Warped Dimensions at the Cosmological Collider

Extra dimensions are present in many beyond the Standard Model scenarios, most notably in string theory. However, direct signatures of extra dimensions are difficult to observe in many cases. This is the situation, for example, if the energy scales associated with extra dimensions are close to the string or Grand Unification scale. The energetic early universe provides an exciting opportunity to overcome this challenge, since the heavy states associated with high-scale extra dimensions, such as scalar moduli and Kaluza-Klein (KK) gravitons, could have been produced on-shell at early epochs. In this work, we illustrate this by focusing on how such states can be produced during inflation and leave signatures in primordial non-Gaussianity (NG). Specifically, we consider a 5D spacetime with a warped extra dimension that remains stabilized as inflation proceeds in the four non-compact dimensions. By discussing an explicit stabilization mechanism, we compute the masses and couplings of the radion modulus and the KK graviton modes. Being gravitational degrees of freedom, these unavoidably couple to the field(s) generating curvature perturbation, and can lead to observable NG with a distinctive oscillatory shape and characteristic angular dependence. We give example benchmarks which can already be probed by the Planck data and identify targets for the future. Our study shows that cosmological surveys have the potential to observe on-shell imprints of extra dimensions in the coming years.