Two Micron-Size Dark Dimensions

TL;DR

This paper explores the viability of two micron-scale extra dimensions addressing hierarchy problems, analyzing observational bounds, stability conditions, and potential dark matter candidates within such models.

Contribution

It investigates observational constraints and stability conditions for two large extra dimensions of micron scale, proposing a fine-tuned cosmological scenario with primordial black holes as dark matter.

Findings

Micron-scale extra dimensions must lack isometries to satisfy astrophysical bounds.

A fine-tuned temperature is required for cosmological consistency in these models.

Primordial black holes could constitute all dark matter within this framework.

Abstract

Two extra dimensions of micron scale might simultaneously address the gauge and cosmological hierarchy problems. In our paper we examine various observational bounds in scenarios with one and two large extra dimensions, to see if they are compatible with the micron scale. We show that consistency with astrophysical observations requires that two extra dimensions of micron scale must not admit isometries, whereby conservation of the extra dimensional momentum is violated, allowing the massive Kaluza-Klein modes of the graviton to decay to other lighter graviton modes. However, to remain consistent with cosmological observations two extra dimensions of micron scale require a delicately fine tuning of the temperature at which the universe enters the radiation dominated epoch. Diving into this fine-tuned scenario we also show that primordial black holes with masses in the range $10^8 <M_{\rm BH}/{\rm g} <10^{21}$ could make all cosmological dark matter.