Today in Edworking News we want to talk about The Webb Telescope Further Deepens the Biggest Controversy in Cosmology August 13, 2024
Three new measurements using the James Webb Space Telescope have led some to question if the Hubble tension is real.
The James Webb Space Telescope, a marvel of modern engineering, continues to provide groundbreaking data.
Introduction
Nearly a century ago, Edwin Hubble discovered that the universe is getting larger. Modern measurements of how fast it is expanding disagree, however, suggesting that our understanding of the laws of physics might be off. Everyone expected the sharp vision of the James Webb Space Telescope to bring the answer into focus. But a long-awaited analysis of the telescope’s observations released late Monday evening once again gleans conflicting expansion rates from different types of data, while homing in on possible sources of error at the heart of the conflict.
Two rival teams have led the effort to measure the cosmic expansion rate, which is known as the Hubble constant, or H0. One of these teams, led by Adam Riess of Johns Hopkins University, has consistently measured H0 to be about 8% higher than the theoretical prediction for how fast space should be expanding, based on the cosmos’s known ingredients and governing equations. This discrepancy, known as the Hubble tension, suggests that the theoretical model of the cosmos might be missing something — some extra ingredient or effect that speeds up cosmic expansion. Such an ingredient could be a clue to a more complete understanding of the universe.
Riess and his team released their latest measurement of H0 based on Webb data this spring, getting a value that agrees with their earlier estimates. But for years a rival team led by Wendy Freedman of the University of Chicago has urged caution, arguing that cleaner measurements were needed. Her team’s own measurements of H0 have invariably landed closer than Riess’ to the theoretical prediction, implying that the Hubble tension may not be real.
Since the Webb telescope started taking data in 2022, the astrophysics community has awaited Freedman’s multipronged analysis using the telescope’s observations of three types of stars. Now, the results are in: Two types of stars yield H0 estimates that align with the theoretical prediction, while the third — the same type of star Riess uses — matches his team’s higher H0 value.
A Clashing Universe
The hard part of gauging cosmic expansion is measuring distances to objects in space. The American astronomer Henrietta Leavitt first uncovered a way to do this in 1912 using pulsating stars called Cepheids. These stars flicker at a rate that relates to (and can therefore reveal) their intrinsic luminosity. Once you know how luminous a Cepheid is, you can compare that to how bright or dim it appears to estimate how far away its galaxy is. Edwin Hubble used Leavitt’s method to measure the distances to a handful of galaxies with Cepheids in them, discovering in 1929 that galaxies farther from us are moving away faster. That means the universe is expanding.
The American astronomer Edwin Hubble, discoverer of cosmic expansion, is pictured in 1949 peering into the Schmidt telescope at the Palomar Observatory near San Diego.
Hubble pegged the expansion rate at 500 kilometers per second per megaparsec (km/s/Mpc), meaning that two galaxies separated by 1 Mpc, or about 3.2 million light-years, fly apart at 500 km/s. That was wildly off. Measurements of H0 improved as astronomers got better at calibrating the relationship between Cepheids’ pulsation frequency and their luminosity. Still, the whole approach was limited because Cepheids are only so bright. To measure the distance to galaxies across the vastness of the universe, scientists would need a new approach.
In the 1970s, researchers started using Cepheids to calibrate the distances to bright supernovas, enabling more accurate measurements of H0. Then as now, two research teams led the way, using supernovas anchored to Cepheids and arriving at disagreeing values of 50 km/s/Mpc and 100 km/s/Mpc. “There was no meeting of minds ever; they were just completely polarized,” said George Efstathiou, an astrophysicist at the University of Cambridge.
The 1990 launch of the Hubble Space Telescope gave astronomers a new, crisp view of the universe. Freedman led a multiyear observing campaign using Hubble, and in 2001, she and her colleagues announced an expansion rate of 72 km/s/Mpc, estimating that this was at most 10% off. Riess, who is one of the Nobel Prize-winning discoverers of dark energy, jumped into the cosmic expansion game a few years later. In 2011, his team published an H0 value of 73 with an estimated 3% uncertainty.
Ways of Seeing
Some cosmologists, including Freedman, have suspected that unrecognized errors are to blame for the discrepancy. The most common argument in this vein is that Cepheid stars live in the disks of younger galaxies, in regions crowded with stars, dust and gas. “Even with the exquisite resolution of Hubble, you don’t see a single Cepheid,” Efstathiou said, “you see it superimposed with other stars.” This congestion complicates brightness measurements.
When the house-size Webb telescope launched in December 2021, Riess and his colleagues turned to its powerful infrared camera to pierce the dust in the crowded regions where Cepheids live. They sought to test if crowding has as strong an effect as Freedman and other researchers have claimed.
The 6.5-meter segmented mirror of the James Webb Space Telescope underwent tests at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in 2017, years before its December 2021 launch.
When they compared their new numbers to the distances calculated from Hubble telescope data, “we saw phenomenal agreement,” said Gagandeep Anand, a member of the team based at the Space Telescope Science Institute. “That tells us, basically, that the work that has been done with Hubble is still good.” Their latest results with Webb reaffirm the H0 value that they measured with Hubble a few years ago: 73.0, give or take 1.0 km/s/Mpc.
Given the crowding concern, though, Freedman had already turned to alternative stars that could serve as distance indicators. These are found in the outskirts of galaxies, far from the madding crowd. One type is “tip-of-the-red-giant-branch,” or TRGB, stars. A red giant is an elderly star with a puffed-up atmosphere that glows brightly in red light. As it ages, a red giant will eventually ignite the helium in its core. At that moment, both the star’s temperature and its brightness suddenly drop off, said Kristen McQuinn, an astronomer at the Space Telescope Science Institute who led a Webb telescope project to calibrate distance measurements with TRGBs.
An Evaporating Solution
On March 13, 2024, Freedman, Lee and the rest of their team sat around a table in Chicago to reveal what they had been hiding from themselves. Over the previous months, they had split into three groups. Each was tasked with measuring the distance to the 11 galaxies in their study using one of three methods: Cepheids, TRGBs or JAGBs. The galaxies also hosted the relevant kinds of supernovas, so their distances could calibrate the distances of supernovas in many more galaxies farther away. How fast these farther galaxies are receding from us (which is easily read off from their color) divided by their distances gives H0.
The three groups had calculated their distance measurements with a unique random offset added to the data. When they met in person, they removed each of the offsets and compared the results. All three methods gave similar distances, within 3% uncertainty. It was “sort of jaw-dropping,” Freedman said. The team calculated three H0 values, one for each distance indicator. All came within range of the theoretical prediction of 67.4. At that moment, they appeared to have erased the Hubble tension.
But when they dug into the analysis to write up the results, they found problems. The JAGB analysis was fine, but the other two were off. The team noticed that there were large error bars on the TRGB measurement. They tried to shrink them by including more TGRBs. But when they did so, they found that the distance to the galaxies was smaller than they first thought. The change yielded a larger H0 value. In the Cepheid analysis, Freedman’s team uncovered an error: In about half the Cepheids, the correction for crowding had been applied twice. Fixing that significantly increased the resulting H0 value. It “brought us more into agreement with Adam Riess, which ought to make him a little happier,” Freedman said. The Hubble tension was resurrected.
Tensions and Resolutions
The James Webb Space Telescope is also enabling additional ways to measure H0. For instance, astronomers are in the early phases of using how mottled a galaxy looks as a proxy for its distance. The idea is simple: Closer galaxies look clumpier because you can resolve some of their stars, whereas more distant galaxies appear smoother. “It’s basically a way to turn the crowding into a measure of distance,” said Anand, who is involved with this project in addition to his work with Riess.
A different method also offers some hope: A massive cluster of galaxies acts like a warped magnifying glass, bending and magnifying the image of an object behind it and creating multiple images of the same object as its light takes multiple paths. The University of Arizona astronomer Brenda Frye leads a program to observe seven clusters with the Webb telescope. When Frye and her colleagues looked at their first telescope image last year, featuring the massive galaxy cluster G165, “we all just said, ‘What are those three dots that weren’t there before?’” she recalled. The dots were three separate images of the same supernova that had exploded behind the cluster. After repeatedly observing the image, they could calculate the differences between the arrival times of the three lensed supernova images. The time delay is proportional to, and can be used to infer, the Hubble constant. “[It] is a one-step measurement for H0,” Frye said, “which makes it completely independent.” They measured an expansion rate of 75.4 km/s/Mpc, although with a large uncertainty of +8.1 or −5.5 km/s/Mpc. Frye expects to refine those error bars after a few more years of similar measurements.
Both Riess’ and Freedman’s teams also anticipate that the next few years of JWST observations will enable them to home in on an answer with their traditional, star-based methods. “With the improvement in the data, this is ultimately going to be solved, and I think pretty quickly,” Freedman said. “We’re going to get to the bottom of this.”
Remember these 3 key ideas for your startup:
Embrace Diverse Methods: Just as astronomers use multiple methods to measure cosmic distances, startups should diversify their approaches to problem-solving. This can lead to more robust and reliable solutions. For example, consider exploring ways to replicate the office in a remote work setup to enhance team collaboration.
Continuous Improvement: The ongoing adjustments and refinements in measuring the Hubble constant highlight the importance of continuous improvement. Startups should constantly iterate and refine their products and strategies based on new data and feedback. Learn more about how to effectively assign tasks to team members to streamline your processes.
Leverage Advanced Tools: The James Webb Space Telescope's advanced capabilities have revolutionized cosmology. Similarly, startups should leverage cutting-edge tools and technologies to gain a competitive edge and drive innovation. Discover the benefits of using free productivity software to enhance your team's efficiency.
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