New Webb Telescope data suggests our model of the universe may hold up after all

“Confirming the reality of the Hubble constant tension would have significant consequences for both fundamental physics and modern cosmology,” explained Freedman.

Given the importance and also the difficulty in making these measurements, scientists test them with different methods to make sure they’re as accurate as possible.

One major approach involves studying the remnant light from the aftermath of the Big Bang, known as the cosmic microwave background. The current best estimate of the Hubble constant with this method, which is very precise, is 67.4 kilometers per second per megaparsec.

The second major method, which Freedman specializes in, is to measure the expansion of galaxies in our local cosmic neighborhood directly, using stars whose brightnesses are known. Just as car lights look fainter when they are far away, at greater and greater distances, the stars appear fainter and fainter. Measuring the distances and the speed at which the galaxies are moving away from us then tells us how fast the universe is expanding.

In the past, measurements with this method returned a higher number for the Hubble constant—closer to 74 kilometers per second per megaparsec.

This difference is large enough that some scientists speculate that something significant might be missing from our standard model of the universe’s evolution. For example, since one method looks at the earliest days of the universe and the other looks at the current epoch, perhaps something large changed in the universe over time. This apparent mismatch has become known as the ‘Hubble tension.’

Webb wades in

The James Webb Space Telescope or JWST, offers humanity a powerful new tool to see deep into space. Launched in 2021, the successor to the Hubble Telescope has taken stunningly sharp images, revealed new aspects of faraway worlds, and collected unprecedented data, opening new windows on the universe.

Freedman and her colleagues used the telescope to make measurements of ten nearby galaxies that provide a foundation for the measurement of the universe’s expansion rate.

To cross-check their results, they used three independent methods. The first uses a type of star known as a Cepheid variable star, which varies predictably in its brightness over time. The second method is known as the “Tip of the Red Giant Branch,” and uses the fact that low-mass stars reach a fixed upper limit to their brightnesses. The third, and newest, method employs a type of star called carbon stars, which have consistent colors and brightnesses in the near-infrared spectrum of light. The new analysis is the first to use all three methods simultaneously, within the same galaxies.

In each case, the values were within the margin of error for the value given by the cosmic microwave background method of 67.4 kilometers per second per megaparsec.

“Getting good agreement from three completely different types of stars, to us, is a strong indicator that we’re on the right track,” said Freedman.

“Future observations with JWST will be critical for confirming or refuting the Hubble tension and assessing the implications for cosmology,” said study co-author Barry Madore of the Carnegie Institution for Science and visiting faculty at the University of Chicago.

The other authors on the paper were UChicago research scientist In Sung Jang, Taylor Hoyt (PhD’22, now at Lawrence Berkeley National Laboratory), and UChicago graduate students Kayla Owens and Abby Lee.

Citation: “Status Report on the Chicago-Carnegie Hubble Program (CCHP): Three Independent Astrophysical Determinations of the Hubble Constant Using the James Webb Space Telescope.” Submitted to the Astrophysical Journal.

Funding: NASA.