An article published in Physical Review Letters suggests that the collapse of plasma clouds and turbulent flows greatly accelerated the growth of ordered magnetic fields, potentially resolving the discrepancy between conventional models and observations of young galaxies.
Galactic magnetic fields can appear surprisingly quickly, due to plasma collapse and turbulent flows.
How quickly can a galaxy develop ordered magnetic fields that extend across thousands of light-years? According to traditional models, this process takes billions of years, but observations of real galaxies indicate much faster timelines.
A study published in Physical Review Letters suggests a possible solution to this discrepancy. When plasma clouds collapse during galaxy formation, they can rapidly accelerate the development of magnetic fields.
Most of the visible matter in the universe exists as plasma, a state that responds to forces such as gravity, temperature differences, and rotation. These forces can create turbulent flows, and under such conditions, the dynamo theory predicts that existing magnetic fields become stronger. This theory is still the main tool that scientists use to explain how cosmic magnetic fields are created.
"But the dynamo theory has limitations," says Pallavi, one of the study's authors. "In particular, it has difficulty explaining observations of young galaxies with strong magnetic fields that extend across thousands of light-years."
A new perspective on galaxy formation
The study examines how dynamo processes could have behaved differently in the early stages of galaxy formation. It focuses on a coronal cloud of ionized gas, an important stage in galaxy formation. “When a galaxy forms, gravity itself can spin the plasma, and that can amplify magnetic fields,” says Irshad, the study’s lead author.
Using analytical methods, the researchers show that this gravitational motion can accelerate the development of magnetic fields. Their results suggest that these fields could have formed much earlier than previously thought.
They attribute this effect to changes in turbulent motion during the collapse. The turbulence creates swirling patterns called eddies, similar to currents in flowing water. The rate at which the magnetic fields grow depends on the speed at which these eddies rotate, known as the overturning rate.
As the cloud shrinks, the rate of overturning increases. The team found that this causes a “super-exponential” increase in magnetic strength, which could explain the strong magnetic fields seen in young galaxies. Their numerical results also suggest that these fields can become stronger than predicted based on standard dynamo theory.
The mathematical framework and limitations
To perform the analysis, the researchers used a mathematical approach called “supercomoving coordinates.” In cosmology, this method takes into account the expansion of the universe.
"Using these coordinates, the equations of a collapsing galaxy become similar to a static galaxy, and this greatly simplifies the calculations," says Irshad. "This works well in a spherical system that collapses uniformly, but we will need to extend this research to more realistic cases."
Despite these insights, important questions remain. “There’s still a lot to learn, even on this fundamental question of timescales,” says Pallavi. Scientists are developing computational models to simulate how structures form in the universe, and this study provides predictions that could help test and improve those models.
Magnetic forces are usually much weaker than gravity in shaping objects in the universe, but the findings suggest that strong, ordered magnetic fields may have emerged earlier than expected. Over time, these fields could have played a subtle but persistent role in guiding the evolution of the universe.
for the scientific article DOI: 10.1103/fp1v-xrr5
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