⚡ The magnetism of the Universe: an elegant solution to an astrophysical paradox

Astrophysics faces a paradox: several independent measurements of the speed at which the cosmos is growing do not agree. This discrepancy, known as the Hubble tension, weakens the very foundations of modern cosmology.

To calculate the Hubble constant, scientists primarily use two approaches. On one side, analysis of the cosmic microwave background, which is the residual light from the Big Bang, provides a value of approximately 67 kilometers per second per megaparsec. On the other side, a more direct method, relying on supernovae serving as distance markers, indicates about 73 km/s/Mpc. Although this gap seems small, it is statistically significant and suggests that our standard theoretical framework might be incomplete.


In an attempt to reconcile these measurements, an interesting avenue is emerging: that of primordial magnetic fields. These fields, which could have formed just after the Big Bang, might have influenced the Universe's transition to a transparent state, consequently affecting the cosmic signals we observe. Their existence would slightly alter the moment when light began to travel freely, thus changing the interpretation of the data, which could make the two measurements of the expansion converge.

Recent work, published in Nature Astronomy, used three-dimensional simulations to model the effect of these magnetic fields on the formation of the hydrogen atom, necessary to make the Universe transparent. Using these simulations, researchers can predict how the observed cosmic microwave background would be altered.

Comparing these predictions with actual observations allows testing the robustness of this hypothesis and its potential influence on the Hubble tension. The obtained results indeed show that the presence of primordial magnetic fields could help explain the Hubble tension. The compatible field strengths are on the order of five to ten pico-Gauss.


If confirmed, the existence of these magnetic fields would provide additional information about the physical processes that prevailed in the nascent cosmos. Future observations, conducted with more precise instruments, will allow observationally testing this theory. This discovery would then open a new window onto the events of the first moments, potentially linked to the Big Bang itself.