Ice, Ice, Maybe: A Geometric Mechanism for the Hubble Tension

This research explores what happens if the higher-dimensional bulk of a Randall-Sundrum (RS2) model is not an empty vacuum but contains structured matter, and what expansion history that imprints on our brane. The work builds on the modified Friedmann equation of Apostolopoulos and Tetradis, and imposes a power-law bulk density profile, 𝜌5(𝑟)=𝜌0(𝑟0𝑟)𝑛 to show that the resulting brane term scales as 𝑎−𝑛 rather than the 𝑎−4 of empty-bulk dark radiation. For 3<𝑛<4 the component dilutes faster than matter but slower than radiation, with a fractional contribution that peaks near matter-radiation equality. The argument presented is at the level of the background expansion 𝐻(𝑎) only, and the resulting component does not behave like Early Dark Energy, since it dilutes more slowly than radiation after its peak. The piece asks if a suitably structured bulk, with a localised and sharp cut-off, has the potential to produce expansion behaviour relevant to the tension, and whether it could leave a testable 𝑎−𝑛 scaling for future evaluation.