The galaxy's magnetic field sets a stricter limit on monopoles

Magnetic monopoles would drain energy from the Milky Way's magnetic field as they pass through it — and the field's survival constrains how many monopoles can exist. By anchoring the calculation in the actual lowest eigenmode of the galactic dynamo and accounting for turbulent fields that both seed that mode and accelerate monopoles before they hit the coherent field, the authors substantially revise the standard extended Parker bound. The result is more internally consistent and more robust to assumptions about primordial magnetic fields, which are either already ruled out by Lyman-alpha data or testable by 21-cm surveys.