Aspects of the Dark Dimension in Cosmology

Abstract: It was recently understood that if the swampland conjectures are confronted to experiment they naturally point to a solution of the cosmological hierarchy problem in which the smallness of the dark energy is ascribed to an internal (dark) dimension with characteristic length-scale in the micron range. It was later inferred that the universal coupling of the Standard Model fields to the massive spin-2 Kaluza-Klein (KK) excitations of the graviton in the dark dimension leads to a dark matter candidate. Since the partial decay widths of KK gravitons into the visible sector must be relatively small to accommodate experiment, the model is particularly challenging to probe. We show that the model can accommodate neutrino masses associated to right-handed neutrinos propagating in the bulk of the dark dimension with an additional constraint imposed by neutrino oscillation data. After that, we study the impact of the KK tower in cosmology. We show that the modulation of redshifted 21-cm lines driven by ${\rm KK} \to \gamma \gamma$ could be within the reach of next generation experiments (e.g. SKA and FARSIDE). We also show that indirect dark matter searches could uncover the ${\rm KK} \to \gamma \gamma$ signal. These two observations combined have the potential for model identification. Finally, we explore the global structure of the inflationary phase and demonstrate that the model parameters required for a successful uniform inflation driven by a 5-dimensional cosmological constant (corresponding to a flat region of the 5-dimensional potential) are natural.