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

TL;DR

This paper proposes a gravitational baryogenesis mechanism driven by bulk viscosity and particle creation in the early universe, producing the observed matter-antimatter asymmetry within cosmological bounds.

Contribution

It introduces a sign-definite curvature source via thermodynamic irreversibility, analyzing its viability and parameter space for generating baryon asymmetry.

Findings

Baryon asymmetry proportional to viscosity parameter and temperature.

Viable parameter space consistent with cosmological bounds.

High-scale benchmarks depend on reheating scale and tensor limits.

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 $\mathcal{L}_{\rm int}=(c/M^2)\,\partial_\mu R\,J^\mu_{B-L}$ 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.