As part of the Chicago-Carnegie Hubble Program, astronomers using the NASA/ESA/CSA James Webb Space Telescope have performed new measurements of the Hubble constant. Their results are consistent with the current standard Lambda cold dark matter (ΛCDM) model, without the need for the inclusion of additional new physics.
This artist’s impression shows the evolution of the Universe beginning with the Big Bang on the left followed by the appearance of the Cosmic Microwave Background. The formation of the first stars ends the cosmic dark ages, followed by the formation of galaxies. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
“The new evidence is suggesting that our Standard Model of the Universe is holding up,” said University of Chicago’s Professor Wendy Freedman.
“It doesn’t mean we won’t find things in the future that are inconsistent with the model, but at the moment the Hubble Constant doesn’t seem to be it.”
There are currently two major approaches to calculating how fast our Universe is expanding.
The first approach is to measure the remnant light left over from the Big Bang, which is still traveling across the Universe.
This radiation, known as the Cosmic Microwave Background, informs astronomers about what the conditions were like at early times in the Universe.
Professor Freedman and colleagues specialize in a second approach, which is to measure how fast the Universe is expanding right now, in our local astronomical neighborhood.
Paradoxically, this is much trickier than seeing back in time, because accurately measuring distances is very challenging.
Over the last half century or so, scientists have come up with a number of ways to measure relatively nearby distances.
One relies on catching the light of a particular class of star at its peak brightness, when it explodes as a supernova, at the end of its life.
If we know the maximum brightness of these supernovae, measuring their apparent luminosities allows us to calculate its distance.
Additional observations tell us how fast the galaxy in which that supernova occurred is moving away from us.
An image of the CMB radiation from the Atacama Cosmology Telescope; orange and blue represent more or less intense radiation. Image credit: ACT Collaboration.
Professor Freedman has pioneered two other methods that use what we know about two other types of stars: red giant stars and carbon stars.
However, there are many corrections that must be applied to these measurements before a final distance can be declared.
Astronomers must first account for cosmic dust that dims the light between us and these distant stars in their host galaxies.
They must also check and correct for luminosity differences that may arise over cosmic time.
And finally subtle measurement uncertainties in the instrumentation used to make the measurements must be identified and corrected for.
But with technological advances such as the launch of the much more powerful Webb in 2021, scientists have been able to increasingly refine these measurements.
“We’ve more than doubled our sample of galaxies used to calibrate the supernovae,” Professor Freedman said.
“The statistical improvement is significant. This considerably strengthens the result.”
The team’s latest calculation, which incorporates data from both Hubble and Webb telescopes, finds a value of 70.4 km per second per megaparsec, plus or minus 3%.
That brings her value into statistical agreement with recent measurements from the Cosmic Microwave Background, which is 67.4 km per second per megaparsec, plus or minus 0.7%.
Webb has four times the resolution of Hubble, which allows it to identify individual stars previously detected in blurry groups.
It’s also about 10 times as sensitive, which provides higher precision, and the ability to find even fainter objects of interest.
“We’re really seeing how fantastic Webb is for accurately measuring distances to galaxies,” said Dr. Taylor Hoyt, a researcher at the Lawrence Berkeley Laboratory.
“Using its infrared detectors, we can see through dust that has historically plagued accurate measurement of distances, and we can measure with much greater accuracy the brightnesses of stars,” added Dr. Barry Madore, a reseacher at the Carnegie Institution for Science.
“Astrophysicists have been trying to come up with a theory that would have explained different rates of expansion as the Universe ages,” Professor Freedman said.
“There have been well over 1,000 papers trying to attack this problem, and it’s just turned out to be extraordinarily difficult to do.”
The team’s paper was published on May 27 in the Astrophysical Journal.
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Wendy L. Freedman et al. 2025. Status Report on the Chicago-Carnegie Hubble Program (CCHP): Measurement of the Hubble Constant Using the Hubble and James Webb Space Telescopes. ApJ 985, 203; doi: 10.3847/1538-4357/adce78
