It introduces a dust-driven inflation model in Born-Infeld gravity that naturally ends, includes reheating via unstable dust decay, and predicts a vanishing tensor-to-scalar ratio.

Findings

Inflation is driven by dust components at high energy densities.

The model predicts a small number of e-folds related to dust decay.

Tensor modes are not excited, resulting in zero tensor-to-scalar ratio.

Abstract

In the framework of Born-Infeld inspired gravity theories, which deviates from General Relativity (GR) in the high curvature regime, we discuss the viability of Cosmic Inflation without scalar fields. For energy densities higher than the new mass scale of the theory, a gravitating dust component is shown to generically induce an accelerated expansion of the Universe. Within such a simple scenario, inflation gracefully exits when the GR regime is recovered, but the Universe would remain matter dominated. In order to implement a reheating era after inflation, we then consider inflation to be driven by a mixture of unstable dust species decaying into radiation. Because the speed of sound gravitates within the Born-Infeld model under consideration, our scenario ends up being predictive on various open questions of the inflationary paradigm. The total number of e-folds of acceleration is given by the lifetime of the unstable dust components and is related to the duration of reheating. As a result, inflation does not last much longer than the number of e-folds of deceleration allowing a small spatial curvature and large scale deviations to isotropy to be observable today. Energy densities are self-regulated as inflation can only start for a total energy density less than a threshold value, again related to the species' lifetime. Above this threshold, the Universe may bounce thereby avoiding a singularity. Another distinctive feature is that the accelerated expansion is of the superinflationary kind, namely the first Hubble flow function is negative. We show however that the tensor modes are never excited and the tensor-to-scalar ratio is always vanishing, independently of the energy scale of inflation.