Faint Big Bang Magnetic Fields May Explain Why the Universe's Expansion Doesn't Add Up
Three-dimensional simulations of the early universe's plasma show a mild statistical preference for primordial magnetic fields that could ease the Hubble tension.
Physicists have a long-standing problem: two ways of measuring how fast the universe is expanding give answers that disagree, and nobody has been able to explain why. A team of cosmologists now says extremely weak magnetic fields left over from the first moments after the Big Bang could help close the gap.
The disagreement is called the Hubble tension. Measurements based on the cosmic microwave background, the afterglow of the early universe, imply an expansion rate of about 67 kilometers per second per megaparsec. Direct measurements using distant supernovae and the local universe give about 73. The difference is too large to blame on measurement error, and it has persisted for years as instruments have become more precise. Either some measurements are wrong in a way nobody has found, or the standard model of cosmology is missing something.
Karsten Jedamzik, Levon Pogosian and Tom Abel describe their approach in an article summarizing work published in Nature Astronomy. Their idea is that tiny magnetic fields, present in the plasma that filled the universe shortly after the Big Bang, changed how hydrogen formed about 380,000 years in, a period called recombination. If hydrogen formed slightly differently, the pattern imprinted on the microwave background would change, and the expansion rate inferred from that pattern would shift toward the locally measured value.
To test it, the team ran what they describe as the first full three-dimensional simulations of the primordial plasma with magnetic fields embedded in it, tracking how hydrogen forms. Earlier work used simpler approximations. The simulations let the researchers see how the magnetic fields clump the plasma, which speeds up recombination in some regions.
They then compared the predictions with observational data. The result was a consistent but mild preference for primordial magnetic fields, ranging from about 1.5 to three standard deviations depending on the data used. In physics, a five-standard-deviation result is the usual bar for a discovery, so this is a hint rather than a finding. The favored field strength is about 5 to 10 picogauss today. A picogauss is a trillionth of a gauss, trillions of times weaker than a refrigerator magnet.
Magnetic fields of that size would be very hard to detect directly, but the authors say the results give clear targets for future observations. Astronomers have been looking for evidence of weak magnetic fields in the space between galaxies, and signs of early fields could show up in how the first stars and galaxies formed and in more detailed maps of the microwave background.
The idea also has competitors. Researchers have proposed new particles, an early burst of dark energy and changes to gravity as ways to resolve the tension. What makes the magnetic-field explanation attractive is that it requires no new particle or force, only fields that many theories of the early universe already predict could exist.
If the preference holds up as data improve, it would do two things at once. It would remove one of cosmology's biggest puzzles, and it would give physicists a window into the universe just after the Big Bang. If it fades, the tension will remain, and so will the question of what is wrong with the standard picture.
