Warp factor and the gravitational wave spectrum
  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%.
