Stochastic Particle Acceleration during Pressure-Anisotropy-Driven Magnetogenesis in the Pre-Structure Universe

Abstract

We investigate whether stochastic acceleration associated with pressure-anisotropy-driven magnetogenesis can generate a dynamically significant population of cosmic rays (CRs) prior to nonlinear structure formation. As magnetic fields amplify in the early Universe, the associated increase in gyrofrequency enhances pitch-angle scattering, potentially shortening the stochastic acceleration time. We derive an analytic criterion for efficient cosmological acceleration by comparing the acceleration timescale with the Hubble time, which defines a critical magnetic field and a corresponding CR turn-on redshift $z_{\rm on}$. For representative parameters, we find $z_{\rm on}\sim1.7$. To quantify the resulting particle population, we solve a Fokker-Planck equation for the isotropic ion (proton) distribution in the redshift interval $z=10\rightarrow z_{\rm on}$, including Coulomb energy losses in a fully ionized intergalactic medium. Throughout most of this epoch, adiabatic expansion dominates over stochastic energization, and Coulomb cooling efficiently thermalizes low-energy particles, introducing an effective low-energy threshold at energies of order ${\mathcal O}(10)$ keV. As a result, the distribution remains close to a cooling Maxwellian, and the formation of a suprathermal tail is strongly suppressed even in the presence of a pre-existing nonthermal component. Even under optimistic assumptions corresponding to the strong-scattering limit, the maximum attainable ion energy reaches at most $\mathcal{O}(10^2)$ GeV. These results indicate that efficient CR production in the intergalactic medium is intrinsically tied to the onset of structure-formation shocks, while earlier microinstability-driven stochastic processes can provide at most a modest pre-acceleration.