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We propose a new dark matter contender within the context of the so-called “dark dimension”, an innovative 5-dimensional construct that has a compact space with characteristic length-scale in the micron range. The new dark matter candidate is the radion, a bulk scalar field whose quintessence-like potential drives an inflationary phase described by a 5-dimensional de Sitter (or approximate) solution of Einstein equations. We show that the radion could be ultralight and thereby serve as a fuzzy dark matter candidate. We advocate a simple cosmological production mechanism bringing into play unstable Kaluza–Klein graviton towers which are fueled by the decay of the inflaton.

 

In this short note we comment on the relation between the cosmological and the Kaluza–Klein mass scale in the dark dimension scenario [1], also in view of some recent claims [2] that would raise some doubts about the validity of this scenario. Here we argue that these claims have serious flaws and cannot be trusted.

 

A bstract We argue for a relation between the supersymmetry breaking scale and the measured value of the dark energy density Λ. We derive it by combining two quantum gravity consistency swampland constraints, which tie the dark energy density Λ and the gravitino mass M 3 / 2 , respectively, to the mass scale of a light Kaluza-Klein tower and, therefore, to the UV cut-off of the effective theory. Whereas the constraint on Λ has recently led to the Dark Dimension scenario, with a prediction of a single mesoscopic extra dimension of the micron size, we use the constraint on M 3 / 2 to infer the implications of such a scenario for the scale of supersymmetry breaking. We find that a natural scale for supersymmetry signatures is $$ M=\mathcal{O}\left({\Lambda}^{\frac{1}{8}}\right)=\mathcal{O}\left(\textrm{TeV}\right). $$ M = O Λ 1 8 = O TeV . This mass scale is within reach of LHC and of the next generation of hadron colliders. Finally, we discuss possible string theory and effective supergravity realizations of the Dark Dimension scenario with broken supersymmetry. 

Recently, the idea of using neutrino oscillations to measure the Hubble constant was introduced. We show that such a task is unfeasible because for typical energies of cosmic neutrinos, oscillations average out over cosmological distances and so the oscillation probability depends only on the mixing angles.