Primordial Power Spectra of EiBI Inflation in Strong Gravity Limit

TL;DR

This paper analyzes how strong gravity effects in EiBI theory modify scalar and tensor perturbations during inflation, showing potential suppression of tensor modes and making the $\

Contribution

It demonstrates that strong EiBI gravity can significantly alter tensor perturbations, providing a new perspective on inflationary predictions in modified gravity theories.

Findings

Tensor spectrum can be suppressed to zero in strong EiBI gravity.

Scalar spectrum remains largely unchanged compared to GR.

Tensor-to-scalar ratio can be reduced, making the model compatible with observations.

Abstract

We investigate the scalar and the tensor perturbations of the $\varphi^2$ inflation model in the strong-gravity limit of Eddington-inspired Born-Infeld (EiBI) theory. In order to consider the strong EiBI-gravity effect, we take the value of $\kappa$ large, where $\kappa$ is the EiBI theory parameter. The energy density of the Universe at the early stage is very high, and the Universe is in a strong-gravity regime. Therefore, the perturbation feature is not altered from what was investigated earlier. At the attractor inflationary stage, however, the feature is changed in the strong EiBI-gravity limit. The correction to the scalar perturbation in this limit comes mainly via the background matter field, while that to the tensor perturbation comes directly from the gravity ($\kappa$) effect. The change in the value of the scalar spectrum is little compared with that in the weak EiBI-gravity limit, or in GR. The form of the tensor spectrum is the same with that in the weak limit, but the value of the spectrum can be suppressed down to zero in the strong limit. Therefore, the resulting tensor-to-scalar ratio can also be suppressed in the same way, which makes $\varphi^2$ model in EiBI theory viable.