How Modified Gravity Explains Our World express
In the first second after the Big Bang, the universe was a boiling cauldron of energy. Particle-antiparticle pairs were born and instantly annihilated each other. But for every billion such pairs, one particle of matter survived, and that leftover became stars, planets, and people.
New research blames modified gravity for this. If spacetime isn't just curved but has an extra geometric feature (like stirring soup with a spatula instead of a spoon), then gravity tips the balance in favor of matter. The key turned out to be the expansion rate, which affects entropy (a measure of disorder). At the observed expansion speed, the model yields the required excess without fine-tuning.
Original abstract
as written by the authors arXiv:2605.02008
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.
The energy released during the annihilation of the first particles was trillions of times brighter than all starlight. We are the ashes of that magnificent fire.