The Blazar Floor: How Weak a Field a Bounce May Deliver to the Voids

The strongest evidence that intergalactic voids are magnetised comes from what is not seen: TeV blazars should be surrounded by extended gamma-ray halos from electromagnetic cascades, and the halos' absence implies void fields of at least about 1e-16 gauss for megaparsec coherence. This paper traces that floor backwards through a bounced cosmology. The floor field at the turnaround was 1.45e+24 tesla, carrying an energy fraction of only 9.53e-22 of the turnaround density. The observationally allowed window for a primordial field, between the blazar floor and the microwave ceiling, spans a factor 4.40e+07 in strength, which becomes a factor 1.94e+15 in permitted energy fraction at the turnaround. A bounce therefore does not need an efficient magnetogenesis mechanism to satisfy the void evidence; it needs a mechanism that clears a remarkably low energy bar while achieving megaparsec coherence, which is where the real difficulty lies. Computed from the published floor and stated scaling laws, not measured.