The mini-halos of dark matter dispersed throughout the universe could serve as highly sensitive probes of primordial magnetic fields. This was the finding of a theoretical study conducted by the International School of Advanced Studies (SISSA) in Trieste, Italy.
Cosmological Magnetic Fields
Magnetic fields are everywhere in the cosmos, on very large scales. All of the astrophysical objects are embedded in this vector field. Smaller objects, such as planets and stars, generate their magnetic fields from dynamo actions inside them. In these celestial bodies, swirling flows of electrically charged plasma force weak magnetic fields to fold over themselves.
This is also true in the deep space between galaxies and galactic clusters. They are observed inside nebulas, remnants of supernovas, and protoplanetary disks at large scales.
Magnetic fields are weak, typically around a millionth the strength of Earth's magnetic field, or much more vulnerable than those of a refrigerator magnet. However, they are dynamically significant in that they profoundly affect the dynamics of the cosmos. In some cases, they are also tremendous and stretch for millions of light years.
Despite their abundance in the cosmos, the origin of these magnetic fields is still a subject of debate in the scientific community. One possibility is that magnetic fields are primordial, meaning they existed near the beginning of the universe.
Unveiling Cosmic Secrets
In the paper "Dark Matter Minihalos from Primordial Magnetic Fields," the researchers suggest that if magnetic fields are indeed primordial, then they can cause an increase in dark matter density perturbations on small scales. This process would ultimately lead to the formation of mini-halos of dark matter, which, if detected, would provide information on the primordial nature of magnetic fields. In an apparent paradox, the invisible part of the cosmos could be used in resolving the nature of a component of the visible one.
According to research author Pranjal Ralegankar of SISAA, magnetic fields are ubiquitous in the universe. However, the theory that they were produced in the early stages of our universe lacks explanation in the standard physics model. To better understand this aspect, the researchers proposed an indirect approach that would help them detect primordial magnetic fields.
The new method is based on determining the influence of magnetic fields on dark matter. Although there is no known direct interaction between them, there is an indirect one that occurs through gravity.
Ralegankar and his team found that the growth in baryon density gravitationally induces the growth of dark matter perturbation without possible subsequent cancellation. This results in their small-scale collapse, producing mini-halos of dark matter. Although fluctuations in the density of baryonic matter are canceled, they leave traces through the mini-halos, which occur solely through gravitational interactions.
The researchers further noted that their theoretical findings also suggest that mini-halos' abundance is not determined by the current presence of primordial magnetic fields but by their strength in the primordial universe. This means detecting dark matter mini-halos would reinforce the hypothesis that magnetic fields formed very early, even within one second after the Big Bang.
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