Scientists have found a possible link between one of the earliest mysteries of the Universe and a problem that has troubled cosmologists for decades. Tiny magnetic fields that arose shortly after the Big Bang may have altered the process of hydrogen formation in the early Universe. As a result, the cosmic microwave background may have looked slightly different than commonly assumed, and with it the estimated rate of the Universe’s expansion would also change.

This idea may help explain the so-called Hubble tension — the persistent discrepancy between the two main ways of measuring the Hubble constant. The new study is based on the first full-fledged three-dimensional simulations of the early plasma that take primordial magnetic fields into account. The results do not yet amount to a discovery, but they hint that such fields may indeed have existed, Sciencedaily.com reports.

The Universe expands at different rates — depending on the method

The Hubble constant shows how fast the Universe is expanding. Today, there are two fundamentally different ways to determine its value, and they produce results that differ noticeably.

The first is based on the cosmic microwave background — ancient radiation left over from the early stage of the Universe’s development. Space telescopes, including Planck, measure the tiniest irregularities in this radiation. Scientists then use the standard cosmological model to reconstruct the parameters of the Universe from them and calculate its expansion rate.

This approach gives a value of about 67 km/s/Mpc.

The second method relies on direct observations of distant galaxies. Astronomers determine their distances, in particular by using Type Ia supernovae, which serve as a kind of “standard candles”: their apparent brightness makes it possible to estimate the distance to a galaxy.

Observations made with the Hubble and James Webb telescopes give a higher value — about 73 km/s/Mpc.

The difference may seem small, but statistically it is significant enough to call into question the completeness of the standard model of cosmology. If both methods work correctly, then some important element may be missing from our understanding of the Universe.

Magnetic fields may have altered the birth of hydrogen

The researchers suggested that this missing element could be primordial magnetic fields — extremely weak fields left over from the earliest stages of the Universe’s existence.

Such fields have long been considered a possible source of cosmic magnetism. Today, magnetic fields are observed in galaxies and galaxy clusters, yet the origin of these enormous structures remains not fully understood.

If magnetic fields already existed in the early Universe, they could have influenced charged particles at the moment when neutral hydrogen was forming.

This period is called recombination. Electrons and protons began combining into neutral hydrogen atoms, and the Universe gradually became transparent to light. The radiation that was then able to travel freely is observed today as the cosmic microwave background.

Magnetic fields may have slightly accelerated recombination. They would have affected charged particles and created small changes in the distribution of matter. Where particles became a little denser, the likelihood of collisions and hydrogen formation increased.

This, in turn, could have changed the moment when the Universe became transparent.

The “cosmic yardstick” would change as well

At first glance, such a correction seems too insignificant to have anything to do with modern cosmology. But the cosmic microwave background is extremely sensitive to changes in the recombination process.

If the moment when the Universe transitioned to a transparent state shifted slightly, the nature of the structures observed in the microwave background would also change. And these structures are used as a kind of cosmic ruler to measure distances and reconstruct the parameters of the Universe.

As a result, a small influence of the magnetic field on the early plasma could have changed the value of the Hubble constant that scientists derive from the cosmic microwave background.

That is why primordial magnetic fields could potentially reduce the gap between the values of 67 and 73 km/s/Mpc.

New simulations tested the idea in detail for the first time

Researchers had previously suggested that primordial magnetic fields could affect recombination. In 2020, this effect was demonstrated using a simplified model.

Now scientists have carried out a much more detailed study. They created the first full three-dimensional simulations of the early plasma with magnetic fields and traced how hydrogen forms under such conditions.

They then used the resulting history of hydrogen formation to model what the cosmic microwave background would look like in the presence of primordial magnetic fields. The results were compared with real observations.

And here the idea did not run into any obvious contradiction: the simulated effects are consistent with the available data.

Using different combinations of observational data, the researchers found a weak but persistent preference for the scenario with primordial magnetic fields. The statistical significance of the effect was about 1.5 to 3 sigma.

This is important, but not enough to declare a discovery. In physics, a result usually requires much higher statistical confidence before a hypothesis is considered confirmed.

If the fields exist, they may be a trace of the earliest Universe

The estimated present-day strength of the proposed magnetic fields is about 5–10 picoGauss. According to the authors’ calculations, this is close to the values that could have been sufficient for forming the magnetic fields of galaxies and their clusters from primordial “seeds.”

That is why the hypothesis could potentially solve two cosmological problems at once: it may help explain the Hubble tension and provide a natural source of cosmic magnetism.

But the broader significance of a possible discovery is much greater. If primordial magnetic fields really arose in the first moments of the Universe’s existence, detecting them would provide a way to peer into an era that modern observations describe only indirectly.

So far, scientists have only managed to show that this hypothesis withstands the most detailed test carried out to date. Future observations will have to determine whether a weak magnetic field that arose at the dawn of the Universe really left a noticeable imprint on the cosmic microwave background — and whether it can finally explain why different methods of measuring the Universe’s expansion rate continue to produce different answers.