If these magnetic fields exist, it could settle a debate over the universe's expansion rate

April 24, 2021
Do ancient magnetic fields that hold the universe together exist? It's possible. (Sergio Martin Alvarez)

Do ancient magnetic fields that hold the universe together exist? It's possible. (Sergio Martin Alvarez)

Researchers have shown it's possible that some magnetic fields in the cosmos date back to the beginning of time, and if the prospect proves accurate, it could address a conundrum wreaking havoc among physicists.

In the long and heated debate about quantifying how fast the universe is expanding, so-called Hubble tension, scientists have taken varying mathematical approaches and scrutinized for errors, only to calculate different answers. But now, a paper published April 9 in Monthly Notices of the Royal Astronomical Society offers new data about the early universe's magnetic fields that might be the missing puzzle piece.

Study author Sergio Martin-Alvarez, a postdoctoral researcher at the University of Cambridge, uncovered that some strong magnetic fields could be the consequence of Big Bang-induced quantum fluctuations, making them primordial. The results were pulled from his team's comprehensive simulations of a galaxy much like the Milky Way.

"A collaborator and myself, we developed this idea that is like a code, that gives you a way to see how different species of magnetic fields evolve in simulations," Martin-Alvarez said. He added that the researchers wanted to "see if the magnetic field affects the galaxy by itself and survives the entire formation of the galaxy — just to check."

If a field were able to endure galaxy formation, it would mean the prospect of primordial magnetic fields can't be eliminated.

"That's the tip of the iceberg," Martin-Alvarez told The Academic Times. "Because we know very well that if a magnetic field becomes strong enough, we'll have to consider whether this entire Hubble tension business is just based on the fact that we don't understand the early universe enough." 

According to Scientific American, the persistent dilemma was discussed in 2019 at a conference at the Kavli Institute for Theoretical Physics in Santa Barbara, California, during which David Gross, a particle physicist and the institute's former director, remarked, "Do we have a 'tension,' or do we have a 'problem'?" He later clarified, "We wouldn't call it a tension or a problem, but rather a crisis."

Terminology aside, disagreement over the Hubble constant is raging. Among other attempts, one measurement looks at the universe's expansion rate starting from the early universe going forward to suggest that the Hubble constant is about 67. Another computes the rate starting from the recent universe going backward; it finds the answer to be around 72. Neither side has any reasonable doubt. 

"If you have magnetic fields that are strong enough in the early universe, this body of 67 has to be reinterpreted," Martin-Alvarez explained. "Because you're actually forgetting that there is, like, a clumpiness that is generated as a consequence of this magnetic field."

He continued, "The entire cosmology field is based on the assumption that everything looks the same in every direction, everywhere. If there's a primordial field … things are not supposed to look the same in every direction anymore."

If such fields are real, the dispute might not only cease, but the true Hubble constant could be revealed. That constant is crucial for understanding the way the universe formed and what it will turn into, and could even someday provide insight into dark matter

To reach the potentially groundbreaking finding, the researchers started, almost as a formality, with an attempt to rule out the possibility of primordial black holes. To their surprise, they found that if a primordial magnetic field is at or above a certain threshold of strength, it survives — even if there are non-primordial fields competing with it. 

"There is this assumption that everybody has about magnetic fields — they're potentially not true — but nobody really wants to talk about it, because it's just too hard," Martin-Alvarez said. "So, we just tried to like point the finger and say, 'It's actually this problem, here!'"

He also relayed that while many primordial magnetic fields are likely so weak that they can evade any type of detection, the strongest are strong enough to bend the way gravity collapses, for example.

The other type of magnetic field is the exact opposite of a primordial one: It's formed after the universe's creation. A starting point for such a field is from a star's supernova, or explosion. For instance, the sun appears stringy because its particles follow the fields. 

"When you blow up a star with a supernova, all the magnetic fields have to go somewhere — and it goes into your galaxy," Martin-Alvarez explained.

Supernova-based fields are inevitable, as stars eventually have to collapse, but the team dissected the differences between that type and primordial ones. They hope the parameters can help inform cosmologists in search of the fields.

"At the moment," Martin-Alvarez said, "It's still the theoreticians' kind of work to tell the observers, 'This is the signature associated with each side.'"

The fields' fingerprints included how spatially different they are, where in space they should reside, the heat that likely surrounds them and how many chemical elements they're associated with, among other things. 

Standing in solidarity with those who promote the understanding of magnetic fields' role in the universe's expansion, Martin-Alvarez highlighted a need to address them. 

For "many of the questions that we're trying to solve," he said, "using the same hammer on the same nail, just hitting it harder, is not going to solve these things."

The paper, "Unraveling the origin of magnetic fields in galaxies," published April 9 in Monthly Notices of the Royal Astronomical Society, was authored by Sergio Martin-Alvarez and Debora Sijacki, University of Cambridge; and Harley Katz, Julien Devriendt and Adrianne Slyz, University of Oxford. 

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