NASA’s Webb and Hubble Team Up To Solve Universe Expansion Rate Puzzle

Webb Measurements Shed New Light on a Decade-Long Mystery

One of the three scientific justifications to the U.S. Congress for building the Hubble Space Telescope was to use its observing power to give an exact value for the expansion rate of the universe. Prior to Hubble’s 1990 launch, observations from ground-based telescopes yielded huge uncertainties. Depending on the expansion rate, the universe could be anywhere between 10 to 20 billion years old. For the past 34 years, Hubble has shrunken this value to an accuracy approaching one percent. This has been accomplished by refining the so-called “cosmic distance ladder” by measuring the gold standard of cosmic milepost markers known as Cepheid variable stars.

However, the results have puzzled cosmologists for a decade. The best measurements from Hubble show the universe is now expanding faster than predicted based on observations of how it looked shortly after the Big Bang. These observations were made by the Planck satellite mapping of the cosmic microwave background radiation – sort of a blueprint for how the universe would evolve structure after it cooled down from the Big Bang.

The simple solution to the dilemma is to say that maybe Hubble observations are wrong due to some creeping inaccuracy in its deep-space yardstick. Then along came the James Webb Space Telescope to crosscheck Hubble’s results. Webb’s sharp infrared views of Cepheids agreed with Hubble data. Webb confirmed that the Hubble telescope’s keen eye was right all along.

The bottom line is that the so-called “Hubble Tension” between what happens in the nearby universe compared to the early universe’s expansion remains a fascinating puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand.

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?

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. (See graphic below.)

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.

“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 findings were published in the February 6, 2024 issue of The Astrophysical Journal Letters.

Reference: “JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8σ Confidence” by Adam G. Riess, Gagandeep S. Anand, Wenlong Yuan, Stefano Casertano, Andrew Dolphin, Lucas M. Macri, Louise Breuval, Dan Scolnic, Marshall Perrin and Richard I. Anderson, 6 February 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad1ddd

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.