The Observable Window for a Bounced Primordial Field and the Tests That Will Narrow It

A bounced primordial magnetic field currently lives in a wide observational window: stronger than about 1e-16 gauss in the voids, from blazar cascade arguments, and weaker than 4.4 nanogauss on megaparsec scales, from the microwave background, a factor 4.40e+07 in strength. This paper computes how the window will narrow, test by test, under stated assumptions about next-generation instruments, and what each narrowing would mean for the bounce frame. A Square Kilometre Array class rotation-measure grid with residual precision of 0.1 radians per square metre reaches coherent fields of 5.61e-01 nanogauss, the first Faraday test comparable to the microwave bound. Combined with a next-generation blazar floor near 3e-15 gauss, the window shrinks to a factor 1.87e+05 in strength. The paper also registers the decision logic in advance: which combinations of detection and null result would confirm a primordial field in the window, exclude the bounce's permitted range, or point to the destroyed-field histories that gravitational waves address. Computed from stated instrument assumptions, not measured.