The Hubble tension arises because two precise approaches to determining the Hubble constant give significantly different results, despite drawing on high quality data. One method infers the expansion rate from the cosmic microwave background, while the other uses observations of relatively nearby objects such as supernovae and galaxies.

The SFU led team proposes that tiny magnetic fields generated in the very early universe may have altered the physics of recombination, the epoch when electrons and protons combined to form neutral atoms and the cosmic microwave background was released. If primordial magnetic fields sped up recombination, they would subtly change the pattern of temperature anisotropies in the microwave background sky.

Such changes feed directly into how cosmologists infer the Hubble constant from cosmic microwave background data, potentially shifting the value and easing the tension with late time measurements. The idea suggests that a physical ingredient long present in the cosmos could reconcile the discrepant expansion rates without invoking a completely new cosmological model.

"This is an exciting moment for us and the wider cosmology community because our idea could address two major unsolved puzzles about our universe - the Hubble tension and the origin of cosmic magnetic fields," says Levon Pogosian, professor and department chair at SFU Physics and co author of the study. "Solving these puzzles would be like opening a new window into the early universe. It would help cosmologists to better explain the origin of the universe and everything within it."

Pogosian notes that cosmologists worldwide have been driven to introduce new components into the standard cosmological model to deal with the Hubble tension. In contrast, he argues that primordial magnetic fields could serve as a more natural ingredient that both influences recombination and seeds the magnetic fields observed across galaxies and clusters today.

"It's a major headache for cosmologists across the world. It has sprung an industry of scientists inventing new ingredients in the cosmological model to try to address the Hubble tension," he says. "But what we are saying is that the ingredient, the magnetic fields, could have been there all this time. And, if confirmed, it would also explain the origin of magnetic fields observed throughout the cosmos."

Over the last three years, Pogosian and collaborators Karsten Jedamzik from the University of Montpelier, Tom Abel from Stanford University, and Yacine Ali Haimoud from New York University have used SFU's Cedar supercomputer to run detailed simulations of the recombination era. These large computations track how primordial magnetic fields would modify the interaction between matter and radiation in the early universe.

The team then confronted the simulated signatures with observations from the Hubble Space Telescope, the Planck satellite and other facilities. By comparing model predictions with data, they tested whether the proposed primordial magnetic fields scenario remains compatible with current cosmological measurements.

"Remarkably, our findings show that the idea survives the most detailed and realistic tests available today," says Pogosian. He adds that the work not only preserves agreement with existing observations but also points to specific patterns that future measurements could look for.

According to Pogosian, the analysis defines clear observational targets for upcoming surveys of the cosmic microwave background and large scale structure. Over the next several years, new data should be able to confirm or rule out whether tiny magnetic fields from the dawn of time helped shape the universe visible today and whether they can finally settle the Hubble tension.

SFU's Cedar supercomputer, and its successor Fir, were central to the project. The machines made it possible to decompose the demanding recombination calculations into many smaller jobs and execute them in parallel.

"We would not have been able to carry out our research without the supercomputer. It was crucial for our tests and calculations," Pogosian says. "The supercomputer allowed us to break down our tests into smaller jobs and run them in parallel, which saved us a huge amount of time."

An accessible overview essay, "A Cosmic Clue Hidden in Magnetism: How Primordial Magnetic Fields May Help Resolve the Hubble Tension," offers additional context on how the proposed mechanism connects early universe magnetism to present day cosmological measurements.

Research Report:Hints of primordial magnetic fields at recombination and implications for the Hubble tension

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