Reports that the JWST killed the reigning cosmological model have been exaggerated. But there’s still much to learn from the distant galaxies it glimpses.

The Webb telescope has spotted galaxies surprisingly far away in space and deep in the past. These four, studied by a team called JADES, are all seen as they appeared less than 500 million years after the Big Bang.Illustration: Samuel Velasco

The galaxies’ apparent distances from Earth suggested that they formed much earlier in the history of the universe than anyone anticipated. (The farther away something is, the longer ago its light flared forth.) Doubts swirled, but in December, astronomers confirmed that some of the galaxies are indeed as distant, and therefore as primordial, as they seem. The earliest of those confirmed galaxies shed its light 330 million years after the Big Bang, making it the new record holder for the earliest known structure in the universe. That galaxy was rather dim, but other candidates loosely pegged to the same time period were already shining bright, meaning they were potentially humongous.

How could stars ignite inside superheated clouds of gas so soon after the Big Bang? How could they hastily weave themselves into such huge gravitationally bound structures? Finding such big, bright, early galaxies seems akin to finding a fossilized rabbit in Precambrian strata. “There are no big things at early times. It takes a while to get to big things,” said Mike Boylan-Kolchin, a theoretical physicist at the University of Texas, Austin.

Astronomers began asking whether the profusion of early big things defies the current understanding of the cosmos. Some researchers and media outlets claimed that the telescope’s observations were breaking the standard model of cosmology—a well-tested set of equations called the lambda cold dark matter, or ΛCDM, model—thrillingly pointing to new cosmic ingredients or governing laws. It has since become clear, however, that the ΛCDM model is resilient. Instead of forcing researchers to rewrite the rules of cosmology, the JWST findings have astronomers rethinking how galaxies are made, especially in the cosmic beginning. The telescope has not yet broken cosmology, but that doesn’t mean the case of the too-early galaxies will turn out to be anything but epochal.

Simpler Times

To see why the detection of very early, bright galaxies is surprising, it helps to understand what cosmologists know—or think they know—about the universe.

After the Big Bang, the infant universe began cooling off. Within a few million years, the roiling plasma that filled space settled down, and electrons, protons, and neutrons combined into atoms, mostly neutral hydrogen. Things were quiet and dark for a period of uncertain duration known as the cosmic dark ages. Then something happened.

Most of the material that flew apart after the Big Bang is made of something we can’t see, called dark matter. It has exerted a powerful influence over the cosmos, especially at first. In the standard picture, cold dark matter (a term that means invisible, slow-moving particles) was flung about the cosmos indiscriminately. In some areas its distribution was denser, and in these regions it began collapsing into clumps. Visible matter, meaning atoms, clustered around the clumps of dark matter. As the atoms cooled off as well, they eventually condensed, and the first stars were born. These new sources of radiation recharged the neutral hydrogen that filled the universe during the so-called epoch of reionization. Through gravity, larger and more complex structures grew, building a vast cosmic web of galaxies.

Astronomers with the CEERS survey, who are using the James Webb Space Telescope to study the early universe, look at a mosaic of images from the telescope in a visualization lab at the University of Texas, Austin. Photograph: Nolan Zunk/University of Texas at Austin

Meanwhile, everything kept flying apart. The astronomer Edwin Hubble figured out in the 1920s that the universe is expanding, and in the late 1990s, his namesake, the Hubble Space Telescope, found evidence that the expansion is accelerating. Think of the universe as a loaf of raisin bread. It starts as a mixture of flour, water, yeast and raisins. When you combine these ingredients, the yeast begins respiring and the loaf begins to rise. The raisins within it—stand-ins for galaxies—stretch further apart from one another as the loaf expands.

The Hubble telescope saw that the loaf is rising ever faster. The raisins are flying apart at a rate that defies their gravitational attraction. This acceleration appears to be driven by the repulsive energy of space itself—so-called dark energy, which is represented by the Greek letter Λ (pronounced “lambda”). Plug values for Λ, cold dark matter, and regular matter and radiation into the equations of Albert Einstein’s general theory of relativity, and you get a model of how the universe evolves. This “lambda cold dark matter” (ΛCDM) model matches almost all observations of the cosmos.

One way to test this picture is by looking at very distant galaxies—equivalent to looking back in time to the first few hundred million years after the tremendous clap that started it all. The cosmos was simpler then, its evolution easier to compare against predictions.

Astronomers first tried to see the earliest structures of the universe using the Hubble telescope in 1995. Over 10 days, Hubble captured 342 exposures of an empty-looking patch of space in the Big Dipper. Astronomers were astonished by the abundance hiding in the inky dark: Hubble could see thousands of galaxies at different distances and stages of development, stretching back to much earlier times than anyone expected. Hubble would go on to find some exceedingly distant galaxies—in 2016, astronomers found its most distant one, called GN-z11, a faint smudge that they dated to 400 million years after the Big Bang.

That was surprisingly early for a galaxy, but it did not cast doubt on the ΛCDM model in part because the galaxy is tiny, with just 1 percent of the Milky Way’s mass, and in part because it stood alone. Astronomers needed a more powerful telescope to see whether GN-z11 was an oddball or part of a larger population of puzzlingly early galaxies, which could help determine whether we are missing a crucial piece of the ΛCDM recipe.

Unaccountably Distant

That next-generation space telescope, named for former NASA leader James Webb, launched on Christmas Day 2021. As soon as the JWST was calibrated, light from early galaxies dripped into its sensitive electronics. Astronomers published a flood of papers describing what they saw.

The James Webb Space Telescope, a joint venture of space agencies in the United States, Europe, and Canada that took decades to design, build, and test, was launched into space on December 25, 2021. Courtesy of Northrop Grumman

Researchers use a version of the Doppler effect to gauge the distances of objects. This is similar to figuring out the location of an ambulance based on its siren: The siren sounds higher in pitch as it approaches and then lower as it recedes. The farther away a galaxy is, the faster it moves away from us, and so its light stretches to longer wavelengths and appears redder. The magnitude of this “redshift” is expressed as z, where a given value for z tells you how long an object’s light must have traveled to reach us.

One of the first papers on JWST data came from Naidu, the MIT astronomer, and his colleagues, whose search algorithm flagged a galaxy that seemed inexplicably bright and unaccountably distant. Naidu dubbed it GLASS-z13, indicating its apparent distance at a redshift of 13—further away than anything seen before. (The galaxy’s redshift was later revised down to 12.4, and it was renamed GLASS-z12.) Other astronomers working on the various sets of JWST observations were reporting redshift values from 11 to 20, including one galaxy called CEERS-1749 or CR2-z17-1, whose light appears to have left it 13.7 billion years ago, just 220 million years after the Big Bang—barely an eyeblink after the beginning of cosmic time.

These putative detections suggested that the neat story known as ΛCDM might be incomplete. Somehow, galaxies grew huge right away. “In the early universe, you don’t expect to see massive galaxies. They haven’t had time to form that many stars, and they haven’t merged together,” said Chris Lovell, an astrophysicist at the University of Portsmouth in England. Indeed, in a study published in November, researchers analyzed computer simulations of universes governed by the ΛCDM model and found that JWST’s early, bright galaxies were an order of magnitude heavier than the ones that formed concurrently in the simulations.

Rohan Naidu, an astronomer at the Massachusetts Institute of Technology, was among the first scientists to spot a surprisingly bright early galaxy in JWST images. Courtesy of Michelle L. Peters

Some astronomers and media outlets claimed that the JWST was breaking cosmology, but not everyone was convinced. One problem is that ΛCDM’s predictions aren’t always clear-cut. While dark matter and dark energy are simple, visible matter has complex interactions and behaviors, and nobody knows exactly what went down in the first years after the Big Bang; those frenetic early times must be approximated in computer simulations. The other problem is that it’s hard to tell exactly how far away galaxies are.

In the months since the first papers, the ages of some of the alleged high-redshift galaxies have been reconsidered. Some were demoted to later stages of cosmic evolution because of updated telescope calibrations. CEERS-1749 is found in a region of the sky containing a cluster of galaxies whose light was emitted 12.4 billion years ago, and Naidu says it’s possible the galaxy is actually part of this cluster—a nearer interloper that might be filled with dust that makes it appear more redshifted than it is. According to Naidu, CEERS-1749 is weird no matter how far away it is. “It would be a new type of galaxy that we did not know of: a very low-mass, tiny galaxy that has somehow built up a lot of dust in it, which is something we traditionally do not expect,” he said. “There might just be these new types of objects that are confounding our searches for the very distant galaxies.”

The Lyman Break

Everyone knew that the most definitive distance estimates would require the JWST’s most powerful capability.

The JWST not only observes starlight through photometry, or measuring brightness, but also through spectroscopy, or measuring the light’s wavelengths. If a photometric observation is like a picture of a face in a crowd, then a spectroscopic observation is like a DNA test that can tell an individual’s family history. Naidu and others who found large early galaxies measured redshift using brightness-derived measurements—essentially looking at faces in the crowd using a really good camera. That method is far from airtight. (At a January meeting of the American Astronomical Society, astronomers quipped that maybe half of the early galaxies observed with photometry alone will turn out to be accurately measured.)

But in early December, cosmologists announced that they had combined both methods for four galaxies. The JWST Advanced Deep Extragalactic Survey (JADES) team searched for galaxies whose infrared light spectrum abruptly cuts off at a critical wavelength known as the Lyman break. This break occurs because hydrogen floating in the space between galaxies absorbs light. Because of the continuing expansion of the universe—the ever-rising raisin loaf—the light of distant galaxies is shifted, so the wavelength of that abrupt break shifts too. When a galaxy’s light appears to drop off at longer wavelengths, it is more distant. JADES identified spectra with redshifts up to 13.2, meaning the galaxy’s light was emitted 13.4 billion years ago.

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