The Microwave Background Limit Translated to the Bounce: What Planck Allows at the Turnaround

The Planck satellite limits a scale-invariant primordial magnetic field, smoothed over one megaparsec, to 4.4 nanogauss. That number is quoted at the present epoch, but the mechanisms it constrains operated far earlier. This paper translates the limit through a bounced cosmology to the three epochs where it does work. At recombination the limiting field was 5.33e-07 tesla and carried a fraction 2.96e-07 of the total energy density. Against radiation alone, the same limit corresponds to a fraction 1.85e-06, and because magnetic and radiation energy scale identically, that fraction is also the largest magnetic share permitted at nucleosynthesis and, under adiabatic evolution, at the turnaround itself, where the limiting field was 6.36e+31 tesla. The translation exposes a quiet strength of the microwave limit: expressed as a radiation fraction, it is epoch-independent, so a single present-day number disciplines magnetogenesis at every radiation-dominated moment including the bounce. Computed from the published limit and stated scaling laws, not measured.