Testing string theory with combined cosmological probes: a case study for dark matter gravitons

The string theory Swampland programme has resulted in distinct scenarios for the particle nature of dark matter. In this research note, I use combined cosmological probes to constrain the Dark Dimension scenario, which predicts that dark matter consists of decaying massive gravitons characterised by a time-dependent kick velocity. I first provide updated constraints using a combination of CMB and BAO data. I then produce forecasts for a Stage-IV tomographic survey, and outline strategies for predicting the model's behaviour on nonlinear scales. The results demonstrate the potential of surveys like Euclid and LSST to confirm or rule out string theory models of the dark sector.