Tensor Perturbations from Brane-World Inflation with Curvature Effects

TL;DR

This paper investigates how curvature effects in a modified brane-world inflation model influence primordial gravitational waves, showing suppression of tensor amplitude enhancement and breaking of standard consistency relations at high energies.

Contribution

It introduces a combined Gauss-Bonnet and induced gravity framework in brane-world inflation and analyzes their effects on tensor perturbations and observational signatures.

Findings

Both effects suppress the tensor amplitude enhancement at high energies.

Gauss-Bonnet effect increases the tensor-to-scalar ratio and breaks the consistency relation.

Induced gravity mildly counterbalances the high-energy effects.

Abstract

The brane-world scenario provides an intriguing possibility to explore the phenomenological cosmology implied by string/M theory. In this paper, we consider a modified Randall-Sundrum single brane model with two natural generalizations: a Gauss-Bonnet term in the five-dimensional bulk action as well as an induced gravity term in the four-dimensional brane action, which are the leading-order corrections to the Randall-Sundrum model. We study the influence of these combined effects on the evolution of the primordial gravitational waves generated during an extreme slow-roll inflation on the brane. The background, for the early inflationary era, is then modeled through a de Sitter brane embedded within an anti-de Sitter bulk. In this framework, we show that both effects tend to suppress the Randall-Sundrum enhancement of the amplitude of the tensor perturbations at relatively high energies. Moreover, the Gauss-Bonnet effect, relative to standard general relativity, will abruptly enhance the tensor-to-scalar ratio and break the standard consistency relation at high energies, which cannot be evaded by invoking the induced gravity effect, even though the induced gravity strength would mildly counterbalance these significant changes at high energies. We note that the brane-world model with or without the induced gravity effect fulfills the consistency relation. Finally, we discuss some implications of the observational constraints.