First Star Formation in the Presence of Primordial Magnetic Fields

It has been recently claimed that primordial magnetic fields could relieve the cosmological Hubble tension. Fields of sufficient strength to relieve this tension would result in a magnetic field whose Alfvén velocity, va, is comparable to the speed of sound, cs, at the start of structure formation. We consider the impact of such fields on the formation of the first cosmological objects, minihalos (<106 M⊙), forming stars with zoom-in cosmological simulations tracking a single such minihalo. We seed each simulation with present-day field strengths of 2 × 10−12–2 × 10−10 G corresponding to initial ratios of Alfvén velocity to the speed of sound of va/cs ≈ 0.03−3. We find that when va/cs≪1, the effects are modest. However, when va ∼ cs, the starting time of the gravitational collapse is delayed and the duration extended as much as by Δz = 2.5 in redshift. When va > cs, the collapse is completely suppressed and the minihalos continue to grow and are unlikely to collapse until reaching the atomic cooling limit. Employing current observational limits on primordial magnetic fields we conclude that inflationary-produced primordial magnetic fields could have a significant impact on first star formation.