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NASA’s Webb, Hubble Telescopes Affirm Universe’s Expansion Rate, Puzzle Persists


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NASA’s Webb, Hubble Telescopes Affirm Universe’s Expansion Rate, Puzzle Persists

When you are trying to solve one of the biggest conundrums in cosmology, you should triple check your homework. The puzzle, called the “Hubble Tension,” is that the current rate of the expansion of the universe is faster than what astronomers expect it to be, based on the universe’s initial conditions and our present understanding of the universe’s evolution.

Scientists using NASA’s Hubble Space Telescope and many other telescopes consistently find a number that does not match predictions based on observations from ESA’s (European Space Agency’s) Planck mission. Does resolving this discrepancy require new physics? Or is it a result of measurement errors between the two different methods used to determine the rate of expansion of space?

A spiral galaxy with a small bar of bright-white stars at its core. Two main spiral arms extend outward from each end of the bar. They appear to fork into multiple branches beyond the galaxy's core. The spiral arms have a lavender hue. Bright-white and bright-red stars dot the galaxy. Reddish-brown dust lanes line the inner curves of the spiral arms.
This image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the farthest galaxy in which Hubble has identified Cepheid variable stars. These are important milepost markers for measuring the expansion rate of the universe. The distance calculated from Cepheids has been cross-correlated with a type Ia supernova in the galaxy. Type Ia supernovae are so bright they are used to measure cosmic distances far beyond the range of the Cepheids, extending measurements of the universe’s expansion rate deeper into space.

Hubble has been measuring the current rate of the universe’s expansion for 30 years, and astronomers want to eliminate any lingering doubt about its accuracy. Now, Hubble and NASA’s James Webb Space Telescope have tag-teamed to produce definitive measurements, furthering the case that something else – not measurement errors – is influencing the expansion rate.

“With measurement errors negated, what remains is the real and exciting possibility we have misunderstood the universe,” said Adam Riess, a physicist at Johns Hopkins University in Baltimore. Riess holds a Nobel Prize for co-discovering the fact that the universe’s expansion is accelerating, due to a mysterious phenomenon now called “dark energy.”

As a crosscheck, an initial Webb observation in 2023 confirmed that Hubble measurements of the expanding universe were accurate. However, hoping to relieve the Hubble Tension, some scientists speculated that unseen errors in the measurement may grow and become visible as we look deeper into the universe. In particular, stellar crowding could affect brightness measurements of more distant stars in a systematic way.

The SH0ES (Supernova H0 for the Equation of State of Dark Energy) team, led by Riess, obtained additional observations with Webb of objects that are critical cosmic milepost markers, known as Cepheid variable stars, which now can be correlated with the Hubble data.

“We’ve now spanned the whole range of what Hubble observed, and we can rule out a measurement error as the cause of the Hubble Tension with very high confidence,” Riess said.

The team’s first few Webb observations in 2023 were successful in showing Hubble was on the right track in firmly establishing the fidelity of the first rungs of the so-called cosmic distance ladder.

Astronomers use various methods to measure relative distances in the universe, depending upon the object being observed. Collectively these techniques are known as the cosmic distance ladder – each rung or measurement technique relies upon the previous step for calibration.

But some astronomers suggested that, moving outward along the “second rung,” the cosmic distance ladder might get shaky if the Cepheid measurements become less accurate with distance. Such inaccuracies could occur because the light of a Cepheid could blend with that of an adjacent star – an effect that could become more pronounced with distance as stars crowd together and become harder to distinguish from one another.

The observational challenge is that past Hubble images of these more distant Cepheid variables look more huddled and overlapping with neighboring stars at ever farther distances between us and their host galaxies, requiring careful accounting for this effect. Intervening dust further complicates the certainty of the measurements in visible light. Webb slices though the dust and naturally isolates the Cepheids from neighboring stars because its vision is sharper than Hubble’s at infrared wavelengths.

A horizontal, two-panel image of pixelated, black-and-white star fields. The left image label is: “Webb Near-IR.” It holds a few dozen points of light of varying brightness. At the center of the image, a circle marks one bright point. The right image label is: “Hubble Near-IR.” It holds more indistinct, blurry patches whose overall brightness is similar to the more defined regions in the left image. At center, a circle marks a light gray pixel.
At the center of these side-by-side images is a special class of star used as a milepost marker for measuring the universe’s rate of expansion – a Cepheid variable star. The two images are very pixelated because they are a very zoomed-in view of a distant galaxy. Each of the pixels represents one or more stars. The image from the James Webb Space Telescope is significantly sharper at near-infrared wavelengths than Hubble (which is primarily a visible-ultraviolet light telescope). By reducing the clutter with Webb’s crisper vision, the Cepheid stands out more clearly, eliminating any potential confusion. Webb was used to look at a sample of Cepheids and confirmed the accuracy of the previous Hubble observations that are fundamental to precisely measuring the universe’s expansion rate and age.
NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)

“Combining Webb and Hubble gives us the best of both worlds. We find that the Hubble measurements remain reliable as we climb farther along the cosmic distance ladder,” said Riess.

The new Webb observations include five host galaxies of eight Type Ia supernovae containing a total of 1,000 Cepheids, and reach out to the farthest galaxy where Cepheids have been well measured – NGC 5468 – at a distance of 130 million light-years. “This spans the full range where we made measurements with Hubble. So, we’ve gone to the end of the second rung of the cosmic distance ladder,” said co-author Gagandeep Anand of the Space Telescope Science Institute in Baltimore, which operates the Webb and Hubble telescopes for NASA.

Hubble and Webb’s further confirmation of the Hubble Tension sets up other observatories to possibly settle the mystery. NASA’s upcoming Nancy Grace Roman Space Telescope will do wide celestial surveys to study the influence of dark energy, the mysterious energy that is causing the expansion of the universe to accelerate. ESA’s Euclid observatory, with NASA contributions, is pursuing a similar task.

At present it’s as though the distance ladder observed by Hubble and Webb has firmly set an anchor point on one shoreline of a river, and the afterglow of the big bang observed by Planck’s measurement from the beginning of the universe is set firmly on the other side. How the universe’s expansion was changing in the billions of years between these two endpoints has yet to be directly observed. “We need to find out if we are missing something on how to connect the beginning of the universe and the present day,” said Riess.

These finding were published in the February 6, 2024 issue of The Astrophysical Journal Letters.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. Goddard also conducts mission operations with Lockheed Martin Space in Denver, Colorado. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations for NASA.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

More Webb News: https://science.nasa.gov/mission/webb/latestnews/

More Hubble News: https://science.nasa.gov/mission/hubble/hubble-news/

More Webb Images: https://science.nasa.gov/mission/webb/multimedia/images/

More Hubble Images: https://science.nasa.gov/mission/hubble/multimedia/hubble-images/

Webb Mission Page: https://science.nasa.gov/mission/webb/

Hubble Mission Page: https://science.nasa.gov/mission/hubble/

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Media Contacts:

Claire Andreoli – claire.andreoli@nasa.gov
Laura Betz – laura.e.betz@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, MD

Ray Villard, Christine Pulliam
Space Telescope Science Institute, Baltimore, MD

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Mar 11, 2024
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      Last Updated May 14, 2025 Editor Marty McCoy Contact Laura Betz laura.e.betz@nasa.gov Related Terms
      James Webb Space Telescope (JWST) Astrophysics Goddard Space Flight Center Planets Saturn Saturn Moons Science & Research The Solar System Titan View the full article
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      Preparations for Next Moonwalk Simulations Underway (and Underwater)
      Advanced Composites Consortium team members gathered during May 2025 at NASA’s Langley Research Center in Virginia for a technical review of activities in the Hi-Rate Composite Aircraft Manufacturing project.NASA NASA and its partners in the Advanced Composites Consortium gathered at the agency’s Langley Research Center in Hampton, Virginia, on April 29-May 1, 2025.
      Team members from 22 organizations in the public-private partnership are collaborating to increase the production rate of composite aircraft, reduce costs, and improve performance.
      The team discussed results from the Technology Development Phase of NASA’s Hi-Rate Composite Aircraft Manufacturing (HiCAM) project.
      The project is evaluating concepts and competing approaches at the subcomponent scale to determine technologies with the greatest impact on manufacturing rate and cost. The most promising concepts will be demonstrated on full-scale wing and fuselage components during the next four years. 
      Through collaboration and shared investment, the team is increasing the likelihood of technologies being adopted for next-generation transports, ultimately lowering costs for operators and improving the U.S. competitive advantage in the commercial aircraft industry.
      Want to Learn More About Composite Aircraft Research?
      Go to the HiCAM project page here Facebook logo @NASA@NASAaero@NASA_es @NASA@NASAaero@NASA_es Instagram logo @NASA@NASAaero@NASA_es Linkedin logo @NASA Explore More
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      Last Updated May 13, 2025 EditorJim BankeContactShannon Eichornshannon.eichorn@nasa.gov Related Terms
      Hi-Rate Composite Aircraft Manufacturing View the full article
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