Cosmic Cloaking of Rich Extra Dimensions

TL;DR

The paper argues that large or dynamic extra dimensions in the universe are hidden inside black holes, making them difficult to detect, with a threshold size around 10^{-19} meters.

Contribution

It provides a theoretical analysis linking extra dimension size to black hole formation, proposing a size threshold that conceals large extra dimensions from observation.

Findings

Extra dimensions larger than ~10^{-19} meters are hidden inside black holes.

Dynamic or twisted extra dimensions increase the likelihood of gravitational collapse.

Large extra dimensions in the early universe could have been observable.

Abstract

We present arguments that show why it is difficult to see \emph{rich} extra dimensions in the Universe. More precisely, we study the conditions under which significant size and variation of the extra dimensions in a Kaluza-Klein compactification lead to a black hole in the lower dimensional theory. The idea is based on the hoop (or trapped surface) conjecture concerning black hole existence, as well as on the observation that dimensional reduction on macroscopically large, twisted, or highly dynamical extra dimensions contributes positively to the energy density in the lower dimensional theory and can induce gravitational collapse. We analyze these conditions and find that in an idealized scenario a threshold for the size exists, on the order of $10^{-19}m$, such that extra dimensions of length above this level must lie inside black holes, thus shielding them from the view of outside observers. The threshold is highly dependent on the size of the Universe, leading to the speculation that in the early stages of evolution truly macroscopic and large extra dimensions would have been visible.