It’s well established that the universe is expanding, but there’s serious disagreement among scientists over how fast it’s happening. Two of our best ways of measuring the cosmic expansion rate, the Hubble constant, give answers that are stubbornly at odds. This presents a major problem in modern cosmology known as the Hubble tension.
However, we wondered if an idea originally proposed to solve another cosmic mystery — the origin of cosmic magnetic fields — could help us unlock the mystery of the Hubble tension. Our recently published research explores whether extremely weak magnetic fields left over from the earliest moments after the Big Bang might help us unpack the Hubble tension, while offering a glimpse into physics at energies far beyond anything achievable on Earth. Astronomers use the Hubble constant as a measure of how fast the universe is expanding.
It is named after the American astronomer Edwin Hubble who first discovered that the universe is expanding. There are two conceptually different approaches to measuring the Hubble constant. One is indirect, based on predictions of our cosmological model tuned to match the patterns in the cosmic microwave background, the faint afterglow of the Big Bang.
Telescopes such as the Planck Space Telescope have measured tiny fluctuations in this ancient light, predicting a Hubble constant of about 67 kilometers per second per megaparsec (km/s/Mpc). A parsec is a unit of distance used in astronomy equal to about 3.26 light years, or 30.9 trillion kilometers. A megaparsec is one million parsecs.
The second method is more direct, similar to the one used by Hubble in the 1920s when he first demonstrated that the universe is expanding. It measures how fast distant galaxies are moving away from our home galaxy, the Milky Way, by observing the brightness of supernovae explosions in these far away galaxies. Type Ia supernovae are known to be “standard candles” because we know that their luminosity is the same wherever they are.
That means we can judge the distance to them from how dim they appear to us. To determine their intrinsic brightness, astronomers use other standard candles, such as Cepheid stars, in the galaxies nearby. These observations, which use the Hubble and James Webb space telescopes, give a higher value of around 73 km/s/Mpc.
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