Buoyancy vs. Surface-Tension Dominance in Ekpyrotic Brane Convection

Determine whether buoyancy-driven Rayleigh instability or surface-tension-driven Marangoni instability dominates the proposed Rayleigh–Bénard–Marangoni type convection in the thin cosmic fluid confined to the visible brane within the ekpyrotic universe scenario, where uneven heating arises from the collision of the bulk brane, in order to identify the primary driver of the resulting density inhomogeneities.

Background

The paper hypothesizes that, in the ekpyrotic universe scenario where the hot Big Bang results from the collision of two branes, a thin cosmic fluid on the visible brane may undergo Rayleigh–Bénard–Marangoni type convection due to extreme and uneven heating from the colliding bulk brane. Such convection could generate macroscopic density perturbations that act as heavy seeds for early supermassive black hole formation.

In classical fluid dynamics, buoyancy (Rayleigh mechanism) and surface-tension gradients (Marangoni mechanism) are distinct instability drivers that can coexist, with Marangoni effects often dominating in thin layers with free surfaces. The authors explicitly state uncertainty about which mechanism would dominate in the proposed brane-based cosmological setting, making this determination crucial for assessing the viability and characteristics of the hypothesized density perturbation source.