Magnetic fields in the young universe revealed as incredibly faint

Magnetic fields in the young universe revealed as incredibly faint
by Erica Marchand
Paris, France (SPX) Sep 03, 2025

The first magnetic fields that emerged after the Universe's birth may have been billions of times weaker than the pull of a refrigerator magnet, with intensities comparable to the magnetism created by neurons in the brain. Despite their weakness, researchers have found that these fields left detectable traces in the cosmic web that spans the Universe.

The conclusions come from a collaboration led by SISSA (International School for Advanced Studies, Trieste), working with teams from Hertfordshire, Cambridge, Nottingham, Stanford, and Potsdam. The scientists ran roughly 250,000 computer simulations to explore the behavior of primordial magnetic fields, validating the results with astronomical observations. The findings, published in Physical Review Letters, define the possible and maximum strengths of these early fields and offer insights into how the first galaxies and stars arose.

"The cosmic web, of which much remains to be discovered, is a filamentary structure connecting galaxies across the Universe," explained lead author Mak Pavicevic, a SISSA PhD student, with co-author Matteo Viel. "One of its mysteries is why it is magnetised even in the most remote and sparsely populated regions. Our hypothesis was that this could be a legacy of processes in the primordial Universe, either during inflation before the Big Bang or in later cosmic phase transitions."

By comparing simulations with data, the team demonstrated that models including weak primordial fields better match observations. Pavicevic and Viel note that a Universe with a magnetic field of about 0.2 nanogauss aligns closely with measured data. Co-author Vid Irsic of the University of Hertfordshire emphasized: "These are the most realistic and largest suite of state-of-the-art simulations of the influence of primordial magnetic fields on the intergalactic cosmic web."

The study establishes a new upper limit on the strength of primordial magnetic fields, significantly lower than prior estimates. This strict constraint is also consistent with independent measurements from the cosmic microwave background. According to the researchers, such magnetic fields would have influenced the density of the cosmic web, speeding up star and galaxy formation. Future observations by the James Webb Space Telescope could provide further confirmation.

Research Report:Constraints on Primordial Magnetic Fields from the Lyman Forest