A hidden magnetic force from the early universe may finally explain the Hubble tension and why cosmic expansion doesn’t add up

For many years now, one of the challenges that cosmology has faced has been based on two values: 67 and 73. They both refer to the Hubble constant, which is used to measure the speed at which the universe is expanding. These values are both the result of highly precise methods and observations. However, they do not seem to reconcile. What scientists have started considering is that there might be something that existed when the universe was very young.

These new simulations in 3D show that there could have been magnetic fields in the universe that existed just after the Big Bang and were responsible for changing the formation of hydrogen in the early universe. This is important because it would affect the CMB, which is used by scientists to understand how the universe expanded over time.

This study, reported in the journal Nature Astronomy, does not resolve the Hubble tension problem.


The evidence for primordial magnetic fields remains tentative. But the calculations show that the idea survives a much more detailed test than earlier models — and could connect two major puzzles in cosmology.

Why 67 and 73 have become a problem for cosmology

The Hubble constant can be determined through observations that go back into the distant history of the universe.

Observation of the cosmic microwave background radiation, which is emitted when the universe became transparent, can be calibrated with the standard model of cosmology. Using data collected from spacecraft like the Planck Space Telescope gives an expansion rate of around 67 kilometres per second per megaparsec.

One megaparsec equals 3.26 million light-years.

Astronomers can also measure expansion using relatively nearby objects. In this way, a “cosmic distance ladder” is constructed by using objects whose distances can be found, such as Cepheid variables and Type Ia supernovae, which are either known to have absolute luminosities or can be calibrated.

The measurement that uses the Hubble Space Telescope and James Webb Space Telescope places the number closer to 73 km/s/Mpc.

A gap of six might sound modest, but its statistical significance has made it difficult to dismiss as an ordinary measurement discrepancy. If the disagreement persists, scientists may need either an overlooked systematic effect or new physics beyond the standard cosmological model.

A magnetic field could have changed the universe’s first hydrogen

The proposed explanation begins hundreds of thousands of years after the Big Bang, during a crucial transition known as recombination.

The young universe was initially filled with a hot plasma of charged particles. As it expanded and cooled, electrons were eventually able to combine with protons to make neutral hydrogen. Photons could then travel much more freely through space.

That ancient light survives today as the cosmic microwave background.

Primordial magnetic fields could have changed how smoothly recombination unfolded. The fields could alter matter density by affecting charged particles in the plasma. Dense regions offer more opportunities for collisions between protons and electrons, thus allowing for the creation of hydrogen atoms.

It means that even the weakest magnetic field can affect the opacity of the universe. It is essential because the patterns on the CMB are used by cosmologists to set up the ruler of the cosmos.

Change the physics responsible for those patterns, and the expansion rate inferred from them can shift as well.

New 3D simulations put the idea through a tougher test

The possibility that primordial magnetism could affect recombination is not new. What has changed is scientists’ ability to model the process in far greater detail.

Earlier work showed with simplified calculations that magnetic fields could make primordial matter clumpier and accelerate hydrogen formation.

The authors of the new study simulated the magnetised plasma in 3D and took into account the role of the effect on hydrogen formation. After that, they estimated how the observed changes would be reflected in the cosmic microwave background.

That is a demanding test. Measurements of the CMB are precise enough that a model producing the wrong recombination history could quickly run into trouble.

Instead, the magnetic-field scenario remained compatible with the data.

Depending on which datasets were combined, the researchers found a mild statistical preference for primordial magnetic fields of roughly 1.5 to three standard deviations. That falls well short of the level physicists would regard as a discovery, so the result should be treated as a clue rather than evidence that the mystery has been solved.

The same clue could explain another cosmic mystery

The simulations open up an interesting possibility since magnetism is not limited to planets and stars alone.

Magnetism can occur on a galactic or even a cluster of galaxies scale, whereas observation has led scientists to probe for magnetism in the vast space in between. Exactly where such large-scale fields originated remains uncertain.

One possibility is that today's cosmic magnetism grew from tiny magnetic “seeds” produced in the early universe.

The new analysis favours present-day primordial field strengths of roughly five to 10 pico-Gauss. This is what the team says is in line with what might be needed if the observed magnetic fields in galaxies and galaxy clusters had originated from primordial seeds.

The study may help in answering two questions simultaneously – how the large-scale cosmic magnetic fields were formed and why there is such a difference between the estimates of the Hubble constant obtained using different measurements.

At the moment, however, the Hubble tension persists, as the statistical signature is too weak to prove the existence of the primordial magnetic fields.

What the new simulations provide is a more specific target. An extremely weak field might have been important in the hot plasma early in the Universe's evolution, affecting the first neutral hydrogen formation and leaving a mark in the radiation that travelled through space for more than 13 billion years.

Frequently Asked Questions

What is the Hubble tension?

The Hubble controversy concerns the discrepancy in the measurement of the universe’s current rate of expansion. The value of Hubble’s constant derived from the Cosmic Microwave Background radiation is around 67 km/s/Mpc, whereas that obtained from the distance ladder using stars and supernovae is 73 km/s/Mpc.

What are primordial magnetic fields?

These are theoretical magnetic fields that might have originated in the very early universe, when there were no stars and galaxies present. Researchers are investigating whether they could have acted as seeds for the origin of larger magnetic fields in space.

What is the effect of magnetic fields on the Hubble constant?

A primordial magnetic field could cause matter to be clumpier during the formation of the universe and cause the rate at which the electrons and protons combine to form neutral hydrogen to change. It may also cause changes in the structure of the cosmic microwave background.

Have scientists solved the Hubble tension?
No. The new analysis finds only a mild statistical preference for primordial magnetic fields, ranging from about 1.5 to three standard deviations depending on the datasets used. The result shows that primordial magnetism remains a viable explanation worth testing, not that it has been confirmed.