The universe expands over time, but how fast it's expanding seemed to differ depending on whether you looked early in the universe's history or the present day. If true, this would have presented a major problem to the gold-standard model that represents our best understanding of the universe.
But thanks to the new James Webb Space Telescope, scientists from the University of Chicago have been able to take new and better data-suggesting there may be no conflict after all.
"This new evidence is suggesting that our Standard Model of the universe is holding up," said UChicago Prof. Wendy Freedman, a leading figure in the debate over this rate of expansion, known as the Hubble Constant.
"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," she said.
The new results are published in the May 27th issue of The Astrophysical Journal.
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.
Freedman, the John and Marion Sullivan University Professor in Astronomy and Astrophysics, specializes 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. Freedman has also 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. Scientists 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 James Webb Space Telescope 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," Freedman said. "The statistical improvement is significant. This considerably strengthens the result."
Freedman's latest calculation, which incorporates data from both the Hubble Telescope and the James Webb Space Telescope, finds a value of 70.4 kilometers 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, plus or minus 0.7%.
Webb has four times the resolution of the Hubble Telescope, 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 the James Webb Space Telescope is for accurately measuring distances to galaxies," said co-author Taylor Hoyt of 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 co-author Barry Madore, of the Carnegie Institution for Science.
"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," she said.
Scientists are still trying to find cracks in the Standard Model that describes the universe, which could provide clues to the nature of two big outstanding mysteries-dark matter and dark energy. But the Hubble Constant increasingly seems not to be the place to look.
Freedman and her team will be using the Webb Telescope next year to get measurements in a group of galaxies called the Coma cluster, which should provide more data from a different angle, she said. "These measurements will allow us to measure the Hubble constant directly, without the additional step of needing the supernovae."
"I am optimistic about resolving this in the next few years, as we boost the accuracy to make these measurements," she said.
Other UChicago coauthors included In Sung Jang, Abigail Lee and Kayla Owens.
Related Links
University of Chicago
Understanding Time and Space
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