Gravitational baryogenesis in scalar-nonmetricity $f(Q,\phi)$ gravity

Abstract

In this work, we investigate gravitational baryogenesis in the framework of scalar-nonmetricity theories by considering two classes of modified gravity models, namely $f(Q,\phi)=Q+\xi Q \phi^2$ and $f(Q,\phi)=\alpha Q^n + \beta \phi Q$. These models extend standard $f(Q)$ gravity through the inclusion of nonminimal couplings between the scalar field and the nonmetricity scalar, leading to nontrivial modifications of the cosmological dynamics. We analyze the evolution of the baryon-to-entropy ratio in terms of the cosmic expansion parameter $\gamma$, assuming a power-law behavior of the scale factor. For the first model, we show that the baryon-to-entropy ratio decreases monotonically with increasing $\gamma$, reflecting the impact of the expansion rate on the efficiency of baryogenesis. The observed baryon asymmetry, of order $10^{-11}$ to $10^{-10}$, is successfully reproduced for $\gamma \approx 0.2$--$0.3$ without requiring fine-tuning of the model parameters. For the second model, we explore the parameter space of $\alpha$ and $\beta$, and demonstrate that the correct order of magnitude of the baryon asymmetry can be achieved for physically reasonable values of the parameters. In particular, we find that the baryon-to-entropy ratio lies within observational bounds for $\alpha \sim 10^{-3}$ to $10^{-2}$ and $\beta \sim 10^{-2}$ to $10^{-1}$, with specific combinations yielding excellent agreement with observations. Overall, our results show that scalar-nonmetricity gravity provides a viable and robust framework for explaining the origin of the baryon asymmetry of the Universe. The interplay between nonlinear geometric terms and scalar field couplings plays a crucial role in controlling the baryogenesis mechanism, opening new perspectives in the study of modified gravity and early Universe cosmology.