There is a big controversy in astronomy

This story’s initial publication was in Quanta Magazine.

Edwin Hubble found almost a century ago that the universe is expanding. However, there is disagreement among current measurements regarding how quickly it is expanding, which raises the possibility that our interpretation of the laws of physics is incorrect. Everyone anticipated that the James Webb Space Telescope’s keen vision would clarify the solution. However, a much-awaited examination of the telescope’s observations, which was made public late on Monday night, once more reveals divergent expansion rates from various kinds of data while focusing on potential error sources at the center of the dispute.

The measurement of the cosmic expansion rate, or H0, as it is also called, has been spearheaded by two competing teams. Using the known components and governing equations of the universe, one of these teams, headed by Adam Riess of Johns Hopkins University, has routinely measured H0 to be roughly 8% higher than the theoretical prediction for how quickly space should be expanding. This difference, referred to as the Hubble tension, raises the possibility that there is something missing from the theoretical model of the universe—an additional component or effect that accelerates cosmic expansion. Such a component might hold the key to a deeper comprehension of the cosmos.

This spring, using Webb data, Riess and colleagues published their most recent measurement of H0, obtaining a value consistent with their previous estimates.

However, a rival group headed by University of Chicago’s Wendy Freedman has long advocated for caution, claiming that more precise measurements were required. It appears that the Hubble tension may not be real because her team’s measurements of H0 have consistently come in closer to the theoretical prediction than Riess’ did.

The astrophysics community has eagerly anticipated Freedman’s multipronged analysis utilizing the telescope’s observations of three different types of stars since the Webb telescope began collecting data in 2022. The findings are as follows: H0 estimates from two kinds of stars agree with the theoretical prediction, while the third type of star, which Riess also uses, agrees with the higher H0 value found by his team.

The fact that the three approaches diverge, according to Freedman, “is not telling us about fundamental physics.”. This indicates to us that one or more of the distance methods contain some systematic [error]. “.

Freedman has sent his findings to The Astrophysical Journal, but they haven’t yet gone through the official peer review process, in which other scientists examine the information and analysis in an anonymous manner. The University of California, Berkeley’s Saul Perlmutter, a cosmologist who won the Nobel Prize, was shown the team’s preprint before it was published, and he told Quanta that the findings suggested “we may have a Hubble tension just within the [star-based] measurements.”. More than trying to create new [cosmological] models, we actually need to be attempting to resolve that tension. “.

Following review of the preprint, Riess informed Quanta that he disagrees with the small sample of supernovas used in one step of the analysis by Freedman’s team, arguing that this could skew the results. “The distance measurements appear under control, as the new measurements are lovely and actually show excellent agreement with the same measurements obtained by our group several years ago,” he stated. But I worry that this analysis of such a small sample of supernovae presents a somewhat false picture of the Hubble constant’s value. “.

The results are the culmination of months of behind-the-scenes drama; Freedman had believed that her analysis had put an end to the Hubble tension, only to have it resurface explosively. She remarked, “It’s been really… not dull, to put it that way.”.

That is routine operations. “The Hubble constant has such a long and glorious tradition of being an impossible decades-long problem,” claims Perlmutter. “.

A universe that clashes.

Measuring distances to space objects is a challenging aspect of estimating cosmic expansion. Using pulsating stars known as Cepheids, American astronomer Henrietta Leavitt discovered how to accomplish this for the first time in 1912. These stars flicker at a rate that corresponds to their inherent luminosity, which can be revealed. Once a Cepheid’s luminosity is known, its galaxy’s distance can be calculated by comparing it to how bright or dim it appears.

Edwin Hubble discovered in 1929 that galaxies that are farther away from us are moving away faster after using Leavitt’s method to measure the distances to a small number of galaxies that contain Cepheids. This indicates that the cosmos is expanding. Hubble estimated the expansion rate to be 500 km/s/Mpc, or 500 km/s for two galaxies separated by 1 Mpc, or roughly 3 point 2 million light-years.

It was completely incorrect.

As astronomers became more adept at calibrating the relationship between the luminosity and pulsation frequency of Cepheids, measurements of H0 improved. However, Cepheids are limited in their brightness, so the entire approach was constrained. Scientists would require a novel method to gauge the separations between galaxies throughout the vast cosmos.

When Cepheids were first used in the 1970s, scientists were able to more precisely measure H0 by calibrating the distances to bright supernovas. In the past as well as currently, two research groups have led the way, using supernovas anchored to Cepheids to arrive at values that differ by 50 and 100 km/s/Mpc. Astrophysicist George Efstathiou of the University of Cambridge said, “There was no meeting of minds at all; they were just completely polarized.”.

Astronomers were able to see the universe in a new, clear light after the Hubble Space Telescope was launched in 1990. In 2001, Freedman and her colleagues announced an expansion rate of 72 km/s/Mpc, estimating that this was at most 10% off. Freedman oversaw a multiyear Hubble observation campaign.

A few years later, Riess—one of the dark energy discovery team members who won the Nobel Prize—entered the cosmic expansion fray. His group released an H0 value of 73 with an estimated 3 percent uncertainty in 2011.

Cosmologists soon invented a completely different approach. They were able to ascertain the precise form and makeup of the early universe in 2013 by utilizing data from the Planck telescope on light remnants from the early universe. Following that, they entered those components into Einstein’s general theory of relativity and advanced the theoretical model by almost 14 billion years to forecast the universe’s current state. Based on this extrapolation, the universe should be expanding at a rate of 67.4 km/s/Mpc at the moment, with an uncertainty of less than 1%.

Even as accuracy increased, Riess’ team’s measurement remained at 73. This greater number suggests that galaxies are currently separating more quickly than theory predicts. The Hubble tension started to develop. “It’s telling us that we’re missing something in the cosmological model if it’s a real feature of the universe,” Riess stated.

The first novel component of the universe to be found since dark energy would be this missing element. Historians have conjectured regarding its nature: could it be an extra type of repulsive energy that persisted briefly in the early universe? Or could it be primordial magnetic fields produced during the Big Bang explosion?

Alternatively, perhaps the missing element relates to us rather than the cosmos.

Perspectives.

Freedman and other cosmologists have surmised that the discrepancy could be the result of overlooked errors.

Typically, the argument goes like this: Cepheid stars reside in regions densely packed with gas, dust, and stars in the disks of younger galaxies. Efstathiou stated, “Even with [Hubble’s] superb resolution, you don’t see a single Cepheid—rather, you see it superimposed with other stars.”. Measurements of brightness are complicated by this congestion.

Following the launch of the massive Webb telescope in December 2021, Riess and his associates utilized the telescope’s potent infrared camera to cut through the densely populated areas where Cepheids reside. They aimed to investigate whether crowding has the same potent effect as suggested by Freedman and other researchers.

Gagandeep Anand, a team member from the Space Telescope Science Institute, said that “we saw phenomenal agreement” when their new numbers were compared to the distances determined from Hubble telescope data. This basically indicates that the Hubble research that has been conducted is still relevant. “.

The H0 value, measured with Hubble a few years ago, is 73.0, give or take 1.0 km/s/Mpc, and their most recent Webb results confirm this.

But Freedman had already shifted to other stars that might be used as distance markers due to concerns about crowding. These can be found far from the throng, on the periphery of galaxies.

“Tip-of-the-red-giant-branch,” or TRGB, stars are one kind of these. A red giant is an old star that emits a bright red light due to its puffed-up atmosphere. Over time, the helium within the core of a red giant star will ignite. That’s when the star abruptly loses brightness and temperature, according to Kristen McQuinn, an astronomer at the Space Telescope Science Institute who oversaw a Webb telescope project to use TRGBs to calibrate distance measurements.

Red giants are common in galaxies. You can see the point at which these stars’ brightness decreases if you plot their brightness against their temperatures. Since every galaxy’s star population has a similar distribution of luminosities, the population of stars immediately preceding the drop serves as a reliable distance indicator. Astronomers can determine relative distances between these stellar populations by comparing their observed brightness.

To calibrate the entire scale, physicists need to determine the precise distance of a minimum of one “anchor” galaxy using any available technique. Using a parallax-like effect, Riess, Freedman, and other groups use an unusual nearby galaxy whose absolute distance has been determined geometrically as their anchor. ).

However, compared to Cepheids, using TRGBs as distance indicators is more complicated. To precisely determine how their brightness depends on their color, McQuinn and her colleagues employed nine of the wavelength filters available on the Webb telescope.

A new distance indicator that astronomers are starting to use are giant stars rich in carbon that are part of the J-region asymptotic giant branch (JAGB). These stars emit a great deal of infrared light and are situated far from the bright disk of a galaxy. According to Abigail Lee, Freedman’s graduate student, the technology needed to observe them at a great distance wasn’t sufficient until the Webb era.

In order to observe TRGBs and JAGBs in 11 galaxies in addition to the more well-known distance markers, the Cepheids, Freedman and her team applied for time on the Webb telescope. She declared, “I am a strong supporter of various methods.”.

A Solution That Evaporates.

To confess what they had been keeping from one another, Freedman, Lee, and the other members of their team gathered around a table in Chicago on March 13, 2024. They had divided into three groups during the preceding months. Each was given the task of determining the separation between the 11 galaxies in their study using one of three techniques: JAGBs, TRGBs, or Cepheids. The distances of the galaxies could be used to calibrate the distances of supernovas in many more galaxies that are farther away because they also hosted the relevant types of supernovas. H0 is the product of the distances between these farther galaxies and their rate of receding from us, which can be easily calculated from their color.

With a distinct random offset added to the data, the three groups computed their distance measurements. They eliminated each offset and compared the outcomes when they were face-to-face.

The distances obtained by all three methods were comparable, with an uncertainty of only 3%. Freedman described it as “sort of jaw-dropping.”. For every distance indicator, the team determined three H0 values. All results fell between the theoretically predicted range of 67 points.

Hubble tension seemed to have been eliminated at that very moment. However, upon further examination of the analysis to compose the report, they discovered issues.

The other two analyses weren’t correct, but the JAGB analysis was fine. On examining the TRGB measurement, the team saw that there were significant error bars. By adding more TGRBs, they attempted to make them smaller. When they eventually did, however, they discovered that the distance to the galaxies was closer than they had initially believed. An increased H0 value was obtained from the modification.

Freedman and colleagues found a mistake in the Cepheid analysis: the crowding correction had been applied twice in roughly half of the Cepheids. Making that correction greatly raised the H0 value that was obtained. Freedman stated that it “brought us more into agreement with Adam [Riess], which ought to make him a little happier.”. The Hubble tension reappeared.

Freedman, however, believes that the H0 measurement based on Cepheids is not as reliable as the rest. It is very sensitive to assumptions about, say, the neighborhood of each star and the elemental makeup of the Cepheids. Cepheids can become dimmed by light absorption from dust in the galactic disks where they reside. The dust is penetrated by the Webb’s infrared vision, but in order to account for the dust, astronomers need to know how much of it is absorbing light. In order to do this, Freedman and her associates used Hubble telescope archive data, which although capturing the “dust depth,” is not as high-resolution as Webb data. She claimed that this increased the uncertainty in the calculated distances.

Another problem revealed itself. All four of the relevant objects (JAGBs, TRGBs, Cepheids, and the relevant type of supernova) are found in the 11 galaxies that are closest to Earth and that they studied with the Webb telescope. However, supernovae in the galaxies appeared to be inherently brighter than those in distant galaxies, according to Freedman. The possibility that this sample is skewed and deceptive worries Riess and his associates. The H0 value is also impacted by this mystery, which cosmologists have not yet solved. In the coming years, Freedman predicted, “this is going to be where we really have to focus our attention.”.

Three distinct H0 values are reported in their work. 67.96 km/s/Mpc, give or take 1.71 km/s/Mpc, is the result of the JAGB measurement, which was carried out entirely blindly and without any further correction. That fits right in with the theoretical prediction and appears to support the standard model of cosmology.

TRGBs produce a result with comparable error margins of 69.85. Additionally, the Hubble tension is lessened by the outcome.

The Cepheid method resulted in a higher value of H0, at 72.05, but it also introduced more subjectivity: the value varied between 69 and 73 due to different assumptions about the characteristics of the stars. Riess’ measurements agree with the high end of the range, while the Hubble tension virtually vanishes at the low end.

According to Freedman, “I don’t think we can just say that the Hubble constant is 73.”. “I believe that this is the first test of the Cepheid distance scale,” indicating that the more proven approach is being verified by JAGBs and TRGBs. “And when we test the Cepheids, we’re not getting the same result. Therefore, I believe it’s crucial. “.

An average H0 value of 69.96 with a 4% uncertainty was obtained by combining the methods and uncertainties. The theoretical estimate for the cosmic expansion rate and the higher value determined by Riess’ team overlap with that margin of error.

“I don’t think we have enough evidence to conclude definitively that there is a [Hubble] tension at this time,” Freedman stated. “I simply don’t get it. “.

Finding all of these systematic errors is crucial, according to Perlmutter.

Resolutions and Tensions.

Further methods of measuring H0 are also made possible by the James Webb Space Telescope. In the early stages of research, for example, the appearance of a galaxy’s mottling may be used by astronomers to estimate its distance. The basic concept is that farther-off galaxies appear smoother, while closer galaxies appear clumsier because you can resolve some of their stars. Anand, who is working on this project in addition to his work with Riess, described it as “basically a way to turn the crowding into a measure of distance.”.

There’s also some hope in an alternative approach: a massive cluster of galaxies functions as a kind of warped magnifying glass, bending and magnifying the image of an object behind it and producing multiple images of the same object as its light travels in different directions. A program to observe seven clusters with the Webb telescope is headed by astronomer Brenda Frye of the University of Arizona. “We all just said, ‘What are those three .s that weren’t there before?'” recalls Frye and her colleagues when they saw their first telescope image of the massive galaxy cluster G165 last year. The .s represented three different photos taken of the same supernova explosion that occurred behind the cluster.

They could determine the differences in the arrival times of the three lensed supernova images by looking at the image again and over. We can infer the Hubble constant from the time delay, which is proportional to it. According to Frye, “it is a one-step measurement for H0,” meaning that it is entirely independent. With a significant degree of uncertainty, approximately +8 points or −5 points5 km/s/Mpc, they measured an expansion rate of 75 points4 km/s/Mpc. Several more years of similar measurements are anticipated by Frye, who plans to refine those error bars.

The teams of Riess and Freedman also hope that their traditional, star-based techniques will help them narrow down on an answer over the course of the next few years of JWST observations.

“This will be solved eventually, and I believe fairly quickly, with the improvement in the data,” Freedman stated. We’ll investigate this thoroughly. “.

Reprinted from the original work by permission of Quanta Magazine, an editorially independent publication of the Simons Foundation whose goal is to improve public science literacy through reporting on scientific advances and trends in mathematics, the physical sciences, and the biological sciences.