For decades, astrophysics has been trying to solve one of the biggest mysteries of modern cosmology — the so-called Hubble tension. Scientists established long ago that the Universe is continuously expanding. However, the two main methods used to measure the rate of this process stubbornly produce different results. A new study offers a possible way out of this impasse: the discrepancy may be caused by extremely weak magnetic fields that emerged in the very first moments after the Big Bang. The study results were published in Nature Astronomy.
A Cosmological Dead End
The Hubble constant determines the rate at which the Universe is expanding. Today, scientists try to calculate it in two fundamentally different ways:
- Indirect method (early Universe). It is based on measuring fluctuations in the cosmic microwave background — the thermal echo of the Big Bang. Data from the Planck space telescope, interpreted within the standard cosmological model, give a value of about 67 km/s per megaparsec (one parsec is approximately 3.26 light-years).
- Direct method (modern Universe). It is based on observations of distant galaxies and so-called “standard candles” — Type Ia supernovae and Cepheid stars. Measurements made with the Hubble and James Webb telescopes indicate a rate of about 73 km/s per megaparsec.
The difference between 67 and 73 may seem small. But given the high precision of the instruments, it is statistically significant. If both methods are correct, the current standard model of cosmology contains a substantial gap.
Magnetic Seeds of the Primordial Cosmos
Magnetic fields permeate the entire Universe: they exist around planets, stars, galaxies, and even in giant cosmic voids. One hypothesis suggests that primordial magnetism arose long before the first stars appeared.
In 2011 and 2020, researchers drew attention to how primordial magnetic fields could have affected the epoch of recombination. This was the period when the Universe cooled enough for electrons and protons to combine into neutral hydrogen atoms, turning the cosmic medium from opaque to transparent to light.
By acting on charged particles, magnetic fields caused the plasma to compress into denser regions. Under these conditions, electrons and protons found one another more quickly. Faster recombination shifted the moment when light began to travel freely through the Universe, changing the characteristic scale of patterns in the cosmic microwave background. As a result, the “cosmic ruler” used to calculate the Hubble constant is adjusted, removing the contradiction between 67 and 73.
What the Three-Dimensional Modeling Showed
In the new study, scientists for the first time carried out full 3D simulations of the behavior of primordial plasma permeated by magnetic fields. The resulting history of hydrogen formation was compared with real observations of the cosmic microwave background — and the hypothesis fully withstood the test against the data.
The observations not only fail to rule out primordial magnetism, but also show moderate support for this model at a level of between 1.5 and 3 standard deviations. The required field strength, recalculated for the present day, is only 5–10 picogauss. This is just enough to explain the origin of present-day galactic magnetic fields without invoking additional mechanisms.
Confirmation of this theory in future observations would not only resolve the Hubble tension problem, but would also provide direct access to studying the physics of extreme energies that existed in the first fractions of a second after the birth of the Universe.
Previously, we reported on how NOIRLab data on the expansion of the Universe challenge modern physics.
Based on materials from Science Daily
