Some novel observations may have solved the mystery of why the expansion rate of the Universe appears to change depending on how you measure it.
Astronomers assess the Universe’s expansion in two ways: by looking at radiation from the Big Bang, which gives a value of 67.4km per second per megaparsec; or from the motion of nearby galaxies, which gives 74km per second per megaparsec.
The rate of the expansion of the Universe changes depending on how you measure it, and this is known as the Hubble tension, named after Edwin Hubble, the American astronomer who in 1929 presented the first observational evidence that the Universe is expanding.
How the Universe's expansion rate is measured
It’s unclear if the disagreement is because our assumptions about the Universe are wrong, or if it’s our measurements of galactic distances.
Many astronomers believe the latter is the culprit.
These measurements are made using standard candles, objects with a known intrinsic brightness.
By comparing how bright they are with how they appear, we can determine their distance.
To calibrate a ‘distance ladder’, we need to find galaxies that contain two or more different kinds of objects.
Where Webb comes in
This new study looked at two well-used methods: Cepheid variable stars whose variance is linked to their brightness; and low-mass stars, which have a fixed limit on how bright they are.
Researchers also used a new method, looking at carbon stars that have a strong colour-to-brightness relationship in the near-infrared.
Using the James Webb Space Telescope, the team found eight galaxies that overlapped between these methods, which they used to measure a Hubble constant of 70km per second per megaparsec.
“Based on these new James Webb Space Telescope data and using three independent methods, we do not find strong evidence for a Hubble tension,” says Wendy Freedman from the University of Chicago, who led the study.
While the results are promising, the sample size is currently very small, so the team plan to look for more galaxies.
“Getting good agreement from three completely different types of stars is a strong indicator that we’re on the right track,” says Freedman.
