Incredibly weak magnetic fields, possibly originating in the very first moments of the Universe, could hold one of the keys that cosmologists have been searching for for years. The discovery does not yet solve the mystery, but it opens up a fascinating possibility: that a signal originating practically from the birth of the cosmos could help explain why we still do not know exactly how fast the universe is expanding.
The problem is known as the Hubble tension and has become one of the major headaches of modern cosmology.
TWO MEASUREMENTS THAT DON'T MATCH
We know that the universe is expanding. The problem arises when we try to determine exactly how fast it is expanding.
When scientists calculate this expansion using the cosmic microwave background—the ancient radiation left over from the Big Bang—they obtain a value close to 67 kilometers per second per megaparsec. But when they observe the nearby Universe using Cepheid variables and Type Ia supernovae, the result is around 73 kilometers per second per megaparsec. The difference seems small. But for cosmologists, it isn't.
If both methods are correct, it could mean that the model we use to describe the universe is missing a piece.
A CLUE THAT ORIGINATED AT THE BEGINNING OF THE COSMOS
A team consisting of Karsten Jedamzik, Levon Pogosian, and Tom Abel studied an extraordinary possibility: that extremely small magnetic fields existed in the early universe.
We're not talking about the magnetic fields produced later by stars or planets. These would be much older: primordial magnetic fields, possible remnants of the physical conditions that existed shortly after the Big Bang.
The researchers conducted detailed three-dimensional simulations of the plasma in the early universe to study how those fields would have affected the formation of hydrogen. And that's where things get interesting.
THE MOMENT THE UNIVERSE BECAME TRANSPARENT
During its first few hundred thousand years, the Universe was a hot mass of charged particles through which light could not travel freely. As it cooled, electrons and protons began to combine to form neutral hydrogen. This period is known as recombination.
From that point on, light was able to travel freely through the cosmos. That ancient light still exists, and today we observe it as the cosmic microwave background. According to simulations, primordial magnetic fields would have produced small concentrations of matter that facilitated the formation of hydrogen, slightly accelerating recombination.
A tiny change that occurred billions of years ago can alter the way we currently interpret that light. And, as a result, it also changes the rate of expansion of the universe that we infer from it.
A VERY WEAK SIGNAL, BUT NOT RULED OUT
The researchers compared the model with different sets of cosmological observations. The results do not allow us to conclude that these magnetic fields have been discovered. The statistical preference found ranges approximately between 1.8 and 3 sigma, depending on the data used.
In physics, that's interesting, but it's still not enough to claim a discovery. What's remarkable is that the hypothesis withstood much more rigorous testing than had been done previously.
The data suggest that current fields are extremely weak, at around 5 to 10 picogauss. Interestingly, that intensity is also close to what is needed to explain how the enormous magnetic fields that today span galaxies and galaxy clusters could have originated. The same idea could, therefore, touch on two different problems in cosmology.
THE BIG BANG MIGHT STILL HOLD THE ANSWER
The research does not prove that we have definitively found the solution to the Hubble tension. New measurements of the cosmic microwave background will have to confirm or rule out this possibility. But it paints an extraordinary picture.
Something so weak that today it would barely reach a few picogauss could have come into being when the universe was extremely young, slightly altering the way the first hydrogen formed and leaving a trace that we can still detect billions of years later. Perhaps the answer to one of the great mysteries of the present-day universe isn’t right in front of us. Perhaps it has been there since the Big Bang.
