Ice Giant Interiors in the Neptune Class: Layer Densities Against the Density Bound

This paper computes where Neptune-class centre sits relative to the density bound that arises in Einstein-Cartan gravity when the spin of fermionic matter sources torsion. The bound on the neutron branch is 9.72e+57 kilograms per cubic metre, and on the electron branch 2.87e+51 kilograms per cubic metre. The primary system examined, Neptune-class centre, has a density of 1.60e+04 kilograms per cubic metre, which is a fraction 1.65e-54 of the neutron-branch bound — a margin of 53.8 orders of magnitude. The fractional correction that the bound makes to the effective density, and hence to structure quantities built from it, is that same ratio to leading order. Further systems examined here are Neptune-class centre at 1.60e+04 kilograms per cubic metre; icy mantle layer at 3.00e+03 kilograms per cubic metre; hydrogen envelope at 5.00e+02 kilograms per cubic metre. The conclusion is stated plainly: at these densities the correction is present in the equations and absent from every digit that observation or modelling can reach, and this paper quantifies that absence system by system rather than asserting it. The study is computed from stated assumptions; it is not a measurement.