Bianchi-I Cosmology with Radiation in Asymptotically Safe Gravity

Abstract

We study the late-time evolution of an anisotropic Bianchi-I universe with radiation in the framework of asymptotically safe gravity. We first discuss the radiation-dominated universe for the perfect fluid with the equation of state $p=\rho/3$, and find that the classical evolution involves logarithmic terms, which lead to a slow approach toward isotropy. The quantum effects introduce subleading corrections that soften the anisotropy in the intermediate stage. Next we discuss the universe with magnetic fields. For a vanishing classical cosmological constant, we find that the universe in general evolves toward a Kasner-type regime with persistent anisotropy while the expansion rate is enhanced by quantum effects, leading to a faster decay of the magnetic field. In contrast, for a nonzero classical cosmological constant, the late-time dynamics are dominated by the cosmological constant, and the universe asymptotically approaches an isotropic de Sitter phase with exponential decay of both anisotropies and the magnetic field. Finally, we employ Hodge duality to demonstrate that these cosmological findings apply equally to environments dominated by electric fields.