Primordial Correlators from a Kaluza-Klein Graviton Continuum

Abstract

Cosmological collider signals are usually discussed for isolated massive particles, whose exchange produces characteristic logarithmic oscillations in primordial correlators. In this work, we study how this signal is modified when the exchanged states form a continuous mass spectrum. We first develop a spectral representation for inflationary correlators mediated by a continuum field. In the soft limit, the non-analytic part of the seed function is expressed as a Fourier--Laplace transform of the spectral weight with respect to logarithmic momentum variables. This representation shows that a continuum superposes clock signals with different frequencies and can dephase the sharp oscillation associated with a single massive particle. We then realize this mechanism in an RS2-like inflationary braneworld, where the inflaton is localized on a de Sitter brane in a five-dimensional AdS bulk. The tensor sector contains a localized massless graviton and a continuum of Kaluza-Klein gravitons starting at \(m=3H/2\). We derive the KK wavefunctions and identify the brane spectral weight fixed by the continuum wavefunction on the brane. Applying this weight to the inflaton four-point function, we find a smooth seed function rather than a persistent logarithmic clock oscillation. This behavior follows from the fact that the continuum starts at zero clock frequency, while the KK spectral weight vanishes near threshold. Our results provide a concrete higher-dimensional example in which deviations from the standard cosmological collider signal encode the structure of a continuum high-energy spectrum.