New analysis of Webb data measures the expansion rate of the universe, finds there may be no ‘Hubble tension’

We know a lot about the universe, but astronomers still argue about how fast it is expanding. In fact, over the past two decades, two main methods for measuring this number—known as the “Hubble constant”—have come up with different answers, leading some to wonder if there’s something missing in our model of how the universe works.

But new measurements from the powerful James Webb Space Telescope suggest there may not be a conflict, also known as the “Hubble tension,” after all.

In a paper presented to Astrophysical Journalcurrently available on Arcxiv In a recent study, University of Chicago cosmologist Wendy Freedman and her colleagues analyzed new data captured by NASA’s powerful James Webb Space Telescope. They measured the distances to 10 nearby galaxies and calculated a new value for the universe’s present-day expansion rate.

Its measurement, of 70 kilometers per second per megaparsec, overlaps with the other main method for the Hubble constant.

“Based on the new James Webb data and using three independent methods, we find no strong evidence for the Hubble tension,” said Friedman, a renowned astronomer and professor of astronomy and astrophysics at the University of Chicago. “On the contrary, our standard cosmological model for explaining the evolution of the universe appears to hold.”

Hubble tension?

We’ve known that the universe is expanding over time since 1929, when Edwin Hubble (a 1910 graduate of the University of Chicago, Ph.D., 1917) made measurements of stars that suggested that galaxies farther from Earth were moving away from us faster than those closer to us. But it’s surprisingly difficult to pin down exactly how fast the universe is expanding today.

This number, known as the Hubble constant, is essential to understanding the back story of the universe. It is a fundamental part of our model of how the universe has evolved over time.

“Confirming the Hubble constant tension would have major consequences for fundamental physics and modern cosmology,” Friedman explained.

Because these measurements are so important and difficult to make, scientists test them in different ways to make sure they are as accurate as possible.

One of the main methods involves studying the light left over from the aftermath of the Big Bang, known as the cosmic microwave background. The current best estimate of the Hubble constant in this way, which is very accurate, is 67.4 kilometers per second per megaparsec.

The second main method, and one that Friedman specializes in, is to measure the expansion of galaxies in our local cosmic neighborhood directly, using stars whose brightness we know. Just as car lights appear dimmer when they are far away, stars appear dimmer at greater and greater distances. By measuring the distances and speeds at which galaxies are moving away from us, we can infer how fast the universe is expanding.

In the past, measurements made this way have given 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 important may be missing from our standard model of the evolution of the universe. For example, since one method looks at the early days of the universe and the other looks at the present age, something big may have changed in the universe over time. This apparent discrepancy has become known as the “Hubble tension.”

Web delves into

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

Friedman and her colleagues used the telescope to make measurements of ten nearby galaxies that provide the basis for measuring the universe’s expansion rate.

To verify their results, they used three independent methods. The first used a type of star known as a Cepheid variable, which changes its brightness predictably over time. The second method, known as the “red giant branch tip,” uses the fact that low-mass stars reach a fixed upper limit on their brightness.

The third and newest method uses 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 of the value given by the cosmic microwave background method, which is 67.4 kilometers per second per megaparsec.

“Getting good agreement from three very different types of stars, for us, is a strong indication that we’re on the right track,” Friedman said.

“Future observations with the James Webb Space Telescope will be crucial to confirm or refute the Hubble tension theory and assess the implications for cosmology,” said Barry Madore, a co-author of the study from the Carnegie Institution for Science and a visiting professor at the University of Chicago.