Warped Dimensions at the Cosmological Collider

TL;DR

This work presents a warped 5D framework with a stabilized radion that yields a KK graviton tower and a radion with masses $m_{\varphi}, m_1\sim H$ during inflation. In the cosmological collider picture, both radion-mediated spin-0 and KK-graviton-mediated spin-2 NG arise, with amplitudes suppressed by large-$N$ and Boltzmann factors, yet potentially observable in current Planck data or future 21-cm surveys for suitable parameter choices. The analysis combines 5D gravity with a 4D CFT perspective, showing that an IR-localized curvaton can maximize NG while maintaining EFT control, whereas the inflaton is UV-localized. Overall, the results demonstrate that cosmological NG measurements can probe the presence of high-scale extra dimensions, linking UV completions to observable primordial signals.

Abstract

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.

Figures (4)

• Figure 1: Figures showing the Goldberger-Wise field profile (top) and warp factor (bottom) for the benchmark parameter points in eqs. \ref{['eq:B1']} and \ref{['eq:B2']}. Units are set so that $k=1$. The grey dashed lines in the bottom plots show the $H=0$ warp factor, which deviate from $n_0(y)$ at large $y$. We observe that, except in a region close to the IR boundary, $n_1$ represents a small correction to $n_0$.
• Figure 2: Plots showing the extra dimensional profiles for the first 3 KK modes for each of the benchmark points. The co-ordinate used is the homogeneous co-ordinate $z$, with the metric given in \ref{['eq:metric_z']}. Units are such that $k=1$.
• Figure 3: In-in diagrams showing the radion ($\varphi$)-mediated (left) and KK graviton ($\tilde{h}_{\mu\nu,l}$) mediated (right) trispectra. Time flows vertically upwards and the horizontal lines denote the end of inflation, parametrized by $t_0$. Both the radion and the KK graviton decay into curvaton ($\sigma$) fluctuations.
• Figure 4: Trispectrum mediated by the radion (top) and the KK graviton (bottom). The oscillations capture the on-shell production of the radion and the KK graviton during inflation.