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.

Some astronomers and media outlets claimed that 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 JWST’s most powerful capability.

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.

As soon as the data was downlinked, JADES researchers began “freaking out” in a shared Slack group, according to Kevin Hainline, an astronomer at the University of Arizona. “It was like, ‘Oh my God, oh my God, we did it we did it we did it!’” he said. “These spectra are just the beginning of what I think is going to be astronomy-changing science.”

Brant Robertson, a JADES astronomer at the University of California, Santa Cruz, says the findings show that the early universe changed rapidly in its first billion years, with galaxies evolving 10 times quicker than they do today. It’s similar to how “a hummingbird is a small creature,” he said, “but its heart beats so quickly that it is living kind of a different life than other creatures. The heartbeat of these galaxies is happening on a much more rapid timescale than something the size of the Milky Way.”

But were their hearts beating too fast for ΛCDM to explain?

Theoretical Possibilities

As astronomers and the public gaped at JWST images, researchers started working behind the scenes to determine whether the galaxies blinking into our view really upend ΛCDM or just help nail down the numbers we should plug into its equations.

One important yet poorly understood number concerns the masses of the earliest galaxies. Cosmologists try to determine their masses in order to tell whether they match ΛCDM’s predicted timeline of galaxy growth.

A galaxy’s mass is derived from its brightness. But Megan Donahue, an astrophysicist at Michigan State University, says that at best, the relationship between mass and brightness is an educated guess, based on assumptions gleaned from known stars and well-studied galaxies.

One key assumption is that stars always form within a certain statistical range of masses, called the initial mass function (IMF). This IMF parameter is crucial for gleaning a galaxy’s mass from measurements of its brightness, because hot, blue, heavy stars produce more light, while the majority of a galaxy’s mass is typically locked up in cool, red, small stars.

But it’s possible that the IMF was different in the early universe. If so, JWST’s early galaxies might not be as heavy as their brightness suggests; they might be bright but light. This possibility causes headaches, because changing this basic input to the ΛCDM model could give you almost any answer you want. Lovell says some astronomers consider fiddling with the IMF “the domain of the wicked.”