Webb Telescope Measures Universe’s Expansion

For the past decade, a discrepancy in the measurement of the current expansion rate of the universe, called the Hubble constant, has mystified cosmologists. Now, in a new study, a team of astronomers makes the case that high-resolution data from the James Webb Space Telescope (JWST) has allowed them to rule out one possible reason for the discrepancy.

In measuring the Hubble constant, measurements of relatively nearby astronomical objects give a faster current expansion rate (73 kilometers per second per megaparsec), while measurements of the early universe give a slower rate (67.5 km/s/Mpc). This mismatch is known as the Hubble tension. Either there’s a misunderstanding in the standard model of cosmology — or there’s an error in interpreting the observations that inform it.

In the Astrophysical Journal Letters, Adam Riess (Johns Hopkins University and the Space Telescope Science Institute) and other astronomers of the SH0ES collaboration (short for Supernova H0 for the Equation of State) have published a study ruling out one of those errors.

The collaboration has been measuring the expansion rate using Cepheid variable stars for almost 20 years, first with the Hubble Space Telescope and now with JWST. Cepheids pulse in a way that reveals their intrinsic brightness, which means astronomers can measure their apparent brightness to figure out their distance. However, Riess says that one of the biggest challenges with this technique has been all the other stars in the foreground and background of the Cepheids, which makes it difficult to measure the pulsations.

“We account for that statistically by adding in what are called ‘artificial stars,’” Riess explains. “Basically, we would put fake images of stars into the real images, and then figure out what the effect was so we could correct for it.” But concerns lingered that the fake-star technique might be somehow flawed.

“JWST is such an amazing telescope we don’t need to do that anymore,” Riess says. The SH0ES collaboration used JWST to image more than 1,000 Cepheids spanning the distance range of those previously used to measure the current expansion rate with Hubble. The new images were able to better resolve the variable stars, and the team discovered no significant differences in the stars’ distance measurements.

The scientific community, as a rule, tends to be cautious until results have been independently verified. Wendy Freedman (University of Chicago) co-led the Hubble Space Telescope Key Project, which made the first accurate measurement of the Hubble constant in 2001, and she currently leads the Carnegie-Chicago Hubble Program, which has also been working to constrain the current expansion rate using other methods, such as via observations of red giant stars.

Using those measurements, Freedman has found a lower Hubble constant, more in line with measurements of the early universe. Still, she thinks more information is needed from many different sources before we can say for sure whether the Hubble tension stems from measurement problems and, if so, whether certain techniques might be implicated or exonerated.

“Measuring distant objects is very challenging,” she says. “That's why we set out to measure the Hubble constant in different ways, because any method you use will have its own set of potentially systematic uncertainties.”

“New technology, like JWST, allows us to make real progress,” she adds. “One of the things that my group is doing now is to measure not just the Cepheids, but also the red giant stars that we measured with Hubble, and also a new kind of measurement using carbon stars. And they're all three being measured in the same galaxies, all of which have previously known Cepheids and supernovae.”

Riess agrees that more distance measurements, from different sources and using different instruments and techniques, are valuable. His team is involved in similar work, using JWST to study other astronomical objects (including red giant stars, asymptotic giant stars, and Mira variable stars).

However, Riess maintains that if measurement errors are to blame, it’s odd that measurements of objects in the nearby universe are all “wrong” in the same direction. He’s therefore steadfast in suggesting that the scientific community might consider giving a harder look at the theoretical assumptions of the standard cosmological model. 

“Granted, it's not satisfying unless you can figure out how to fix [the standard model] to better match the data,” Riess says. “Still, I would say it looks and smells like the cosmological model is the problem.”