Magnetic fields are everywhere in the Universe. Planets and stars generate their own fields through well‑understood internal processes. The real mystery begins on much larger scales. Galaxies are threaded with magnetic fields stretching across hundreds of thousands of light‑years, and clusters of galaxies show magnetism extending over even larger distances. There are also tantalizing hints of magnetic fields in the near‑empty cosmic voids, although the evidence there remains debated. Where did these vast cosmic magnetic fields come from?
One exciting possibility is that they originated in the very early Universe, long before the first stars or galaxies formed. These so‑called primordial magnetic fields (PMFs) have been studied for decades, but only recently have we been able to test their effects with sufficient precision.
In our recent paper, published in Nature Astronomy, we present the most detailed test to date of what would happen if PMFs were present during the early evolution of the Universe. Remarkably, our findings suggest that they could help resolve one of the biggest puzzles in modern cosmology: the Hubble tension.
The Big Puzzle: Why Do We Disagree About the Expansion Rate of the Universe?
The Universe has been expanding since the Big Bang, and the rate of that expansion today is known as the Hubble constant, or H₀. We can measure it in two different ways and they disagree. Experiments such as the Planck satellite measure the Cosmic Microwave Background (CMB), the oldest light we can observe. These measurements allow us to infer the value of H₀ indirectly and result in a lower number. Astronomers can also measure distances to galaxies using Type Ia supernovae, calibrated with observations of nearby stars with the Hubble and James Webb Space Telescopes. These measurements consistently yield a higher value of H₀. The disagreement between these measurements has now reached a statistical significance of more than 5 standard deviations, which makes it very unlikely to be a coincidence. Something may be missing in the standard cosmological model.
Where Magnetic Fields Come Into the Story
To understand how PMFs enter the picture, we need to go back to a time when the Universe was filled with a hot plasma of electrons, protons, and photons. Light could not travel freely because photons were constantly scattering off charged particles.
Only when protons and electrons combined into neutral hydrogen did space become transparent. This moment is called recombination, and the light released then is what we see today as the CMB.
If PMFs existed at that time, even extremely weak ones, they would push and pull on the charged particles in the plasma. This would make the matter slightly clumpy, denser in some places, thinner in others. When protons and electrons are more crowded, they are more likely to meet and form hydrogen. As a result recombination would happen slightly earlier, the Universe would become transparent sooner, and the characteristic patterns in the CMB would shift.
A key consequence is that the inferred value of H₀ from the CMB would increase, moving closer to the value measured from supernovae. In 2020, two of us showed this effect using simplified models, work that was awarded the 2021 Buchalter Cosmology Prize. But to establish whether the idea was truly viable, a much more realistic treatment of recombination was required.
A Breakthrough: First Detailed Simulations of Recombination With PMFs
In our new paper, we use the first full 3‑dimensional magnetohydrodynamic (MHD) simulations of recombination in the presence of PMFs. These simulations track how magnetic fields interact with the plasma and how hydrogen forms, including detailed modeling of how photons escape or become trapped.
Using the results of these simulations, we modified standard cosmology codes and tested the predictions against high‑precision observations of CMB from Planck, Baryon Acoustic Oscillations (BAO) from the DESI survey, and Type Ia supernovae data, including SH0ES calibrations.
Because the CMB is extraordinarily sensitive to changes in recombination, this is an extremely demanding test. If PMFs altered the CMB in a way that disagreed with observations, the idea could be ruled out. Instead, the data showed that our proposal is still alive.
What We Found
Across multiple combinations of datasets, we find a consistent, mild preference for PMFs. Depending on which data are included, the preference ranges from around 1.5 to 3 standard deviations.
Equally important, the field strengths favored by the data, about 5 to 10 pico‑Gauss today, are very close to what would be needed for galaxy and cluster magnetic fields to originate from primordial seeds alone, without help from astrophysical dynamos or stellar processes. Some studies even suggest that magnetic fields may exist in the otherwise empty voids between galaxies. If confirmed, PMFs are the most natural way to explain them.
In other words, a single idea may simultaneously explain the Hubble tension, magnetic fields in galaxies and clusters, and potentially magnetic fields in cosmic voids.
The convergence of evidence makes this scenario especially compelling.
Why This Matters
Unlike many proposed solutions to the Hubble tension, this one is highly testable. There is only a single free parameter, the PMF strength. Everything else follows from standard physics.
Future CMB experiments will be able to decisively check this idea by studying tiny fluctuations on very small angular scales, the region where PMF‑induced changes to recombination leave clear signatures. Projects such as the Simons Observatory will provide data of the required precision.
If PMFs were present in the early Universe, we should soon know.
A Window Into the Earliest Physics
If primordial magnetic fields are confirmed, it would open an entirely new window into physics at enormous energies, possibly linked to phenomena such as cosmic phase transitions or even Inflation. It would not only help resolve a major cosmological puzzle, but also reveal a new piece of the Universe’s history, encoded in the oldest light we can measure.
Our results show that the idea survives the most detailed and realistic test available today. More importantly, they provide clear targets for future observations. Over the next several years, we will learn whether tiny magnetic fields from the dawn of time really helped shape the Universe we see today, and whether they hold the key to resolving the Hubble tension once and for all.