Enhanced gravitational scattering from large extra dimensions

TL;DR

This paper explores the potential for enhanced gravitational scattering in models with large extra dimensions to thermalize particles in the early Universe, considering classical and quantum effects and their dependence on extra-dimensional physics.

Contribution

It provides a detailed calculation of relativistic energy transfer rates in models with large extra dimensions, highlighting the impact of quantum effects and trans-Planckian physics.

Findings

Classical gravitational scattering can thermalize ultra-light WIMPs.

Quantum effects significantly reduce scattering efficiency below the Compton wavelength.

Extra-dimensional models may still be sensitive to trans-Planckian physics.

Abstract

We investigate whether enhanced gravitational scattering on small scales (< 0.1mm), which becomes possible in models with large extra dimensions, can establish statistical equilibrium between different particle species in the early Universe. We calculate the classical relativistic energy transfer rate for two species with a large ratio between their masses for a general elastic scattering cross section. Although the classical calculation suggests that ultra-light WIMPs (e.g., axions) can be thermalized by gravitational scattering, such interactions are considerably less efficient once quantum effects are taken into account on scales below the Compton wavelength. However the energy transfer rate in models with several extra dimensions may still be sensitive to trans-Planckian physics.