The early Universe hosted ultra-weak magnetic fields that still shaped cosmic structures. New simulations establish stricter limits on their strength.
Magnetic fields that originated during the earliest moments of the Universe may have been billions of times weaker than the pull of a household fridge magnet, with strengths on the scale of the magnetism produced by neurons in the human brain. Despite being so faint, measurable evidence of these fields can still be detected in the cosmic web, the vast network of structures linking galaxies across the Universe.
This conclusion comes from a study involving about 250,000 computer simulations carried out by researchers at SISSA (the International School for Advanced Studies in Trieste) in collaboration with teams from the Universities of Hertfordshire, Cambridge, Nottingham, Stanford, and Potsdam.
The simulations were further supported by observational data. Published in Physical Review Letters, the work establishes both potential and upper limits for the strength of primordial magnetic fields and provides new insights into the early Universe, including the processes that shaped the first stars and galaxies.
A magnetic cosmic web
“The cosmic web, of which much remains to be discovered, is a filamentary structure connecting the galaxies that permeates the Universe. One of its many unsolved mysteries is why it is magnetized, not only near galaxies, where this might be expected, but also in distant regions that are sparsely populated and constitute the bulk of the cosmic web. This is harder to explain.”
These comments come from Mak Pavičević, a SISSA PhD student and lead author of the research, and Matteo Viel, his supervisor and co-author of the study.
“Our hypothesis was that this could be a legacy of events occurring in cosmic epochs during the birth of the Universe, and that magnetism was linked essentially to physical processes in the primordial Universe. For example, the filaments would have become magnetized during the inflation process before the so-called ‘Big Bang’ or through events in later epochs, called phase transitions. This is what we sought to ascertain with our work. We also wished to assess the magnitude of these primordial magnetic fields through our investigations, establishing an upper limit and attempting to measure their strengths.”
At the origin of the Universe with a quarter of a million simulations
An international team conducted more than 250,000 computer simulations to investigate the cosmic web and the role of primordial magnetic fields in shaping it. According to Vid Iršič of the University of Hertfordshire, a co-author of the study, “these are the most realistic and largest suite state-of-the-art simulations of the influence of primordial magnetic field on the intergalactic cosmic web.”
Lead author Mak Pavičević and supervisor Matteo Viel add: “By comparing these simulations with observational data, we saw that our hypotheses were correct. When the influence of primordial fields is included in the picture, the cosmic web looks different and more in agreement with observed data. In particular, we can say that a standard model of the Universe with a very weak magnetic field of around 0.2 nano-gauss actually fits experimental data much better.”
The magnitude of primordial magnetic fields: a new upper limit
The scientists have derived a particularly low value for the magnitude of the primordial magnetic fields, establishing a new upper limit several times lower than previously estimated.
Pavičević and Viel continue: “Our research thus places strict limits on the intensity of magnetic fields formed in the very early moments of the Universe and is consistent with recent results obtained in independent data and studies on the cosmic microwave background.
The two scientists explain: “This evidence will help us to improve our understanding of events in the early Universe. The magnetic field would have increased the density of the cosmic web, in turn accelerating the process of star and galaxy formation. It will be possible to further validate our results through observations made by the James Webb Space Telescope.”
Vid Iršič concludes: “Not only will these new limits help us understand the impact of the primordial magnetic fields on the evolution of the Cosmo, but they also hold important implications for other theoretical models that enhance structure formation”.
Reference: “Constraints on Primordial Magnetic Fields from the Lyman- Forest” by Mak Pavičević, Vid Iršič, Matteo Viel, James S. Bolton, Martin G. Haehnelt, Sergio Martin-Alvarez, Ewald Puchwein and Pranjal Ralegankar, 13 August 2025, Physical Review Letters.
DOI: 10.1103/77rd-vkpz
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