Hubble tension may trace to magnetic fields born after Big Bang

The universe is expanding, and astronomers can measure how fast. The problem is that their two best methods give answers that refuse to agree, and the gap between them has become one of the most stubborn puzzles in modern cosmology. New research suggests the Hubble tension may come down to something almost invisible: faint magnetic fields generated moments after the Big Bang.

The rate of cosmic expansion is called the Hubble constant, named after Edwin Hubble, who first showed in the 1920s that the universe is growing. One way to pin down that rate is indirect. Astronomers take the standard model of cosmology, tune it to match tiny fluctuations in the cosmic microwave background, and calculate what expansion rate the model predicts. Measurements from the Planck Space Telescope point to about 67 kilometers per second per megaparsec.

The other approach measures expansion more directly. Distant Type Ia supernovae serve as standard candles: because their intrinsic brightness is known, how dim they look reveals how far away they are, and their redshifts show how fast they are receding. Cepheid stars in nearby galaxies calibrate that brightness scale. These observations, using the Hubble and James Webb space telescopes, give a higher value of around 73 kilometers per second per megaparsec, deepening the Hubble tension.

The gap between the two values may look modest, but it is statistically highly significant. If both measurement chains are sound, the standard cosmological model is missing something real. That disagreement is known as the Hubble tension, and it has resisted easy explanation for years.

Where magnetism enters the story

Magnetic fields are everywhere in the universe. Planets and stars generate their own, but that does not explain the vast fields threading entire galaxies and galaxy clusters, and possibly the voids between them. One long-studied possibility is that magnetism arose in the very early universe, before any star or galaxy existed. These primordial magnetic fields could have seeded every field astronomers observe today.

In 2011, two of the new study's authors, Karsten Jedamzik and Tom Abel, proposed that such fields would have left a mark on recombination, the moment when electrons and protons first combined to form neutral hydrogen and the universe turned from opaque to transparent. The light freed at that moment is what astronomers now detect as the cosmic microwave background, so anything that shifted recombination would have reshaped that ancient signal.

The mechanism works through clumping. Magnetic fields push and pull on charged particles, and according to the authors, that would have made the early plasma slightly lumpy. Where particles were more crowded, they were more likely to meet and form hydrogen. Speeding up recombination shifts the moment the universe becomes transparent, which changes the apparent size of patterns in the cosmic microwave background. That in turn alters the cosmic ruler used to measure distances, and with it the expansion rate inferred from the model. A universe with primordial magnetic fields would read as expanding at a slightly different rate, and the authors report that this shift could ease the Hubble tension.

A hint, not a discovery

Jedamzik and Levon Pogosian tested a simplified version of this Hubble-tension explanation in 2020. The new paper goes much further: the team ran the first full three-dimensional simulations of the primordial plasma with magnetic fields embedded in it, tracking how hydrogen actually formed. They then used that simulated hydrogen formation history to predict what the cosmic microwave background should look like if primordial fields existed, and compared those predictions against real observations.

The cosmic microwave background is extraordinarily sensitive to changes in recombination, so the comparison was a serious test of the Hubble tension proposal. If primordial magnetic fields altered recombination in ways the data could not accommodate, the idea would have been ruled out. Instead, the data kept it alive. Across multiple combinations of datasets, the researchers found a consistent, mild preference for primordial magnetic fields, ranging from 1.5 to 3 standard deviations. That counts as a hint, the authors emphasize, not a discovery.

Equally important is the strength the data favor. The fields preferred by the analysis, the researchers report, measure about five to 10 pico-Gauss today, close to what would be needed for the magnetic fields of galaxies and clusters to have grown from primordial seeds alone. If primordial fields are confirmed, they would open a new window into the universe when it was only split seconds old, offering a glimpse of physics at energies far beyond anything achievable on Earth.

The proposal now has clear targets for future observations. The authors write that over the next several years, new data will show whether tiny magnetic fields from the dawn of time helped shape the universe, and whether they hold the key to resolving the Hubble tension. The study, by Jedamzik, Pogosian and Abel, is published in Nature Astronomy under the title "Hints of primordial magnetic fields at recombination and implications for the Hubble tension" (DOI: 10.1038/s41550-025-02737-x), and the researchers summarized their findings in an explainer republished by ScienceDaily. Related space coverage on genznewz.com includes the first radio signal detected from an exoplanet beyond Earth and the 2026 Nobel Prize season preview.