How fast can a galaxy build ordered magnetic fields spanning thousands of light-years? Existing theories say several billion years, but observations of galaxies in our universe imply shorter timescales.
In a study published in the Physical Review Letters and highlighted in the Physics magazine, scientists propose an explanation that resolves this contradiction. They say that the collapse of plasma clouds during the formation of galaxies could significantly accelerate the growth of these magnetic fields.
Almost all visible matter in our universe is in the form of plasma, which can be stirred by forces related to gravity, temperature gradients and rotation. If these lead to turbulent flow, the dynamo theory predicts that the existing magnetic fields in the plasma are amplified. The dynamo theory is our primary framework for understanding the origin of cosmic magnetic fields.
"However, dynamo theory has its limitations", says Pallavi, an assistant professor at the International Centre for Theoretical Sciences (ICTS) and an author of the study. "In particular, it struggles to explain observations of young galaxies with robust magnetic fields across thousands of light-years".
The new study explores how dynamos might operate differently during galaxy formation. It considers an ionized gas cloud collapsing under gravity - the stage during which galaxies assemble. "When the galaxy is forming, gravity itself can stir the plasma, which can amplify magnetic fields", says Irshad, a graduate student at ICTS and the lead author of the paper.
Through analytic calculations, the team was able to show that such stirring of plasma that occurs during the collapse can accelerate the formation of magnetic fields. As a result, the observed magnetic fields could be established significantly faster than was earlier thought.
The reason for this, they say, is how the turbulent flow of plasma is altered during the collapse. One characteristic of turbulent flows are eddies, not unlike the swirls we see in water streams. How fast the magnetic field grows depends on the 'turnover rate' of these eddies — how fast they swirl.
The team found that this turnover rate increases as the cloud collapses, leading to an accelerated 'super exponential' growth in magnetic fields, explaining how magnetic fields could have formed faster in the younger galaxies. Through numerical analysis, the team also showed that the field thus formed is stronger than one would expect from the standard dynamo theory.
In their study, the team uses a mathematical framework called 'supercomoving coordinates'. In cosmology, this is used to absorb the expansion of the universe. "These coordinates essentially make the equations of a collapsing galaxy the same as a static galaxy, making the calculations very straightforward", says Irshad. "This works well for a uniformly collapsing spherical system, but we would need to extend this study for more realistic cases".
There's still much to learn in this "zeroth-order question you ask about the timescale" too, says Pallavi. For example, there have been efforts to create computational models for the structure formation in the universe. This study can predict how fast the magnetic fields are set up in the universe, enabling scientists to test and refine their models accordingly, she says.
Although magnetic forces are typically much weaker than gravity in shaping cosmic structures, the new study suggests that strong, ordered magnetic fields may have appeared earlier, slowly nudging the evolution of our universe for longer than we thought.
