Imprints of primordial magnetic fields on the late-time Universe

TL;DR

This study uses high-resolution simulations to explore how primordial magnetic fields evolve during gravitational collapse and identifies the scales at which their signatures can survive in the universe today.

Contribution

It demonstrates the importance of resolving the Jeans scale and turbulence to understand the survival of primordial magnetic field signatures in large-scale structures.

Findings

Turbulence during collapse can trigger a small-scale dynamo amplifying magnetic fields.

Dynamo amplification depends on the competition between growth time and free-fall time.

Resolving the Jeans scale is crucial for accurate cosmological MHD simulations.

Abstract

Primordial magnetic fields (PMFs) generated in the early Universe may leave observable imprints in the present-day large-scale structure. However, it remains unclear on which spatial scales primordial signatures can survive the nonlinear processes accompanying structure formation. The aim of this study is to investigate the evolution of PMFs during gravitational collapse and to determine the spatial scales on which primordial signatures can persist. We perform a suite of high-resolution direct numerical simulations of self-gravitating, magnetized halos. By varying the viscosity, we probe different Reynolds-number regimes and follow the coupled evolution of gravitational collapse and magnetohydrodynamic turbulence. At sufficiently high Reynolds numbers, turbulence generated during collapse triggers the onset of a small-scale dynamo, which amplifies magnetic energy below the Jeans scale and modifies the magnetic energy spectrum significantly. Whether dynamo amplification dominates the magnetic field evolution is determined by the competition between the dynamo growth time and the free-fall time. Our results highlight the importance of resolving the Jeans scale and the associated turbulent inertial range in cosmological MHD simulations to accurately capture the interplay between gravitational compression and dynamo amplification and to assess which structures retain memory of primordial fields.