Webb Relieves the Hubble Tension

Sometimes, when scientists measure things differently, they get different results. Whenever that happens with something as crucial to humanity’s long-term future as the universe’s expansion rate, it can draw much attention. Scientists have thought for decades that there has been such a difference, known as the Hubble Tension, in measurements of the speed at which the universe is expanding. However, a new paper by researchers at the University of Chicago and the Carnegie Institution for Science using data from the James Webb Space Telescope (JWST) suggests that there wasn’t any difference at all.

To understand this more, let’s first look at the Hubble tension. Edwin Hubble, the namesake of the Hubble Space Telescope, JWST’s predecessor, first found the universe was expanding when he looked at the speed at which galaxies travel. He found galaxies that were farther away from us were traveling faster than those nearest to us, and the best answer that we have as to why is that the universe itself is expanding.

It does not do so on a scale that we would notice in our daily lives, but on the scale of the space between galaxies, it is definitely noticeable, and in a number of ways. Historically, there have been two different ways to measure this Hubble Constant, as the rate of expansion is known. One involved studying the Cosmic Microwave Background (CMB), and one involved looking at the speed of galaxies, as Hubble did.

Data on the CMB have been consistent and precise for a long time. Studies have shown that it points to an expansion rate of 67.5 kilometers per second per megaparsec. To put that into perspective, the universe adds a little under an hour of highway drive time every second but does so on the scale of 3.2 million light years. Again, that expansion is not noticeable on our own scale, but on the immense scales of the universe, it is very noticeable.

However, calculations of that expansion value differ for the second method of measuring galaxies. Traditionally, the value is higher by about 9% and is estimated at 74 kilometers per second per megaparsec. That measurement is typically done using data from two different kinds of stars in those far and near galaxies – Cepheid variables and “Tip of the Red Giant Branch.” 

Dr. Wendy Freedman, one of the paper’s authors, is an expert in using Cepheid variables to measure the distance of things, so getting a chance to use JWST’s even more precise instrumentation was likely an excellent moment for her and her team. But they didn’t stop there. They added data from another type of star, whose use in calculating distance to an object has recently become more popular. Carbon stars are known for their consistent brightness and wavelengths in near-infrared – exactly the wavelengths JWST was designed to study. Using those known properties, the researchers could calculate redshift and other variables, allowing them to use this new technique to validate their version of the Hubble Constant.

The number they found was much closer to that calculated by the CMB method – 70 kilometers per second per megaparsec, a difference of only 3.5%. That’s within the bounds of estimations for most astronomical calculations, so the authors suggest there might not be a Tension between the two measurements. 

That claim will undoubtedly spark some controversy in the astronomical community, as there are some theories with plenty of proponents to explain the difference in measurements. But, as instruments like JWST provide more and more detailed data and researchers are better able to constrain some of the astronomically large values, one day, we might prove that this existential crisis that has been sitting at the center of cosmology for decades might never have been a thing at all.

Learn More:
University of Chicago – New Webb Telescope data suggests our model of the universe may hold up after all
Freedman et al. – Status Report on the Chicago-Carnegie Hubble Program (CCHP): Three Independent Astrophysical Determinations of the Hubble Constant Using the James Webb Space Telescope
UT – Astronomers Rule Out One Explanation for the Hubble Tension
UT – If Our Part of the Universe is Less Dense, Would That Explain the Hubble Tension?

Lead Image:
Scientists used new data taken by the James Webb Space Telescope to make a new reading of the rate at which the universe is expanding over time, by measuring light from 10 galaxies including the one known as NGC 3972, above.
Credit – Yuval Harpaz, data via JWST

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