The dark geometry of a null extra dimension
We introduce a dynamical formulation of vacuum gravity based on an extra dimension of vanishing proper length. The most general solution to the field equations are presented, leading to an emergent Einsteinian theory associated with a stress-tensor. As an ideal fluid composite, the latter admits a generic decomposition into a unique scalar resembling a running vacuum energy, pure radiation and a vectortensor multiplet exhibiting a bounded equation of state. As one of the most important phenomenological imports of the null extra dimension, this multiplet is shown to provide a potential resolution to the ‘dark matter’ problem. Based on these nonpropagating geometric fields, we suggest an explanation of the stability of the galactic halo and of the flat rotation curves. The proposed theory also predicts the purely axionic and tensorial fraction of the halo mass density, and is open to experimental tests.