Planck 2018 Analysis Sets First Limits on Warped Extra-Dimension Signatures via Non-Gaussianity
First dedicated Planck search for KK-graviton and radion non-Gaussianity yields null result below 1.8 sigma while identifying natural 5D parameter space accessible to next-generation CMB experiments. The analysis bridges string-inspired extra-dimension models with concrete bispectrum observables.
The study computes the full bispectrum templates arising from radion and KK-graviton exchange in a five-dimensional warped geometry, then confronts them with Planck data for the first time. No template exceeds 1.8 sigma; the strongest excursion occurs near m_KK approximately 1.6 H. The same framework identifies a viable 5D parameter region that naturally produces f_NL between 1 and 50, placing the signal inside the forecasted sensitivity of CMB-S4 and LiteBIRD.
Prior searches for extra-dimensional particles relied on collider missing-energy signatures or indirect fifth-force bounds, both insensitive to Planck-scale moduli. Cosmological production during inflation evades those limits because the particles couple only gravitationally to the inflaton sector yet leave a scale-dependent bispectrum. The present work therefore supplies the first direct cosmological constraint on this class of models.
The key limitation remains cosmic variance on the largest angular scales together with the restricted range of mass ratios explored; extending the template bank to lower masses and including lensing reconstruction will tighten bounds by a factor of three to five. Future surveys reaching sigma(f_NL) less than 1 will either detect the predicted signal or push the KK scale above 10 H.
CMB-S4: f_NL measurement above 5 from radion exchange within 68 percent CL by 2032 if 5D scale below 3 H
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2607.02651)
- [2]Planck 2018 Results on Non-Gaussianity(https://arxiv.org/abs/1905.05697)
- [3]Warped Extra Dimensions and Inflationary Observables(https://arxiv.org/abs/2003.05448)