Gravitational baryogenesis beyond the spectator approximation

Abstract: The standard gravitational-baryogenesis operator $λ\,\nabla_μR\,J^μ$, with $λ\equiv ε/M_\ast^{2}$, is usually treated as a spectator interaction that generates an effective chemical potential in a prescribed background. When included in the gravitational action, however, it defines a genuine curvature--matter-coupling variational problem, relevant for the baryon, lepton, and $B!-!L$ currents, whether described microscopically by particle-physics operators or macroscopically by a fluid current $J^μ=n_Xu^μ$. Up to a boundary term the interaction is equivalent to $-λR\nabla_μJ^μ$, making its $f(R,{\rm matter})$ character manifest, but the metric equations remain open unless the metric dependence of $J^μ$ is specified. For an arbitrary local realization $J^μ(Ψ,g)$ we derive the universal part of the field equations and isolate the realization-dependent tensor generated by $δg J^μ$. In the vector-density realization the explicit $J^α\nablaαR$ term cancels, but an algebraic term $-λg_{μν}R\nabla_αJ^α$ survives, so the theory admits only a partial effective-Planck-mass interpretation, $M_{\rm eff}^2=M_{\rm Pl}^2-2λ\nabla_μJ^μ$, and a time-dependent effective gravitational coupling during baryogenesis. Specializing to flat FRW with a homogeneous current $J^μ=n_Xu^μ$, we obtain the modified Friedmann and Raychaudhuri equations, the associated continuity relation, and dimensionless diagnostics that quantify when the spectator approximation is controlled. We also discuss the implications for gravitational-baryogenesis studies in modified theories of gravity, providing a consistent GR-side baseline for implementations in both standard cosmology and modified-gravity frameworks.