In an epic cosmology clash, rival scientists begin to find common ground 

The biggest clash in cosmology might be inching closer to resolution, thanks to the James Webb Space Telescope. 

Scientists disagree over the universe’s expansion rate, known as the Hubble constant. There are two main methods for measuring it — one based on exploding stars called supernovas and the other on the universe’s oldest light, the cosmic microwave background. The two techniques have been in conflict for a decade, in what’s known as the “Hubble tension” (SN: 3/21/14). If this tension is real, and not the result of an error in one of the measurements, it would demand a drastic shift in how scientists understand the universe. 

New papers published by two of the central players are raising hopes that additional observations from the James Webb Space Telescope, or JWST, of certain types of stars and supernovas could solve the question of whether the discord is real, once and for all.

The two teams disagree about whether that tension exists at all. One team says there’s no strong evidence for the Hubble tension from the JWST data. But the other group says the JWST data strengthen the case that the two types of measurements are in conflict. “I’m even more intrigued by the Hubble tension,” says cosmologist Adam Riess of Johns Hopkins University, leader of one of the teams.

The different camps are finally seeing eye to eye on one piece of their measurements: distances to nearby galaxies, which are necessary to deduce the expansion rate of the universe from supernovas. “This is really new — we’re agreeing on distances, and that’s real progress,” says cosmologist Wendy Freedman of the University of Chicago, who leads the other team.

“If you told me 10 years ago that all this would be agreeing at this level, I would just be jumping up and down,” says cosmologist Daniel Scolnic of Duke University, a member of Riess’s team.

That agreement gives scientists newfound confidence that the longstanding dispute is close to resolution. “I’m pretty optimistic that in the next couple of years, the questions that we’re talking about now, we will have resolved those,” Freedman says.

Coming to consensus on distances

Scientists’ theory of the universe, called the standard cosmological model, is based largely on unknowns. Dark matter, a substance that adds unseen mass to galaxies, has never been directly detected. And dark energy, a phenomenon that causes the universe’s expansion to accelerate, is likewise a total question mark. But the model has proven extremely successful in describing the cosmos.

Starting from the ancient light of the cosmic microwave background, scientists can use the standard cosmological model to determine today’s expansion rate. That technique finds that space is expanding at 67 kilometers per second per megaparsec. (One megaparsec is about 3 million light-years.)

But measurements of supernovas by Riess and colleagues peg the number at about 73 km/s/Mpc — putting the two results in direct conflict. That could hint that something is wrong with the standard cosmological model.

To determine the expansion rate via the supernova technique, cosmologists must measure the distances to many distant supernovas. That requires a technique called a distance ladder, to translate nearby distances to those further out. 

Under particular scrutiny is the second rung of this ladder, in which scientists observe certain types of stars — most commonly, pulsating stars called Cepheids — to determine the distances to the galaxies they reside in, as well as to supernovas that occurred in the same galaxies. Observing these stars with JWST, which has better resolution than the Hubble Space Telescope, could suss out flaws in the measurements for that rung.

In addition to Cepheids, Freedman and colleagues used two other types of stars for their distance measurements. Using JWST data on all three, Freedman and colleagues find an expansion rate of about 70 km/s/Mpc. Given the uncertainties on the measurements, that’s close enough to the cosmic microwave background number that it doesn’t require physicists to rethink the cosmos, the team reports in a paper submitted August 12 to arXiv.org. But it also doesn’t fully rule out the existence of the Hubble tension. “We need more data to answer the question definitively,” Freedman says.

The three distance measurement techniques were generally in agreement, Freedman says. The Cepheid measurements result in a slightly higher value of the Hubble constant than the other two methods, but not enough to conclude something’s wrong with the technique. “There is an offset, but the uncertainties are large enough that you can’t say definitely, ‘This is the way it’s going to turn out,’” Freedman says.

Constant Hubble hubbub

Despite agreeing on distances, the teams still differ on the Hubble constant. That could be due to the small number of measurements made with JWST so far, Riess, Scolnic and colleagues report in a paper submitted to arXiv.org on August 21. If Freedman’s team picked different galaxies to observe with JWST, they would’ve gotten a larger value of the Hubble constant, the team argues. (Neither paper has been peer reviewed, and the results could change under further scrutiny.)

Scientists are working with just the first tidbits of data from JWST. To resolve the puzzle, “the best thing we can do is use a whole lot more JWST time to study the distance scale,” says astronomer John Blakeslee of NOIRLab in Tucson, Ariz., who was not involved with the research. 

Freedman wants to keep looking for unidentified issues known as systematic uncertainties that could be artificially pushing estimates of the Hubble constant higher. One concern is crowding — many stars lumped together in the same place, throwing off measurements of the Cepheids. Last year, Riess’s team found no evidence of crowding in JWST data (SN: 9/28/23). But that effect might be more prominent at larger distances than have been studied so far with JWST, Freedman suggests.

If scientists find that different distance measurements disagree, says cosmologist Saul Perlmutter of the University of California, Berkeley, who was not involved with the research, “then it may suggest that we still have to get to the bottom of systematic uncertainties first before we get as concerned about a major problem with the cosmological model.”

But many physicists are bullish about the Hubble tension. For one thing, various other methods have also found higher-than-expected expansion rates, says cosmologist Eleonora Di Valentino of the University of Sheffield in England, who was not involved with the research. “The Hubble tension is still very robust.”

“I see these results as supporting … the fact that we have this difference between what we expect from our standard cosmological model and what we see from these measurements,” says cosmologist Lloyd Knox of the University of California, Davis, who is not involved with either team. 

The standard cosmological model, he notes, rests on mysterious dark energy and dark matter. “Perhaps this is a clue about the dark universe, and we just need to figure out how to interpret it.”