A collaboration of astronomers has used the James Webb Space Telescope to observe Cepheid variable stars, ruling out one source of the prevailing “Hubble tension.”

Spiral galaxy NGC 5468
This image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the farthest galaxy in which Hubble has identified Cepheid variable stars, which enable astronomers to measure the universe's current rate of expansion.
NASA / ESA / CSA / STScI / Adam G. Riess (JHU, STScI)

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.

Cepheid variable star in Webb vs. Hubble views
These two images by Webb and Hubble show the same Cepheid variable star in a distant galaxy. The stars are crisper in Webb's view, resulting in more accurate measurements.
NASA / ESA / CSA / STScI / Adam G. Riess (JHU, STScI)

“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.

Cepheid variable stars' period-magnitude relation
These six plots show the relation between Cepheid variable stars' apparent brightness, or magnitude, and the period of the variation in that brightness. From this relation, astronomers can derive the star's true brightness and thus its distance. The red points are new measurements from JWST, while the gray points are from the Hubble Space Telescope. JWST's measurements are visibly more precise.
Riess et al. / Astrophysical Journal Letters 2024

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.”


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March 22, 2024 at 7:53 pm

If more distant objects, thus earlier objects, were measured I'd expect an even slower Hubble Constant due to Dark Energy's smaller acceleration at an earlier time.

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Monica Young

March 26, 2024 at 2:26 pm

The "Hubble constant" is a constant because it measures the universe's present-day expansion rate. The expansion rate did change in the past, but the Hubble constant doesn't measure that change, it is only a measure of the expansion that's happening right now.

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Brian of DRAA

March 25, 2024 at 9:02 am

Please clarify: What is Reiss' problem with the current cosmological model, Perlmutter and Reiss are resposible for 70% of the current models mass/energy and their contribution made Omega = 1 which gave Inflation theory more validation. What modification of the current model does he have in mind?
Also. if Chepeids can only take you back 130 million years, but Reiss' Dark Energy didn't start to take hold until 5 billion years ago, what is the fuss about a measurement that goes back in time only 1/70 of the way to where Dark Energy begins to reveal its power. Why does Reiss care about the "local" 130 million year old history when his measurements of type 1A supernovas goes back over 5 billion years?

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Brian of DRAA

March 28, 2024 at 10:47 am

Correction: I meant 1/35 as the fraction between Cepheids data and Reiss' measurements of type 1A Super Novas. 1/70 is wrong.
Also, to clarify, I mean, regardless of the local expansion rate (the "current" Hubble constant) the Lambda CDM Inflationary Big Bang model would still hold, only the age of the universe would be a function of the variable H-naught. Is Reiss is considering the CMB BAO data as the information that doesn't fit the current model? Or, perhaps the JWST galaxies that are too big, too bright too early? Any clarification would be appreciated.

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