Physicists have been puzzling over conflicting observational results pertaining to the accelerating expansion rate of our Universe—a major discovery recognized by the 2011 Nobel Prize in Physics. New observational data from the James Webb Space Telescope (JWST) has confirmed that prior measurements of distances between nearby stars and galaxies made by the Hubble Space Telescope are not in error, according to a new paper published in The Astrophysical Journal. That means the discrepancy between observation and our current theoretical model of the Universe is more likely to be due to new physics.
As previously reported, the Hubble Constant is a measure of the Universe's expansion expressed in units of kilometers per second per megaparsec (Mpc). So, each second, every megaparsec of the Universe expands by a certain number of kilometers. Another way to think of this is in terms of a relatively stationary object a megaparsec away: Each second, it gets a number of kilometers more distant.
How many kilometers? That's the problem here. There are basically three methods scientists use to measure the Hubble Constant: looking at nearby objects to see how fast they are moving, gravitational waves produced by colliding black holes or neutron stars, and measuring tiny deviations in the afterglow of the Big Bang known as the Cosmic Microwave Background (CMB). However, the various methods have come up with different values. For instance, tracking distant supernovae produced a value of 73 km/s Mpc, while measurements of the CMB using the Planck satellite produced a value of 67 km/s Mpc.
Just last year, researchers made a third independent measure of the Universe's expansion by tracking the behavior of a gravitationally lensed supernova, where the distortion in space-time caused by a massive object acts as a lens to magnify an object in the background. The best fits of those models all ended up slightly below the value of the Hubble Constant derived from the CMB, with the difference being within the statistical error. Values closer to those derived from measurements of other supernovae were a considerably worse fit for the data. The method was new, with considerable uncertainties, but it did provide an independent means of getting at the Hubble Constant.
Earlier this year, JWST researchers reported on measurements built on last year's confirmation based on Webb data that Hubble's measurements of the expansion rate were accurate, at least for the first few "rungs" of the "cosmic distance ladder." But there was still the possibility of as-yet-undetected errors that might increase the deeper (and hence further back in time) one looked into the Universe, particularly for brightness measurements of more distant stars.
Additional observations of Cepheid variable stars—a total of 1,000 in five host galaxies as far out as 130 million light-years—correlated with the Hubble data helped JWST see past the interstellar dust that has made Hubble's own images of those stars more blurry and overlapping. This enabled astronomers to more easily distinguish between individual stars. Those results further confirmed the accuracy of the Hubble data and allowed astronomers to rule out measurement error with a high degree of confidence.
A critical cross-check
This latest study serves as a critical cross-check to the April paper, using three different measurements to determine distances to galaxies known to be hosts to supernovae. "Cross-checking Hubble might sound prosaic, but the Hubble results demonstrate a profound tension in the Universe between how fast it is expanding now (measured by Hubble) versus the prediction from the standard model, LambdaCDM (calibrated by the Cosmic Microwave Background)," lead author Adam Riess, of the Space Science Telescope Institute at Johns Hopkins University, told Ars. "So Webb confirming Hubble means we are really seeing something amiss in the Universe."
The new study, which includes data from two independent groups working to refine the Hubble constant, encompasses about a third of the full galaxy sample collected by Hubble. The authors used the distance to the galaxy NGC-4258 (aka Messier 106) as a reference point, since that distance is known. They used pulsating stars known as Cepheid variables to calculate distances and cross-checked their work with complementary distance measurements based on carbon-rich stars and red giants. They ended up with a Hubble constant value of 72.6 km/s/Mpc, very close to Hubble's value of 72.8 km/s/Mpc.
Now it's up to the theorists to come up with novel hypotheses to explain these findings. “One possible explanation for the Hubble tension would be if there was something missing in our understanding of the early Universe, such as a new component of matter—early dark energy—that gave the universe an unexpected kick after the Big Bang,” said JHU cosmologist Marc Kamionkowski, who was not involved in the new study. “And there are other ideas, like funny dark matter properties, exotic particles, changing electron mass, or primordial magnetic fields that may do the trick. Theorists have license to get pretty creative.”
As for how these new results compare to the 2023 measurements made using supernovae, "That result is based on a different and far more indirect route," said Riess. "We are measuring distances and redshifts, which is a direct measure of how fast the Universe is expanding. The [2023] measurement is based on modeling a phenomenon called strong lensing (seeing multiple images) determining where the intervening mass is. A byproduct of that is an estimate of the Hubble constant, but the result is rather imprecise, formally not inconsistent with the present work or any past work."
