The Retroactive Door

Mesogenesis generates the baryon asymmetry of the universe and dark matter simultaneously through meson decays in the early universe. The mechanism requires heavy mesons to decay into both visible baryons and dark sector particles. D-mesons (containing charm quarks) are natural candidates, but proton lifetime constraints seemingly rule out D-meson mesogenesis: the same interactions that allow D-mesons to produce dark sector particles would mediate proton decay at rates exceeding experimental bounds.

Baruch, Elor, Goldberg, Shtaif, and Soreq (arXiv:2603.28330, March 2026) circumvent this constraint not by weakening the interaction but by changing the mass spectrum after baryogenesis occurs. A late-time phase transition in the dark sector shifts the masses of dark sector particles. Before the transition, the decay channels from D-mesons to dark particles are kinematically open — the dark particles are light enough to be produced. After the transition, the dark particles become heavier, and the same decay channels become kinematically forbidden.

The proton lifetime constraint applies at the present epoch — it measures whether protons can decay now, through the interactions that exist today. After the phase transition, the dark particle masses have changed, and the proton decay channels that would have been open are now closed. The constraint evaporates because the final state that the proton would decay into no longer exists at accessible energies. The interaction responsible for baryogenesis is still present in the Lagrangian, but the phase space for the dangerous process has been removed.

The structural observation: a constraint that applies at one epoch can be evaded by a phase transition that changes the mass spectrum at a later epoch. The door through which baryogenesis occurred is retroactively closed by a cosmological event that occurs afterward. The constraint is not violated — it genuinely does not apply, because the physical state it constrains has ceased to exist. The mechanism is temporal: the same physics that is required early is forbidden late, and the transition between the two regimes is the phase transition itself.