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

The biggest conundrum in cosmology may be closer to being solved, thanks to the James Webb Space Telescope.

Scientists disagree on the expansion rate of the universe, known as the Hubble constant. There are two main methods for measuring it – one based on exploding stars called supernovae 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 is 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 require a drastic change in the way scientists understand the universe.

New papers published by two of the central players are raising hopes that additional observations by the James Webb Space Telescope, or JWST, of several types of stars and supernovae could settle the question of whether the discrepancy is real, once and for all.

The two teams disagree on whether that tension exists at all. One team says there is no strong evidence for Hubble tension from JWST data. But the other group says the JWST data strengthens 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 part of their measurements: the distances to nearby galaxies, which are needed to infer the expansion rate of the universe from supernovae. “This is really new—we’re agreeing on distances, and that’s a real breakthrough,” says cosmologist Wendy Freedman of the University of Chicago, who leads the other team.

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

This agreement gives scientists newfound confidence that the long-standing dispute is close to being resolved. “I’m very optimistic that in the next couple of years, the questions we’re talking about now, we’ll have solved them,” Freedman says.

Reaching consensus on distances

Scientists’ theory of the universe, called the standard cosmological model, is based largely on unknowns. Dark matter, a substance that adds invisible mass to galaxies, has never been directly detected. And dark energy, a phenomenon that causes the universe’s expansion to accelerate, is also a total question mark. But the model has proved remarkably 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 rate of expansion. This technique reveals that space is expanding at 67 kilometers per second per megaparsec. (A megaparsec is about 3 million light years.)

But supernova measurements by Riess and colleagues peg the number at about 73 km/s/Mpc – putting the two results in direct conflict. This may hint that something is wrong with the standard cosmological model.

To determine the rate of expansion through the supernova technique, cosmologists must measure the distances to many distant supernovae. This requires a technique called a distance scale, to translate closer distances to farther ones.

Under special scrutiny is the second rung of this scale, in which scientists observe certain types of stars – most commonly, pulsating stars called Cepheids – to determine the distances to the galaxies where they reside, as well as to the supernovae that occurred in them the same galaxies. Observing these stars with JWST, which has better resolution than the Hubble Space Telescope, could highlight flaws in measurements on that scale.

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

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 indicate anything wrong with the technique. “There’s a trade-off, but the uncertainties are big enough that you can’t say for sure, ‘This is the way it’s going to turn out,'” Freedman says.

Hubble constant

Despite agreeing on the distances, the teams still differ on Hubble’s constant. This may 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 had chosen different galaxies to observe with JWST, they would have obtained a larger value of the Hubble constant, the team argues. (None of the papers have been peer-reviewed, and results may change under further review.)

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

Freedman wants to continue looking for unidentified issues known as systematic uncertainties that could artificially push estimates of the Hubble constant higher. One concern is clumping—too many stars clumped together in the same place, throwing off Cepheid measurements. Last year, Riess’ team found no evidence of clumping in the JWST data (SN: 28/9/23). But this effect may be more apparent at greater distances than has 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 in the research, “then it may suggest that we still have to get to the bottom of the systematic uncertainties first.” before we get to a major problem with the cosmological model.”

But many physicists are satisfied with the Hubble tension. First, 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 in the research. “The Hubble tension is still very strong.”

“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 was not involved. in each team.

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