15.08.2024

Triple measurement of the Hubble constant using JWST suggests unidentified biases may account for disparate results

Red giant stars are becoming a powerful tool for measuring cosmic distances in pursuit of the Hubble constant, a measure of the universe's expansion rate.QA INTERNATIONAL/SCIENCE SOURCE

A new front has opened in the longstanding debate over how fast the universe is expanding. For years astronomers have argued over a gulf between the expansion rate as measured from galaxies in the local universe and as calculated from studies of the cosmic microwave background (CMB), the afterglow of the Big Bang. The disparity was so large and persistent that some astronomers thought the standard theory of the universe might have to be tweaked. But over the past week, results from NASA’s new JWST orbiting observatory suggest the problem may be more mundane: some systematic error in the strategies used to measure the distance to nearby galaxies.

“The evidence based on these data does not suggest the need for additional physics,” says Wendy Freedman of the University of Chicago, who leads a team that calculated the expansion rate from JWST data using three different galactic distance measurements and released the results on the arXiv preprint server. (The papers have not yet been peer reviewed.) The methods disagreed about the expansion rate, known as the Hubble constant, or H0, and two were close to the CMB prediction.

“It looks like there’s reason to believe that it’s not a trivial matter to get an H0, that very reliable techniques disagree,” says astrophysicist Saul Perlmutter of the University of California, Berkeley, who shared the 2011 Nobel Prize in Physics for the discovery that the universe’s expansion is accelerating.

The stakes are high because the Hubble Constant is one of the key parameters defining the universe, telling us its size and age. The disagreement between theoretical predictions and observed values is so entrenched that it has a name: the Hubble tension. The new data don’t resolve it, but they do refocus astronomers on refining their efforts to measure its value in the nearby universe. “I’m really curious how this will turn out because JWST’s capabilities are just superior compared to what we had before,” says astrophysicist Géza Csörnyei of the Max Planck Institute for Astrophysics.

The Hubble tension emerged a dozen or more years ago, when space telescopes began to accurately map tiny fluctuations in the temperature of the CMB. From those maps, cosmologists could estimate the universe’s initial rate of expansion and extrapolate forward to the present day, factoring in two cosmic forces that shaped the expansion: the gravitational pull of mysterious dark matter and the push of an equally mysterious factor known as dark energy.

This calculation of H0 rooted in the early universe produced a value of about 67 kilometers per second (km/s) per megaparsec (Mpc)—meaning that for every 3.25 million light-years farther out into space, galaxies are receding 67 km/s faster. But when astronomers measured H0 more directly, by tracking how fast nearby galaxies are receding, they got a different value, about 72 km/s per Mpc. As techniques improved those two estimates have not converged, remaining stubbornly about 5 km/s apart—the Hubble tension.

But measuring a value for H0 in the local universe is not a simple matter. Astronomers can assess the speed at which a galaxy is receding by measuring the extent to which its light is stretched to longer wavelengths by its motion. Getting the distance to a galaxy is much harder, requiring what’s known as a distance ladder. First astronomers identify some type of star with a predictable brightness—a standard candle—that is close enough in our galaxy to directly measure its distance, for example from its apparent motion as Earth orbits the Sun. Astronomers have long used a type of variable star called a Cepheid, whose pulse rate indicates how bright it is. Then they can look for Cepheids in nearby galaxies. The pulse rate of each star reveals its true brightness, so comparing that with how bright the stars appear gives their distances.

To go farther, astronomers need a brighter standard candle. For that, they have turned to exploding stars known as type Ia supernovae—each one a white dwarf star that grows too large and ignites in a thermonuclear blast, which reaches a predictable maximum brightness. By finding these supernovae in galaxies at a known distance—calculated using Cepheids—they can add another rung to the distance ladder and then look for similar supernovae in more distant galaxies.

Adam Riess of Johns Hopkins University and the team he leads, known as SH0ES, have turned this method into a fine art, deriving H0 by measuring hundreds of Cepheids in 37 nearby galaxies that are host to 42 type Ia supernovae and then extending measurements out to 277 more distant supernovae. In 2022, using the Hubble Space Telescope, the team put H0 at 73 with a 1.4% precision.

Although SH0ES has championed high values of H0 for many years, in 2019 the Carnegie-Chicago Hubble Program (CCHP), led by Freedman, challenged that with a different standard candle: red giant stars, which attain a predictable maximum brightness as they burn the last of their hydrogen. CCHP used the brightest red giants in galaxies to gauge distance and derived an H0 of about 70. To Freedman and others, that disagreement was a hint that some sort of undetected systematic bias lurked in one or both methods.

Now the disagreement has deepened. The CCHP team has turned to JWST to measure the distance to the same 10 local galaxies using three standard candles simultaneously: Cepheids, the brightest red giants, and a new one, carbon stars. These are a type of red giant whose atmosphere contains a large amount of carbon and carbon compounds that gives them a distinctive spectrum and a characteristic brightness. The CCHP team found the distances to the 10 galaxies measured by the brightest red giants and the carbon stars agreed to about 1%, but differed from the Cepheid-based distance by 2.5% to 4%. Combining all three methods, the team derived a value of H0 just shy of 70 km/s per Mpc, closer to the cosmological value than to SH0ES’s value using Cepheids alone.

“There’s something systematic in the measurements,” Freedman says. “Until we can establish unambiguously where the issue lies in the nearby universe, we can’t be claiming that there’s additional physics in the distant universe.”

The Hubble constant debate will not end there. Riess points out that other teams have used JWST to measure distances with all three methods separately and have come up with values closer to the original SH0ES result. He also questions why CCHP excluded data from telescopes other than JWST. “I don’t see a compelling justification for excluding the data they do,” he says.

Perlmutter, however, applauds the CCHP team for its straightforward approach to directly comparing the three distance methods. The team gave all three “the best shot that they could,” he says. “The biggest message coming out of all this is that the story is not over, that there’s variables to be hunted down that are in play here.” Given the cosmically huge stakes, “We do need to get to the bottom of it.”

Quelle: AAAS