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When you’re trying to solve one of the biggest puzzles in cosmology, you have to triple-check your homework. The puzzle, called the Hubble Stress, is that the current rate of expansion of the Universe is faster than astronomers expect it to be, based on the initial conditions of the Universe and our current understanding of the evolution of the Universe. 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 space expansion?
This image of NGC 5468, a galaxy located about 142 million light-years away in the constellation Virgo, combines data from Hubble and Webb. Image credit: NASA / ESA / CSA / STScI / A. Riess, JHU & STScI.
One of the scientific justifications for building Hubble was to use its observational power to provide an accurate value for the expansion rate of the Universe.
Before Hubble’s launch in 1990, observations from ground-based telescopes yielded enormous uncertainties. Depending on the values deduced for the expansion rate, the Universe could be between 10 and 20 billion years old.
Over the past 34 years, Hubble has reduced this measurement to an accuracy of less than 1%, dividing the difference with an age value of 13.8 billion years.
This has been achieved by refining the so-called “cosmic distance ladder” by measuring important landmarks known as Cepheid variable stars.
However, Hubble’s 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 mapping the Cosmic microwave background (CMB) radiation.
The simple solution to the dilemma would be to say that perhaps Hubble’s observations are wrong, as a result of some inaccuracy in its measurements of deep space criteria.
Then the James Webb Space Telescope appeared, allowing astronomers to verify Hubble’s results.
Webb’s infrared views of the Cepheids matched Hubble’s optical light data.
Webb confirmed that Hubble’s keen eye was right all along, erasing any lingering doubts about Hubble’s measurements.
The bottom line is that Hubble’s tension between what’s happening in the nearby Universe and the expansion of the early Universe remains a persistent enigma 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 the result of measurement errors between the two different methods used to determine the space expansion rate?
Hubble and Webb have now teamed up to produce definitive measurements, bolstering the argument that something else, not measurement errors, is influencing the expansion rate.
“Once measurement errors are denied, 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 SH0ES (Supernova H0 for the Equation of State of Dark Energy). ) equipment.
As a cross-check, an initial observation by Webb in 2023 confirmed that Hubble’s measurements of the expanding Universe were accurate.
However, hoping to ease the strain on Hubble, some scientists speculated that invisible errors in measurements may increase and become visible as we look deeper into the Universe.
In particular, stellar crowding could systematically affect brightness measurements of more distant stars.
The SH0ES team obtained additional observations with Webb of objects that are critical markers of cosmic milestones, Cepheid variable stars, which can now be correlated with Hubble data.
“We have now covered the full range of what Hubble observed and can rule out measurement error as the cause of the Hubble strain with very high confidence,” Dr. Riess said.
The team’s first Webb observations in 2023 managed to demonstrate that Hubble was on the right track to firmly establish the fidelity of the first rungs of the so-called ladder of cosmic distances.
Astronomers use several methods to measure relative distances in the Universe, depending on the object being observed.
Together, these techniques are known as the ladder of cosmic distances: each rung or measurement technique builds on the previous step for calibration.
But some astronomers suggested that, moving outward along the second rung, the cosmic distance scale could become unstable if the Cepheid measurements become less precise with distance.
Such inaccuracies could occur because a Cepheid’s light could mix with that of an adjacent star, an effect that could become more pronounced with distance as stars crowd together in the sky and become harder to distinguish from one another.
The observational challenge is that previous Hubble images of these more distant Cepheid variables appear more crowded and overlapping with neighboring stars at increasing distances between us and their host galaxies, requiring careful explanation of this effect.
The presence of dust further complicates the safety of measurements in visible light.
Webb cuts through the dust and naturally isolates the Cepheids from neighboring stars because their vision is sharper than Hubble’s at infrared wavelengths.
“The combination of Webb and Hubble gives us the best of both worlds. “We found that Hubble measurements remain reliable as we move up the cosmic distance scale,” said Dr. Riess.
Webb’s new observations include five host galaxies for eight type Ia supernovae that contain a total of 1,000 Cepheids and reach the farthest galaxy where Cepheids have been well measured: NGC 5468at a distance of 130 million light years.
“This covers the entire range in which we make measurements with Hubble. So we have reached the end of the second rung of the cosmic distance ladder,” said Dr. Gagandeep Anand, an astronomer at the Space Telescope Science Institute.
The teams paper was published in Letters from astrophysical journals.
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Adam G. Riess et al. 2024. JWST observations reject unrecognized crowding from Cepheid photometry as an explanation for the Hubble stress with 8σ confidence. ApJL 962, L17; doi:10.3847/2041-8213/ad1ddd
