Rosa Rubicondior: How Science Works - And Why Religions Don't

New JWST images of abnormally well-developed galaxy cluster open up the “cosmic noon” frontier | IPAC

This two-panel image shows a distant galaxy cluster as it has been observed by NASA's Hubble Space Telescope and James Webb Space Telescope (JWST). At over 10 billion light-years away, the cluster XLSSC 122 is the most distant known example of a cluster found to act as a strong gravitational lens, magnifying and distorting images of yet more distant galaxies behind it.

Credit: NASA, ESA, CSA; Kyle Finner (Caltech/IPAC)
Image processing: Robert Hurt (Caltech/IPAC-SELab)

Young-Earth creationism, with its insistence on the literal, inerrant truth of Genesis, is a prisoner of its own conclusions. Evidence may be disputed, dismissed or reinterpreted, but the sacred account must remain beyond correction. Science proceeds on a fundamentally different basis: even its most successful explanations must remain open to revision when observations expose their limitations. Where creationism demands that reality conform to an ancient text, science requires its explanations to conform to reality.

A striking example comes from new James Webb Space Telescope observations of the distant galaxy cluster XLSSC 122. First identified in an X-ray survey in 2014, this cluster is now seen in extraordinary detail, revealing an unexpectedly high concentration of mass in its core. The problem is not that gravity should have been incapable of binding galaxies together, but that such a concentrated structure appears to have developed earlier than standard models would lead astronomers to expect.

JWST revealed arcs of light from more distant galaxies, distorted by the cluster’s gravity. This phenomenon, known as strong gravitational lensing, allows astronomers to investigate how mass, including invisible dark matter, is distributed within the cluster. The resulting measurements suggest that massive cosmic structures may sometimes assemble more rapidly than current models predict.

That is a challenge worth investigating, not a declaration that cosmology has collapsed. An unusual object can expose limitations in our modelling, assumptions or understanding of how structures grow without overturning an entire theoretical framework. The scientific response is to check the measurements, examine competing explanations and study more examples. The conclusions must follow the evidence, wherever it leads.

Notice, too, what happened to these potentially awkward findings. They were published in three papers in The Astrophysical Journal Letters by an international team including researchers at Caltech’s IPAC and Yonsei University, and presented at a press conference on 17 June 2026 during the 248th meeting of the American Astronomical Society. Far from being suppressed for challenging scientific orthodoxy, the results were publicised as an opportunity to test and improve our understanding.

The researchers are not announcing that everything we thought we knew was wrong. They are doing something more useful: identifying a possible mismatch between observation and prediction, explaining how they measured it, and inviting further scrutiny. A scientific model earns its standing by surviving such tests; it has no entitlement to protection from them.

For biblical literalism, the difficulty is more fundamental. Once an ancient creation narrative has been declared infallible, there is no equivalent freedom to conclude that its chronology or cosmology is mistaken. Interpretations may shift, but the requirement to vindicate the text remains. Science can advance by discovering where its explanations fail; creationism must continually explain away the possibility that its starting assumptions were wrong.

How Galaxy Clusters Grow — And Why XLSSC 122 Is Surprising. A galaxy cluster is much more than a collection of galaxies. It also contains enormous quantities of hot, diffuse gas and an even larger component of dark matter, whose presence is inferred from its gravitational effects. Together, these components form some of the largest gravitationally bound structures in the universe.

From small irregularities to enormous structures

The early universe was remarkably uniform, but not perfectly so. Some regions contained slightly more matter than others. Their extra gravitational attraction drew in surrounding material, allowing these initial differences to grow. Dark matter accumulated into structures called haloes, within which ordinary matter could collect and galaxies could develop.

Over billions of years, gravitational attraction, continuing infall and mergers assembled larger structures from smaller ones. Galaxy groups combined into clusters, which continued to grow as they collected more matter. This process is known as hierarchical structure formation. It does not imply that every cluster follows an identical timetable: individual histories depend on their surroundings and the structures with which they interact.

Looking into the past

Because light takes time to travel, observing distant galaxies means seeing them as they were in the past. XLSSC 122 has a redshift of approximately 1.98, placing it in the era often called cosmic noon, when the universe’s overall rate of star formation was near its peak.

Observations of clusters from this period allow astronomers to compare earlier stages of structure formation with the mature clusters around us today. They also provide a test of whether simulations reproduce how rapidly matter assembled and how it became distributed.

Weighing a cluster with bent light

Gravity curves spacetime, changing the paths followed by light. When a massive cluster lies in front of more distant galaxies, it can distort and magnify their images. This is gravitational lensing.

Strong lensing produces conspicuous distortions, such as stretched arcs or multiple images of the same background galaxy. It can reveal how mass is distributed in a cluster’s central region.

Weak lensing produces subtler distortions that must be measured statistically across many background galaxies. It helps astronomers map mass over a wider area.

Both methods respond to the cluster’s total gravitating mass, including dark matter. They therefore allow astronomers to investigate matter that cannot be mapped simply by adding up the light from visible galaxies.

What is unusual about XLSSC 122?

JWST observations revealed strong-lensing arcs around this previously known cluster. Analysis of those arcs indicated an exceptionally concentrated distribution of mass in its core. The surprise concerns how such a concentrated structure developed so early, rather than whether gravity could bind galaxies together at all.

Other observations indicate that the cluster is still undergoing merger activity. Its unusually developed core therefore does not mean that the whole system is fully settled or has finished assembling.

A challenge to investigate, not a verdict on all cosmology

An unexpected cluster does not, by itself, disprove the Big Bang or establish that the entire standard cosmological model is wrong. Astronomers must assess measurement uncertainties, modelling assumptions, merger history and how representative the object is. Observations of more clusters from the same period will help establish whether XLSSC 122 is an unusual case or part of a broader pattern requiring changes to our understanding of structure formation.

This is precisely why such discoveries matter: they turn an apparent mismatch between theory and observation into a question that further evidence can answer.

Glossary

Dark matter

Matter detected through its gravitational effects rather than emitted light. Its physical nature remains unresolved.

Dark matter halo

An extended concentration of dark matter associated with a galaxy, galaxy group or cluster.

Redshift

The stretching of light to longer wavelengths. For distant galaxies, cosmological redshift principally reflects the expansion of the universe while their light was travelling.

Cosmic noon

The broad period, roughly 10 billion years ago, when the universe’s overall rate of star formation was highest.

Gravitational lensing

The distortion and magnification of background objects by the gravity of intervening matter.

IPAC’s account of the XLSSC 122 observations.
strong gravitational lensing.
weak lensing and merger activity.

The three papers, available here, here and here, are discussed in this news release from IPAC:

New JWST images of abnormally well-developed galaxy cluster open up the “cosmic noon” frontier
New observations from NASA’s James Webb Space Telescope add to the intrigue surrounding a young, but surprisingly mature galaxy cluster from the early universe that challenges prevailing theories of cosmic evolution.
A stunningly concentrated and hefty galaxy cluster, from a time in the universe’s history when such massive structures aren’t expected to have fully formed yet, is challenging cosmic evolution theories. Across a series of three recent papers, a team led by researchers from IPAC—a science and data center for astrophysics and planetary science at Caltech—have revealed that the cluster is the most distant example of strong gravitational lensing with a galaxy cluster. The new results, based on observations from NASA’s James Webb Space Telescope (JWST), were presented in a press conference on June 17, 2026 at the 248th meeting of the American Astronomical Society.

When astronomers first laid eyes on this galaxy cluster dubbed XLSSC 122, they knew they had found something special. The cluster appeared highly evolved—meaning big and organized like a galaxy cluster in the nearby, modern universe—despite being more than 10 billion light-years away, back in an era when other galaxy groupings had only just started to come together.

Now thanks to the unmatched resolving and light-gathering power of JWST, researchers have discovered that XLSSC 122 is luckily aligned with one or more even more distant galaxies. This chance alignment is causing the giant cluster’s gravity to warp the far-off galaxy’s light—an ultra-rare phenomenon called strong gravitational lensing—in a way that enables newly precise mass measurements of XLSSC 122.

When we got those first images back from JWST, we said, ‘wow, look at this, there's strong lensing coming from this cluster!’ XLSSC 122 has now set the record for the most distant galaxy cluster displaying strong lensing, which is a valuable tool for astronomers.

Kyle Finner, lead author of the first paper.
IPAC, California Institute of Technology
Pasadena, CA, USA;

The serendipitous lensing has provided the most detailed look yet at mass distribution in an early galaxy cluster from the period known as “cosmic noon,” around 10 billion years ago. In this critically formative era, galaxy clusters began to form in bulk as the universe hit its peak in star formation, cranking out stars at a torrid place [sic] as much as 100 times faster than the present-day cosmos.

In keeping with XLSSC 122’s iconoclastic reputation, its mass turned out to be extremely concentrated toward the cluster’s center. Such a feature at so early a time in cosmic history challenges conventional cosmological models that chronicle a far slower buildup of massive structure.

XLSSC 122 is one of the first clusters we know of that formed in the universe, and it has a mass concentration that doesn't agree with our cosmological model predictions.

Kyle Finner.

Following up their strong lensing discovery, Finner and colleagues have recently published two more papers, first authored by Zachary Scofield and Hyungjin Joo at Yonsei University, examining other aspects of XLSSC 122 with JWST. Their slew of new results have positioned XLSSC 122 as a trailblazer of galaxy clusters at cosmic noon.

Seeing in the dark

XLSSC 122 first literally came to light in 2014 during an x-ray survey conducted by the European Space Agency’s XMM-Newton spacecraft. Subsequent observations by the Hubble Space Telescope helped firm up the cluster’s distance, about 10.4 billion light-years away, and its unexpectedly mature features.

Hubble data, however, showed no definitive signs of strong lensing, which appears as arcs of light around the cluster center in JWST images. Capturing these arcs with JWST thus proved a very welcome surprise.

Before JWST, we couldn’t do this level of science in the early, distant universe.

Kyle Finner.

The directly observable mass of XLSSC 122—its bright stars, its illuminated gases, and so on—actually contributes little to the strong lensing effect, however. Instead, the overwhelming source is dark matter—astronomers’ term for an invisible, little-understood, and yet deeply evidenced substance that exerts gravity but otherwise no detectable signatures. Estimates hold that dark matter outweighs regular matter—the stuff of stars, planets, and us—five times over.

Dark matter is a cornerstone of cosmology, responsible for holding galaxies together and creating the large-scale structure of the universe, where galaxies glom together in groups that scale up into gargantuan filaments stretching across space. Gauging the dark matter distribution in XLSSC 122 is a robust test of this framework and how well it describes the origins of structure since the big bang 13.8 billion years ago.

Strong lensing is a way to measure the dark matter without actually seeing the dark matter. It gives us a sensitive probe of our cosmological models.

Kyle Finner.

This image shows the distant galaxy cluster XLSSC 122 as it has been observed by NASA’s James Webb Space Telescope (JWST). The brightest clump of orange-red fuzzy objects at the very center of the image are the central galaxies of the cluster. The gravitationally lensed background galaxies can be seen as a series of blue-gray arcs that extend around the fuzzy bounds of the central galaxies, particularly to the lower right.
Credit: NASA, ESA, CSA; Kyle Finner (Caltech/IPAC)
Image processing: Robert Hurt (Caltech/IPAC-SELab)

A complete view of the galaxy cluster

For their second paper, Finner and colleagues turned to the subtler version of gravitational lensing, called weak lensing. Unlike strong lensing, which jumps off the page, weak lensing involves slight distortions of galaxies caused again by gravity, but so fine that statistical analyses must be run to tease out weak lensing’s effects.

Whereas strong lensing let the research team weigh the core of the cluster, weak lensing offered better coverage of the broader cluster, extending to its peripheral galactic members.

Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area.

Kyle Finner.

This overall perspective, combined with X-ray and radio wave data from a suite of other telescopes, showed that XLSSC 122 is in the process of merging, with its constituent galaxies still coming together. The second study also corroborated the first study regarding XLSSC 122’s whopping, centrally concentrated mass.

In the third and most recent paper, Finner and colleagues used JWST to trace XLSSC’s so-called intracluster light, a glow generated by stars that float freely between galaxies in clusters. This detection—the earliest known instance of intracluster light—further tells the story of XLSSC 122. The broad extent of the intracluster light bolsters that the cluster is undergoing a merger, where stars scattered out of the colliding galaxies have not yet settled into the cluster’s gravitationally powerful core.

Intriguingly, the researchers noticed that the shape of the intracluster light, where centrally located, matches up well with the dark matter mass concentrations disclosed by strong gravitational lensing. This result, if found in other galaxy clusters and especially in the early universe, could prove to be another tool for astronomers to suss out where hidden dark matter resides.

In this cluster, the intracluster light essentially traces the dark matter. That light tells us that the cluster is in a merging state.

Kyle Finner.

The more, the merrier

Looking ahead, Finner and colleagues hope to find and study dozens more ultra-distant galaxy clusters.

Discovery of such rarefied objects does not happen with JWST, which intentionally has a narrow, targeted field of view. Instead, wide-area surveys of the sky conducted in X-rays—like that which turned up XLSSC 122—as well as radio telescope surveys are needed to spot candidates. One particularly promising avenue is the Sunyaev–Zel'dovich effect, where telltale “holes” sometimes show up in observations of the afterglow from the big bang, called the cosmic microwave background. Those holes happen when afterglow light gets scattered by high-energy particles in galaxy clusters, betraying the clusters’ locations.

Should more objects like XLSSC 122 emerge, with skewed dark matter concentrations and otherwise unexplainable precociousness, then cosmologists might have to significantly revamp their foundational frameworks of the universe’s development.

It's still early in the JWST, and if we can start to get data on tens or hundreds of these types of objects at this stage in the universe, then we can really start putting our cosmological models to the test.

Kyle Finner.

More about these results:

IPAC at Caltech is a science and data center for astrophysics and planetary science. IPAC provides science operations, user support, archives and data services, and scientific vision to maximize discovery with observatories both in space and on the ground. JWST is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency). The Space Telescope Science Institute hosts the science and mission operations centers for JWST.

Publications:
Kyle Finner et al 2025
JWST Discovery of Strong Lensing from a Galaxy Cluster at Cosmic Noon: Giant Arcs and a Highly Concentrated Core of XLSSC 122
ApJL 994 L35 DOI 10.3847/2041-8213/ae1d80
Zachary P. Scofield et al 2026
An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes
ApJL 999 L1 DOI 10.3847/2041-8213/ae447a
Hyungjin Joo et al 2026
Mature but Still Growing: JWST Detection of the Earliest Intracluster Light at z ∼ 2
ApJL 1002 L17 DOI 10.3847/2041-8213/ae5c9f

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Abstract
Our observations with the James Webb Space Telescope have made the remarkable discovery of strong gravitational lensing arcs from XLSSC 122 (z = 1.98)—setting the record for the most distant galaxy cluster that exhibits strong lensing. The discovery of giant arcs enables a strong-lensing analysis and a measurement of the concentration of the dark matter halo. We perform a strong-lensing analysis of the cluster and measure the radial projected mass density profile. Our measurements reveal an exceptionally high concentration in the core of XLSSC 122. A Navarro–Frenk–White profile fit to the inner 100 kpc estimates the concentration to be 6.3 ± 0.5. The high concentration of XLSSC 122 contributes to the emerging picture that massive structure formation in the early Universe may proceed more rapidly than standard models suggest. We estimate the mass within 100 kpc to be M(R < 100 kpc) = 6.5 ± 0.7 × 10¹³ M_⊙ and find M_200c = 2.6 ± 1.1 × 10¹⁴ M_⊙.

Figure 1. The left panels show the color-composite image (top) and denoised color-composite image (bottom) of the core of galaxy cluster XLSSC 122 with red, green, and blue colors from F356W, F277W, and F090W+F200W, respectively. The denoised image was produced using a Transformer-based reconstruction method (H. Park et al. 2024; Z. P. Scofield et al. 2025). The middle and right panels show the log-scale, single-filter images as labeled. The cluster galaxies are much fainter in the F090W filter because, at z = 1.98, the filter probes rest-frame wavelengths below the 4000 Å break. The candidate strong-lensing arcs are labeled with purple arrows in the top-left image and are visible in all filters. The intracluster light is prominent in the redder filters and stretches from the BCG to the southwest (H. Joo et al. 2025, in preparation).

Kyle Finner et al 2025
JWST Discovery of Strong Lensing from a Galaxy Cluster at Cosmic Noon: Giant Arcs and a Highly Concentrated Core of XLSSC 122
ApJL 994 L35 DOI 10.3847/2041-8213/ae1d80

Copyright: © 2025 The authors.
Published by IOP Publishing. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

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Abstract
The galaxy cluster XLSSC 122 is a rare system at z = 1.98, hosting surprisingly evolved member galaxies when the Universe was only one-third of its present age. Leveraging deep JWST/NIRCam imaging, we perform a weak-lensing analysis and reconstruct the cluster’s mass distribution, finding a mass peak that coincides with both the X-ray peak and the position of the brightest cluster galaxy. Consistent with recent strong-lensing analyses, we obtain a mass estimate of 1.60 ± 0.30 (stat.) ± 0.26 (LSS) × 10¹⁴ M_⊙ and an implied concentration of c_200c = 6.3 ± 0.4, where the uncertainty represents the propagation of the mass error through the adopted concentration–mass relation and excludes intrinsic scatter. Placing our weak-lensing mass map in the context of Chandra X-ray data, MeerKAT radio imaging, Atacama Large Millimeter/submillimeter Array + Atacama Compact Array/Atacama Cosmology Telescope Sunyaev–Zel’dovich (SZ) mapping, and new JWST intracluster light measurements, we identify consistent NE–SW elongation across datasets and a pronounced offset along the same axis between the SZ and mass/X-ray peaks, pointing to significant merger activity. XLSSC 122 thus serves as a JWST pilot study for high-z lensing, demonstrating the telescope’s unique ability to map cluster mass distributions at z ∼ 2. The concordance of the multiwavelength analysis here, together with the high concentration relative to ΛCDM expectations, motivates a uniform sample of analogous systems with joint lensing, X-ray, SZ, and radio data to probe cluster assembly at cosmic noon.

Figure 1. Color composite image of JWST/NIRCam module A for the XLSSC 122 field. Spectroscopically selected cluster members (green) and photometrically selected members (magenta) are indicated, and the BCG is marked with a yellow square.

Zachary P. Scofield et al 2026
An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes
ApJL 999 L1 DOI 10.3847/2041-8213/ae447a

Copyright: © 2025 The authors.
Published by IOP Publishing. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

Download PDF
Abstract
We present a JWST analysis of intracluster light (ICL) in XLSSC 122 at z = 1.98, currently the most distant known strong-lensing galaxy cluster with an evolved member population. Using deep JWST imaging complemented by Hubble Space Telescope data and careful control of systematics, we robustly detect diffuse emission extending to several hundred kiloparsecs from the brightest cluster galaxy (BCG) down to ~29 mag arsec^-2. Multicomponent point-spread-function-convolved Sérsic modeling separates the surface brightness profiles into three components: a BCG core, a BCG envelope, and an ICL component, with stable Sérsic indices across wavelengths. Nearly flat color profiles indicate minimal radial variation in the stellar populations of the BCG envelope and the ICL. The median ICL fraction measured across seven bands is ∼17%, demonstrating that the buildup of intracluster stars in massive halos was already well underway by z ∼ 2. The ICL fraction peaks near 5000 Å in the rest frame, which provides the first confirmation at z > 1 that this characteristic rise is a feature of dynamically active clusters. We also detect a southern excess of ICL relative to the best-fit Sérsic model and quantify it using wavelet-based modeling, providing additional support that this system is dynamically active. The BCG + ICL light distribution and strong-lensing mass map show strong morphological agreement within ∼100 kpc. These findings establish the ICL as an early forming and dynamically informative component of massive halos.

Hyungjin Joo et al 2026
Mature but Still Growing: JWST Detection of the Earliest Intracluster Light at z ∼ 2
ApJL 1002 L17 DOI 10.3847/2041-8213/ae5c9f

Copyright: © 2025 The authors.
Published by IOP Publishing. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

The significance of XLSSC 122 is not that cosmology has collapsed, but that increasingly precise observations are exposing questions our models must answer. Its unusually concentrated mass may reveal something important about how rapidly cosmic structures assemble. Whether the eventual explanation involves its particular history, refinements to existing models or more substantial changes, the deciding factor will be evidence.

That is precisely what distinguishes science from a belief system committed to defending predetermined conclusions. These researchers have published an awkward finding, explained their methods and invited the scientific community to investigate. The possibility that an established explanation may need revision is a reason to pursue the evidence, not suppress it. Scientific understanding advances because its conclusions remain open to correction.

Nor does uncertainty about how quickly a galaxy cluster assembled provide an escape route for young-Earth creationism. A question about the development of structures billions of years ago supplies no evidence for a universe created a few thousand years ago. Even a substantial revision to models of cluster formation would not, by itself, validate Genesis. Creationism would still need evidence for its own claims; pointing to an unresolved scientific problem cannot supply it.

Science can acknowledge that an explanation is incomplete and then do the work needed to improve it. Biblical literalism begins by declaring its preferred account beyond correction. XLSSC 122 illustrates the productive consequence of allowing observations to challenge expectations: an opportunity to learn something new about the universe, rather than another reason to insist that an ancient text must have been right all along.

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