We don’t know how magnetic fields were formed. Now new theoretical research tells how the invisible part of our Universe could help us discover it, suggesting a primordial genesis, even within a second of the big Bang.
Dark matter minihalos scattered across the Cosmos could function as highly sensitive probes of primordial magnetic fields. This is what emerges from a theoretical study carried out by SISSA and published in the journal Physical examination letters.
Present on immense scales, magnetic fields are found everywhere in the Universe. However, its origins remain a subject of debate among scholars. An intriguing possibility is that the magnetic fields originated near the birth of the universe itself, that is, that they are primordial magnetic fields.
In the study, the researchers showed that if the magnetic fields are truly primordial, they could cause an increase in small-scale dark matter density perturbations. The final effect of this process would be the formation of dark matter minihalos which, if detected, would suggest a primordial nature of the magnetic fields.
Thus, in an apparent paradox, the invisible part of our Universe could be useful to resolve the nature of a component of the visible one.
Shedding light on the formation of magnetic fields
“Magnetic fields are omnipresent in the Cosmos,” explains Pranjal Ralegankar of SISSA, author of the research. “A possible theory about its formation suggests that those observed so far could occur in the early stages of our Universe. However, this proposition lacks explanation in the standard model of physics.
To shed light on this aspect and find a way to detect “primordial” magnetic fields, with this work we propose a method that we could define as “indirect”. Our approach is based on one question: What is the influence of magnetic fields on dark matter? It is known that there is no direct interaction. Still, as Ralegankar explains, “there is an indirect one that occurs through gravity.”
Straight from the primordial universe
Primordial magnetic fields can enhance electron and proton density perturbations in the primordial Universe. When these become too large, they influence the magnetic fields themselves. The consequence is the suppression of small-scale fluctuations.
Ralegankar explains: “In the study we showed something unexpected. The growth of baryon density gravitationally induces the growth of dark matter perturbations without the possibility of subsequent cancellation. This would cause its collapse on a small scale, producing minihalos of dark matter.”
The consequence, the author continues, is that even if fluctuations in the density of baryonic matter are cancelled, they would leave traces through the minihaloes, all of this solely through gravitational interactions.
“These theoretical findings,” concludes Pranjal Ralegankar, “also suggest that the abundance of minihalos is not determined by the current presence of primordial magnetic fields but rather by their strength in the primordial Universe. Therefore, the detection of dark matter minihalos would reinforce the hypothesis that magnetic fields formed very early, even a second after the Big Bang.”
Reference: “Dark Matter Minihalos from Primordial Magnetic Fields” by Pranjal Ralegankar, December 8, 2023, Physical examination letters.