Sign-Locked Gravitational Baryogenesis from Bulk Viscosity and Cosmological Particle Creation

We study a concrete realization of gravitational baryogenesis in which a small bulk-viscous deformation of an otherwise radiation-dominated early universe generates a sign-definite curvature source. The key point is thermodynamic irreversibility: positive entropy production makes the driving term monotonic and therefore avoids the freeze-out cancellation that suppresses rapidly oscillating or sign-changing sources. Motivated by a simple first-order transfer-function diagnostic, we analyze the standard curvature-current operator $\mathcal{L}_{\rm int}=(c/M^2)\,\partial_\mu R\,J^\mu_{B-L}$ in a near-radiation background with effective pressure $p_{\rm eff}=p-3\zeta H$ and $\zeta=\xi \rho/H$. For $\xi>0$ one finds $R\neq 0$, $\dot R>0$, and a baryon asymmetry $\eta \propto \xi T_D^5/(M^2 \bar M_{\rm Pl}^3)$. We derive the viable $(T_D,M,\xi)$ region, include entropy dilution from a finite viscous epoch, and show that the observed $\eta_{\rm obs}\simeq 8.6\times10^{-11}$ can be reproduced in a parameter region consistent with current cosmological bounds while maintaining EFT control. The highest-scale benchmarks should be read conditionally on a very high reheating scale in view of current tensor limits. A particle-creation sector of heavy GUT-scale fields then provides a phenomenological motivation for the required range $\xi\sim10^{-4}$--$10^{-3}$. We also discuss the known higher-derivative instability of gravitational baryogenesis and the role of stabilized or completed embeddings.