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

Abstract

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 ${\mathcal{L}}_{\mathrm{int}}=(c/M^2){\partial }_{\mu }R{J}_{B-L}^{\mu }$ in a near-radiation background with effective pressure $p_{\mathrm{eff}}=p-3\zeta H$ and $\zeta =\xi \rho /H$. For $\xi >0$ one finds $R\ne 0$, $\dot{R}>0$, and a baryon asymmetry $\eta \propto \xi T_D^5/(M^2{\bar{M}}_{\mathrm{Pl}}^3)$. We derive the viable $(T_D,M,\xi )$ region, include entropy dilution from a finite viscous epoch, and show that the observed ${\eta }_{\mathrm{obs}}\simeq 8.6\times 10^{-11}$ 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 $\xi \sim 10^{-4}$--$10^{-3}$. We also discuss the known higher-derivative instability of gravitational baryogenesis and the role of stabilized or completed embeddings.

Comments: 7 pages, 3 figures, 1 table