Comments Concerning a Hypothetical Mesoscopic Dark Dimension

TL;DR

This paper explores two possible string theory realizations of a hypothesized dark fifth dimension, discussing their implications for observable 4D spacetime and potential experimental tests.

Contribution

It identifies and compares two distinct string theory models for a dark fifth dimension, expanding the theoretical landscape of possible dark dimension scenarios.

Findings

Two realizations: dark dimension as a circle or a line interval

Implication of a parallel 4D spacetime in the line interval model

Potential for experimental tests of dark dimension hypotheses

Abstract

Motivated by string-theoretic swampland conjectures, the existence of a dark fifth dimension, whose size is roughly 1 -- 10 microns, has been proposed. A great deal of supporting evidence has been presented, and definitive experimental tests are likely to be carried out. The basic idea is that the four-dimensional spacetime that we observe lives on a brane that is localized in the dark dimension. This short note points out that there are two distinct ways to realize such a scenario in string theory/M-theory. In the one considered previously the dark dimension is topologically a circle and our observable 4d spacetime is confined to a brane that is localized in a GUT-scale region of the circle. An alternative possibility is that the dark dimension is a line interval with branes attached at each end. This option would imply the existence of a parallel 4d spacetime microns away from us!