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The James Webb Space Telescope’s observations of 10 nearby galaxies appear to indicate the Hubble tension—a baffling discrepancy in measurements of the rate of expansion of the universe – maybe it’s not true after all.
The James Webb Space Telescopethe observations place the average value of the Hubble constant (H0), crucial for determining the rate at which the universe is expanding, at 69.96 kilometers per second per megabyte.parsec. This is indeed consistent with predictions arising from the standard model of cosmology, that should seem like the end of the issue — but the results also show critical disagreement.
In 2013, the European Space AgencyThe Planck mission measured the Hubble constant as 67.4 kilometers per second per megapersecond. In other words, this means that each megaparsec (million parsecs, or 3.26 million light year) increasing by 67.4 kilometers (41.9 miles) every second. The Planck science team was able to derive the value of this Hubble constant by measuring the fundamental properties of the universe captured in light of the constant. cosmic microwave background (CMB) and then apply our standard model of cosmology to predict the expansion rate. Assuming the standard model is correct, this method should be accurate to within 1%.
In addition, measurements by a team led by Adam Riess from Johns Hopkins University, who were using the Hubble Space Telescope to measure cosmic expansion using type Ia supernovaof which the explosions white ox stars, beg to differ. Type Ia supernovae have a standardized maximum brightness, which means astronomers can measure how far away they must be based on how bright they are. This distance is compared to theirs redshiftbecause the faster the universe is expanding, the greater the change in matter. That method puts H0 at 73.2 kilometers per second per mega-persecond, suggesting that the Universe is expanding faster than the standard model predicts. It is this discrepancy that scientists describe as the Hubble tension.
Related: The James Webb Space Telescope complicates the expanding universe paradox by checking Hubble’s work
And now, new work led by Wendy Freedman of the University of Chicago raises some difficult questions.
Freedman’s team, working on a project called the Chicago–Carnegie–Hubble Program (CCHP), used the JWST to measure the distance to ten nearby. galaxies which were noted to have been type Ia supernovae. The area measurements were then cross-checked by three independent methods.
It is called “tip of the red giant branch,” which describes the maximum brightness that emerged sun-good stars called red giants can achieve. The second method involves what is known as the J-region asymptotic giant branch, which refers to a cluster of carbon-rich red giant stars with similar intrinsic infrared luminosities. The third cross-check was done with Cepheid variable stars, whose period-luminosity relationship was first discovered by Henrietta Swan Leavitt in 1908, which connects the period of a pulse with the maximum luminosity. In other words, simply by measuring how long it takes for a star to pulsate, we can calculate its maximum brightness and compare that to how bright it is in the the night sky to find out how far away it must be.
The CCHP team measured H0 as 69.85 km/s/Mpc using the tip of the red giant branch, and measured 67.96 km/s/Mpc using the carbon stars. So far, so good – the associated error bars include Planck’s measurement of H0, making them in agreement with the standard model.
The Cepheid variables, however, were not playing ball. Of these, the CCHP team arrived at a value of 72.04 km/s/Mpc, which is inconsistent with the other measurements. Together, the four methods give an average value of 69.96 km/s/Mpc.
“Based on these new JWST data and using independent methods, we do not find strong evidence for the Hubble tension,” Freedman said in statement. “On the contrary, our standard cosmological model appears to the evolution of the universe holding up.”
However, Cepheid variable measurements seem to continue to provide tension. Cepheids are the bottom of the cosmic distance ladder, with type Ia supernovas the next step because they can be seen further out than Cepheids. In the work done by Riess’s group — H0 Supernovae for the Equation of State, or SH0ES for short — cefids are critical to calibrating measurements of type Ia supernovae.
However, Freedman has expressed concern in the past about a potential problem known as “overcrowding.” Although the Hubble Space Telescope with a resolution powerful enough to identify Cepheid variable stars in other galaxies, low-mass stars very close to Cepheid may be unresolved and obscured by Cepheid light, affecting scientific results.
Earlier this year, Riess led a team that used the JWST to double-check Hubble’s observations of Cepheids and concluded congestion was not an issue. However, in their research paper, Freedman and co-researchers point out that the two modes least affected by the crowding—the tip of the red giant branch and the carbon stars—give values in line with the standard model.
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Although the focus will now be on Galactic distance measurements using Cepheid variables, more measurements with the JWST of galaxies with type Ia supernovae will be invaluable in confirming the results for these 10 galaxies. However, type Ia supernovae in galaxies with soluble Cepheids, red giants and carbon stars are relatively rare, meaning that it may take some time to get a large enough sample.
The results from the CCHP team led by Freedman are currently available as a preprintand submitted for peer review by The Astrophysical Journal.
