Resolving the Hubble Tension: Webb’s Precise Measurements Illuminate the Universe’s Expansion Mystery

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. Credit: NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)

The Hubble constant’s measurement discrepancy, known as the Hubble Tension, has been a significant puzzle in cosmology. The James Webb Space Telescope’s confirmation of the Hubble Space Telescope’s data has solidified the cosmic distance ladder’s accuracy and suggested that the discrepancy might hint at unknown cosmic phenomena, demanding further investigation.

The rate at which the Universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was right all along, erasing any lingering doubt about Hubble’s measurements.

Hubble’s Historical Achievements

One of the scientific justifications for building the NASA/ESA Hubble Space Telescope was to use its observing power to provide an exact value for the expansion rate of the Universe. Prior to Hubble’s launch in 1990, observations from ground-based telescopes yielded huge uncertainties. Depending on the values deduced for the expansion rate, the Universe could be anywhere between 10 and 20 billion years old. Over the past 34 years Hubble has shrunk this measurement to an accuracy of less than one percent, splitting the difference with an age value of 13.8 billion years. This has been accomplished by refining the so-called ‘cosmic distance ladder’ by measuring important milepost markers known as Cepheid variable stars.

Addressing the Hubble Tension With Webb

However, the Hubble value does not agree with other measurements that imply that the Universe was expanding faster after the Big Bang. These observations were made by the ESA Planck satellite’s mapping of the cosmic microwave background radiation – a blueprint for how the Universe would evolve structure after it cooled down from the Big Bang.

The simple solution to the dilemma would be to say that maybe the Hubble observations are wrong, as a result of some inaccuracy creeping into its measurements of the deep-space yardsticks. Then along came the James Webb Space Telescope, enabling astronomers to crosscheck Hubble’s results. Webb’s infrared views of Cepheids agreed with Hubble’s optical-light data. Webb confirmed that the Hubble telescope’s keen eye was right all along, erasing any lingering doubt about Hubble’s measurements.

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. Credit: NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)

Cosmic Mysteries and Theoretical Challenges

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 nagging puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand.

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 and Webb have now tag-teamed to produce definitive measurements, furthering the case that something else – not measurement errors – is influencing the expansion rate.

Advancements in Cosmic Observations

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

As a crosscheck, an initial Webb observation in 2023 confirmed that Hubble’s 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.

Collaborative Validation and Future Directions

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

This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong “cosmic distance ladder,” by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This “ladder” is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as “milepost markers” to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the “ladder.” By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy’s light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant. Credits: NASA, ESA and A. Feild (STScI)

The Intricacies of the Cosmic Distance Ladder

Astronomers use various methods to measure relative distances in the Universe, depending on 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 on the sky 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 greater 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 through 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 Adam.

The new Webb observations include five host galaxies of eight Type Ia supernovae containing a total of 1000 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.

Together, Hubble’s and Webb’s confirmation of the Hubble Tension sets up other observatories to possibly settle the mystery, including NASA’s upcoming Nancy Grace Roman Space Telescope and ESA’s recently launched Euclid mission.

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 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 Adam.

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

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