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. Astronomers using the NASA/ESA Hubble Space Telescope and many other telescopes consistently find a number that does not match predictions based on observations from ESA’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?

One of the scientific justifications for building Hubble 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 1%, 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.

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 ESA’s Planck satellite’s mapping of the Cosmic Microwave Background (CMB) radiation.

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’s keen eye was right all along, erasing any lingering doubt about Hubble’s measurements.

The bottom line is that the 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,” the astronomers said.

“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.

“With measurement errors negated, what remains is the real and exciting possibility that we have misunderstood the Universe,” said Dr. Adam Riess, a physicist at Johns Hopkins University and leader of the SH0ES (Supernova H0 for the Equation of State of Dark Energy) team.

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.

The SH0ES team obtained additional observations with Webb of objects that are critical cosmic milepost markers, 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,” Dr. 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 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,” Dr. Riess said.

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 Dr. Gagandeep Anand, an astronomer at the Space Telescope Science Institute.

The team’s paper was published in Astrophysical Journal Letters.

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Adam G. Riess et al. 2024. JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8σ Confidence. ApJL 962, L17; doi: 10.3847/2041-8213/ad1ddd