Primordial magnetic fields may ease the Hubble tension — ScienceDaily

Primordial magnetic fields that may have arisen shortly after the Big Bang could partly explain the discrepancy in measurements of the Universe’s expansion rate. A new study using full three-dimensional simulations of primordial plasma showed that this possibility is consistent with existing observations of the cosmic microwave background, ScienceDaily reports.

Discrepancy in measurements

The Hubble constant describes the rate of the Universe’s expansion. According to indirect calculations based on the cosmological model and cosmic microwave background data, including observations by the Planck telescope, its value is approximately 67 kilometers per second per megaparsec.

At the same time, direct measurements using Type Ia supernovae and Cepheids, conducted with the Hubble and James Webb telescopes, yield about 73 kilometers per second per megaparsec. This statistically significant mismatch is called the Hubble tension. If both approaches are correct, the standard cosmological model may fail to account for an important factor.

Impact on hydrogen formation

The researchers tested the hypothesis that weak magnetic fields in the early Universe affected recombination, the period when electrons and protons combined into neutral hydrogen. Due to the fields’ action, charged particles could have been distributed more unevenly, accelerating hydrogen formation in denser regions.

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Changing the time when the Universe became transparent to light affects the parameters of cosmic microwave background patterns. This, in turn, may alter the value of the Hubble constant inferred from the cosmological model and ease the existing discrepancy.

Not a discovery yet

In several combinations of data sets, the authors obtained a weak preference for the model with primordial magnetic fields, at a level of approximately 1.5 to three standard deviations. The scientists emphasize that this is not yet a discovery, but consider the result a meaningful indication of the existence of such fields.

The data favor a magnetic field strength of about 5–10 picogauss in the present-day Universe. According to the researchers’ estimate, this level may be sufficient for the magnetic fields of galaxies and galaxy clusters to originate from primordial fields.