A Sexual Cosmos

In biology, a cross between two distantly related species almost always fails. The genomes are too different — the combinations they produce are not viable. If universes reproduced by binary black hole mergers, the same question would arise: are most cosmic pairings fertile, or are most of them barren? This paper audits what the fine-tuning literature tells us about the shape of the viable region of physics-constant space — and finds that the relevant question is unanswered. Fine-tuning arguments establish that the viable region is thin: the constants cannot vary much before something essential breaks. What they leave open is whether the viable boundary is curved in the way that would make most cross-combinations fail. Thinness and curvature are logically independent properties. Coadaptation — the tendency of viable constants to be mutually reinforcing rather than independently constrained — lives in the curvature, not the thinness. Tegmark–Aguirre–Rees–Wilczek (2006), Barnes (2012), and Barr–Khan (2007) characterise the viable boundary in low-dimensional cross-sections. What none provides is the full off-diagonal curvature tensor of the viable boundary as a geometric object in the complete \!30-dimensional parameter space. That gap is precisely where the coadaptation question lives. The paper identifies a constraint that cuts against a rarity reading: in this mechanism, the two merging black holes both come from the same parent universe, so their effective constant-sets differ only by small bounce-mutation increments — siblings, not strangers. Whether sibling divergence is enough to reach the viable boundary's fold scale is an open geometric question, the first well-defined calculation target the programme names. One step is deliberately not taken. Rarity, if it holds, is circumstance. The difficulty of crossing does not summon a bridger. We state what the two-parent reproductive mode implies and stop at that edge. Revision history. SC 1.0 → SC 2.0 (June 2026). Coadaptation was defined precisely as the off-diagonal extrinsic curvature of the viable boundary in the projected constant-aligned frame, distinguishing it from the population-genetics usage. The sibling constraint was reframed as the paper’s central structural limit: the merging black holes share the same parent universe, so their constant-sets differ only by small bounce-mutation increments, not by the large divergence that would make most crossings fail. The headline claim — that most cosmic pairings should be sterile — was retracted as overclaiming; the honest statement is that whether pairings fail is an open geometric question whose answer is not settled by fine-tuning alone.