The James Webb Space Telescope has been surveying the sky for only a few weeks and has already yielded a startling discovery: dozens, hundreds, perhaps even 1,000 times more bright galaxies in the early universe than astronomers predicted.
“No one expected anything like this,” says Michael Boylan-Kolchin of the University of Texas at Austin. “Galaxies are bursting out of the woodwork,” says Rachel Somerville of the Flatiron Institute.
Models of galaxy formation may now need a revision, as current ones think clouds of gas must be much slower to coalesce into stars and galaxies than suggested by Webb’s galaxy-rich images of the early universe, less than 500 million years after the big bang. “This is very outside the box of what the models were predicting,” says Garth Illingworth of the University of California (UC), Santa Cruz.
Webb, an orbital observatory led by NASA with contributions from the European and Canadian space agencies, began observing in late June from its vantage point 1.5 million kilometers from Earth. Most of its time so far has been devoted to projects aimed at demonstrating its capabilities, such as the Cosmic Evolution Early Release Science Survey (CEERS). Webb is designed to delve deeper into cosmic history than its predecessor, the Hubble Space Telescope. Its 6.5-meter mirror – six times the area of Hubble – can capture more light from distant sources, and unlike Hubble it operates at infrared wavelengths, making Webb more sensitive to those distant sources. whose light extends to longer and redder wavelengths. cosmic expansion.
Within days of Webb starting observations, he spotted a candidate galaxy that appears to have been glowing when the universe was only 230 million years old, 1.7% of its current age, which would make it the most distant ever seen. Surveys since then have shown the object to be just one of an astonishing plethora of early galaxies, each small by today’s standards but brighter than astronomers expected.
Some researchers warn that the abundance, based on images of a small part of the sky, may be an illusion. Boylan-Kolchin wonders if Webb just got “extra lucky” and looked at a large cluster of galaxies, denser than the rest of the early universe. That question will be resolved when CEERS expands its scope later this year and results come in from other large surveys.
It is also possible for astronomers to misidentify galaxies from slightly more recent times as very early ones. Spectra are the gold standard for measuring the age of a galaxy because they allow the reddening of its light to be measured precisely. But collecting spectra from many galaxies takes time. Instead, Webb’s surveys have so far estimated the ages of galaxies by the color they appear in the images – a relatively crude method. Webb’s near-infrared camera filters their light into several broad wavelength bins, giving astronomers a rough measure of color; redder is more distant. But dust surrounding a galaxy can fool observers, as it can absorb starlight and re-emit it at longer wavelengths, making the galaxy appear redder.
Webb’s early science teams have already identified several such mask galaxies, as they report in several recent preprints. But if the abundance of early galaxies is real, astronomers may have to fundamentally rethink galaxy formation or royal cosmology.
By looking at nearby galaxies, the researchers concluded that the heat inside the gas clouds slows down how quickly gravity would condense matter into stars—making star formation take about 100 times longer than if gravity were responsible alone. As the first stars in a protogalaxy begin to shine, they inject more heat into the gas, pumping the brakes on further star formation. And the first stars are short-lived giants; when they explode as supernovae, they heat clouds of gas even more or blow them completely out of a forming galaxy.
Hubble studies have shown that the rate of star formation has been relatively constant since about 600 million years after the big bang, says Charlotte Mason of the Niels Bohr Institute. But Webb’s results imply that early on its pace was much faster—as fast, Somerville suggests, as if the gas clouds were collapsing freely, without any inhibitions from heat or supernovae.
Indeed, UC Los Angeles’ Tommaso Treu, who leads another Webb survey called GLASS, says his team is seeing these early galaxies “form stars like crazy.” They look, he adds, “like giant balls of star formation and nothing else.”
Theorists do not know whether the higher matter density and higher temperatures of the early universe may have accelerated star formation. Another theory is that the first stars could have formed more quickly because they formed only from the original matter left over from the big bang – hydrogen and helium – without the heavier elements forged by later generations of stars.
Or something may be wrong with the current understanding of how the universe evolves. The prevailing theory of cosmology, known as lambda-CDM (referring to cold dark matter), describes how, shortly after the big bang, the invisible dark matter that makes up most of the stuff in the universe was accreted. together under its gravity in the “halo.” These haloes then pulled in normal matter and created the conditions for it to condense into galaxies. Lambda-CDM predicts the number and size of haloes that must have existed in the early universe, and thus the number of galaxies. “There’s not much room to move, says Boylan-Kolchin.
Somerville says it may be possible to tweak lambda-CDM to create something closer to what Webb is seeing. Or, she says, cosmologists may be forced to reevaluate the early moments of the big bang itself: the inflationary era, a period of rapid growth when quantum fluctuations grew in areas of higher or lower density of matter—the seeds you ate it later. “If inflation is wrong, it can be very fundamental,” she says. “But I wouldn’t bet on that.”
Having discovered the early galaxy problem, Webb can provide the data needed to answer it. So far, Webb is only seeing young, hot, bright stars in newly discovered early galaxies. Follow-up observations of these galaxies at longer wavelengths with Webb’s mid-infrared instrument or ground-based radio telescopes sensitive to submillimeter waves may reveal gas clouds that are actively forming stars. These observations could help astronomers confirm that early galaxies were amazing star factories—and hold clues to how they did it.
“In six months we’ll have a much better picture of all of this,” says Boylan-Kolchin. “It’s a very exciting time.”