D. S. Pereira, B. V. Fernandes, J. P. Mimoso
Abstract
The standard gravitational-baryogenesis operator λ ∇μR Jμ, with λ≡ϵ/M*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 Jμ=nXuμ. Up to a boundary term the interaction is equivalent to −λR∇μJμ, making its f(R, Matter) character manifest, but the metric equations remain open unless the metric dependence of Jμ is specified. For an arbitrary local realization Jμ(Ψ, g) we derive the universal part of the field equations and isolate the realization-dependent tensor generated by δgJμ. In the vector-density realization the explicit Jα∇αR term cancels, but an algebraic term −λgμνR∇αJα survives, so the theory admits only a partial effective-Planck-mass interpretation, Meff2=MPl2−2λ∇μJμ, and a time-dependent effective gravitational coupling during baryogenesis. Specializing to flat Friedmann-Lemaître-Robertson-Walker (FLRW) with a homogeneous current Jμ=nXuμ, 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 General Relativity (GR) baseline for implementations in both standard cosmology and modified-gravity frameworks.
Keywords
Gravity / Baryogenesis / Curvature-matter coupling / Modified gravity / Early universe
Physics Letters B
Volume 877, Number 140471
2026 June
