Warp factor and the gravitational wave spectrum

TL;DR

This paper calculates the gravitational wave spectrum in warped extra dimensions, showing how the warp factor influences the Kaluza-Klein modes and addressing issues near orientifolds to produce a tachyon-free spectrum.

Contribution

It provides a detailed computation of the KK gravitational wave spectrum on warped backgrounds using string theory warp factors, overcoming tachyonic instabilities.

Findings

Warp factor can reduce the first KK mass by at least 69%

Developed tools to eliminate tachyonic modes near orientifolds

Connected spectrum calculations to string compactification models

Abstract

A distinct signature of compact extra dimensions would be a Kaluza-Klein tower of gravitational waves. Motivated by this prospect, we compute the corresponding spectrum on a warped toroidal background. We evaluate in particular the impact of the warp factor on the spectrum. To that end, we use the complete warp factor H of standard string compactifications, generated by D-branes and orientifolds, thus connecting to recent works on stringy de Sitter constructions. The problematic region close to an orientifold where H < 0 leads to unphysical tachyonic modes in the spectrum. We develop tools that overcome this difficulty and lead to a tachyon-free spectrum. We show, in particular, that the warp factor can lower the first Kaluza-Klein mass by at least 69%.