We think of magnetic fields as a part of planets and stars. The Earth and Sun have relatively strong magnetic fields, as do more exotic objects such as neutron stars and the accretion disks of black holes. But magnetic field lines also run throughout galaxies, and even between the vast voids of intergalactic space. Magnetic fields are quite literally everywhere, and we aren’t entirely sure why. One idea is that faint magnetic fields formed during the earliest moments of the Universe. If that’s the case, we might be able to prove it through the distribution of dark matter.
The idea of mapping primordial magnetic fields with dark matter is a bit subtle. As far as we know, dark matter only interacts with regular matter gravitationally. It doesn’t interact with magnetic fields, so the mere presence of a magnetic field shouldn’t affect dark matter in any way. But magnetic fields do interact with charged regular matter such as electrons, and those electrons interact with dark matter gravitationally.
So the idea is that intergalactic magnetic fields would tend to cluster electrons and ionized intergalactic hydrogen along their field lines, making those regions of the intergalactic voids just slightly denser than the rest of the void. This would cause dark matter to cluster a bit along the field lines as well. The gravitational effect would be extremely tiny, but over the entire history of the Universe, it would add up. So if primordial magnetic fields did form in the early Universe, tendrils of dark matter should be present along the same lines.
In a recent work in Physical Review Letters the authors argue that this effect would produce minihalos of dark matter. Just as galaxies are surrounded by a halo of dark matter due to gravitational clustering, faint halos of dark matter should exist around primordial magnetic field lines to do the gravitational tug of ionized matter along the field lines.
What’s interesting about this idea is that over time the charged ions and electrons would interact with the primordial magnetic fields and tend to cancel them out. The ions and electrons could even merge to create neutral hydrogen, so in the modern Universe, there would be no trace of these early magnetic fields in regular matter. But the microhalos of dark matter would still exist, and they could be seen through the gravitational lensing of distant light sources. These tendrils of dark matter could be the only evidence remaining of the earliest magnetic fields in the cosmos.
This study is purely theoretical, and current telescopes aren’t sensitive enough to measure the gravitational lensing effect of microhalos. But it’s interesting to see how dark matter can carry the history of the Universe in its structure, even for things that have long faded from view.
Reference: Ralegankar, Pranjal. “Dark Matter Minihalos from Primordial Magnetic Fields.” Physical Review Letters 131 (2023): 231002.