Easing the tension on Hubble – Sky & Telescope

For nearly a decade, astronomers have been struggling with a persistent mismatch between two different ways of determining the Hubble constant — a measure of the current rate of expansion of the universe. This mismatch, known as Hubble tensionhas led to claims that new physics might be needed to solve the problem. (Read about the “ongoing controversy” in the June 2019 issue of Sky and telescope.)

But a detailed analysis of a new set of observations from the James Webb Space Telescope (JWST) suggests that the problem may not exist. “As Carl Sagan said, extraordinary claims require extraordinary evidence,” says Wendy Freedman (University of Chicago), “and I don’t see extraordinary evidence.”

The Hubble constant (I₀) is expressed in units of kilometers per second per megaparsec (km/s/Mpc), where 1 megaparsec equals 3.26 million light-years. It is the ratio of the apparent velocity of recession of a galaxy (resulting from cosmic expansion) to its distance.

The traditional, so-called “local” way of determining the Hubble constant is to measure precise distances to galaxies using standard candles, whose apparent brightness can be compared to the known true luminosity. This distance can then be compared to that of the galaxies. redshifts – an indicator of the amount of cosmic expansion that occurred as light traveled toward Earth. Over the past decade, a team led by Adam Riess (Johns Hopkins University) has used this method to arrive at a value for I₀ approximately 73 km/s/Mpc, based primarily on observations with the Hubble Space Telescope.

Another way is to analyze the statistical properties of the patterns in the Cosmic microwave background (CMB), also known as the afterglow of the Big Bang. Using cosmological models, astronomers can use temperature fluctuations in the CMB to calculate the current rate of expansion of the universe. Using data from the European Planck mission, this method yields a Hubble constant of 67.4 km/s/Mpc.

The tension between these two methods has seemed real and insurmountable, leading to speculation about possible flaws in our fundamental understanding of the universe. According to cosmologist Richard Ellis (University College London), “it would be surprising if the local value[for the Hubble constant]deviated from the CMB measurements, as it might imply some new, as yet undiscovered physics.”

But in a recent article published in arXiv Astronomy Preprint ServerA team led by Freedman presents new JWST data on 11 galaxies that “do not strongly support the suggestion that fundamental physical elements are missing in the early universe,” as the authors write. (The paper has been submitted to the Astrophysical journalbut has not yet passed peer review). Their thorough analysis points to a value of I₀ between 68.8 and 71.1 km/s/Mpc, which they say is consistent with the current standard model of cosmology.

Riess and his colleagues used Cepheid variable stars as their standard candle of choice for measuring galactic distances, as these bright, relatively young supergiant stars have periods of variability related to their absolute luminosity. Freedman also used these stars.

However, Freedman's team also studied two other types of standard candles. One is a class of old, low-mass stars that experience a sudden “flash” of helium fusion in their cores as they reach the end of their red giant evolutionary phase; these stars are part of the so-called tip of the red giant branch (TRGB). Another particular type of carbon-rich pulsating stars, known as JAGB stars (short for Asymptotic giant branch of the J region).

According to Freedman, these two methods are more accurate than using Cepheids, although he admits that no single perfect method exists. “Cepheids are not just standard candles,” he says. “They are complex.” When analyzing the results, astronomers must correct for their temperatures and abundances of heavy elements.

Furthermore, since Cepheids are relatively young, they are found primarily in the densely populated and dusty spiral arms and inner disks of galaxies, so it is necessary to understand the role of dust absorption and crowdingwhere a Cepheid may appear brighter than it actually is because its light is mixed with the light of other nearby stars, depending on the angular resolution of the telescope.

In contrast, red giant and JAGB stars, though intrinsically fainter than Cepheids, are found in the outer disks and halos of galaxies. Using these stars yields distances between galaxies that are in “superb agreement” with each other, Freedman says, while the Cepheid method arrives at somewhat smaller distances and a correspondingly higher value for the Hubble constant.

JWST's angular resolution is four times greater than that of Hubble, and a recent study… JWST study conducted by Riess' team indicates that crowding effects do not play a decisive role. However, Freedman believes that the divergent results for Cepheids that Riess and his colleagues found could be a result of the many complexities associated with this type of star. “All methods are fraught with systematic errors,” he says, “some of which may be unknown.”

Ellis agrees and is particularly impressed by Freedman’s TRGB work. “JWST is the new kid on the block,” he says. “I really find (their) case compelling. It definitely raises the question of whether the error bar on the Cepheid-based value for I₀ “has been underestimated due to contamination from crowding.” That would make the various distance estimates agree more closely with each other, loosening the Hubble tension.

The Hubble constant value presented by Freedman's team is still a bit higher than the CMB-based value, but the error bars now overlap. “The CMB people are convinced that everything[in their analysis]is understood,” Freedman says, “but it's very difficult to get to one percent accuracy.” Also, converting the distances of local galaxies into a value for I₀ It also involves the precise calibration of Type Ia supernovae as standard candles for much more remote galaxies, which could introduce even more uncertainties. “This is all really hard work.”

The hard work will continue for years. “So far, we’ve only scratched the surface,” Freedman says. “By looking at more distant galaxies with JWST, we’ll get to the bottom of it.”

In the next decade, large facilities such as the European Extremely Large Telescope and NASA's Roman Space Telescope will also be able to measure galactic distances over much larger volumes of space, while the Vera Rubin Observatory is expected to discover some 300,000 new distant supernovae per year.

Freedman says: “The path is clear; we are just getting started.”

Editorial note (August 23, 2024): Since this story was published, Adam Riess' team has posted a rebuttal on the arXiv repository, claiming that the TRGB and JAGB methods result in a Hubble constant that is consistent with that measured with Cepheids. Read their results here: https://www.arxiv.org/abs/2408.11770.