The Absolute Ceiling: Backreaction Limits on the Magnetic Field at a Torsion-Bound Bounce

No generation mechanism can produce a magnetic field whose energy density exceeds the total density of the universe at the time of production. Applied at a bounce whose maximum density is fixed by a spin-torsion bound of 9.72e57 kg per cubic metre, that trivial statement becomes a sharp number: the field at the turnaround cannot exceed 4.69e+34 tesla, or 4.69e+38 gauss, even at full equipartition. Diluted adiabatically to the present, a field that saturated the ceiling at the turnaround would measure 3.24e+03 nanogauss today, which is a factor 7.36e+02 above the cosmic microwave background limit of 4.4 nanogauss. Working backwards, present limits require the field at the turnaround to have carried at most a fraction 1.85e-06 of the total density. The ceiling itself is not the operative constraint on primordial fields; the operative constraint is that any mechanism filling more than about two millionths of the bounce energy budget into magnetic form must dispose of the excess before recombination. Computed from stated assumptions, not measured.