Coherence Length Through a Bounce: The Causal Generation Window That Contraction Opens

A causal generation process can organise a magnetic field only within its horizon. For a field with a present comoving coherence of one megaparsec, this paper computes the latest epoch, in either a contracting or an expanding phase, at which causal generation remains possible: the scale factor must be at least 1.66e-06, corresponding to a temperature of 1.64e+06 kelvin. In a purely expanding history that window closes extremely early and forces generation into the universe's first instants. In a bounced history the same window is open twice, and on the contracting side it is open for a long, cool epoch before the turnaround, at scale factors 1.99e+16 times larger than the turnaround value, where plasma processes have time to act. The turnaround's own horizon, by contrast, is only 1.55e+06 metres comoving, so nothing generated at the turnaround itself explains megaparsec coherence. Coherence is the observable that most cleanly separates what a bounce can do from what it cannot. Computed from stated assumptions, not measured.