Scientists have been debating how fast our universe is expanding, a rate that is called the “Hubble constant”. For the past 20 years, two main methods of measuring this rate have given different results, leading some to wonder if our understanding of the universe was incomplete.

However, new data from the James Webb Space Telescope (JWST) suggests there might not be a conflict after all.

Wendy Freedman, a cosmologist from the University of Chicago, and her team analyzed JWST data from ten nearby galaxies. They measured a new value for the current expansion rate of the universe: about 70 kilometers per second per megaparsec. This measurement overlaps with other main methods, potentially resolving the so-called “Hubble tension”.

An artist's concept showing the expansion of the universe over time since the Big Bang.
An artist’s concept showing the expansion of the universe over time since the Big Bang. Credit: Goddard Space Flight Center / NASA

We’ve known the universe is expanding since 1929, when Edwin Hubble noticed that distant galaxies were moving away from Earth faster than nearby ones. But pinpointing the exact expansion rate has been tricky.

The Hubble constant is crucial for understanding the universe’s history and evolution. Scientists use different methods to measure it, studying leftover light from the Big Bang cosmic microwave background (this method estimates the Hubble constant at 67.4 km/s/Mpc.), or directly measuring the expansion of nearby galaxies using stars with known brightness (Previously, this method gave a higher number, around 74 km/s/Mpc.)

This difference was big enough that some scientists thought our standard model of the universe might be missing something important.

The James Webb Space Telescope, launched in 2021, gives us a powerful new tool for looking deep into space. Freedman’s team used it to measure ten nearby galaxies, providing a basis for calculating the universe’s expansion rate.

The views of stars provided by JWST (at left) are noticeably sharper than the same stars viewed by the Hubble Space Telescope (at right).
The views of stars provided by JWST (at left) are noticeably sharper than the same stars viewed by the Hubble Space Telescope (at right). Credit: Freedman et al.

They used three independent methods to verify their results:

  1. Cepheid variable stars, which change brightness predictably over time.
  2. The “Tip of the Red Giant Branch”, using the fact that low-mass stars reach a fixed upper limit in brightness.
  3. Carbon stars, which have consistent colors and brightness in near-infrared light.

This study 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 cosmic microwave background method’s 67.4 km/s/Mpc.

Freedman says getting good agreement from three completely different types of stars is a strong indicator that they’re on the right track. This suggests that our current model of the universe might be correct after all, and there may not be a “Hubble tension” to resolve.

However, more observations with the JWST will be crucial to confirm or disprove the Hubble tension and assess what this means for our understanding of the cosmos.

SOURCES

University of Chicago

Wendy L. Freedman, Barry F. Madore, et al., Status Report on the Chicago-Carnegie Hubble Program (CCHP): Three Independent Astrophysical Determinations of the Hubble Constant Using the James Webb Space Telescope. doi.org/10.48550/arXiv.2408.06153

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