The James Webb Space Telescope has only been watching the sky for a few weeks, and it has already delivered a startling finding: tens, hundreds, maybe even 1000 times more bright galaxies in the early universe than astronomers anticipated.
“No one was expecting anything like this,” says Michael Boylan-Kolchin of the University of Texas, Austin. “Galaxies are exploding out of the woodwork,” says Rachel Somerville of the Flatiron Institute.
Galaxy formation models may now need a revision, as current ones hold that gas clouds should be far slower to coalesce into stars and galaxies than is suggested by Webb’s galaxy-rich images of the early universe, less than 500 million years after the big bang. “This is way outside the box of what models were predicting,” says Garth Illingworth of the University of California (UC), Santa Cruz.
Webb, a NASA-led orbiting observatory with contributions from the European and Canadian space agencies, began observing in late June from its vantage point 1.5 million kilometers from Earth. Much of its time so far has been devoted to projects meant to show off its capabilities, such as the Cosmic Evolution Early Release Science (CEERS) Survey. Webb is designed to delve deeper into cosmic history than its predecessor, the Hubble Space Telescope. Its 6.5-meter mirror—with six times the area of Hubble’s—can catch more light from distant sources, and unlike Hubble it operates at infrared wavelengths, making Webb more sensitive to those faraway sources, whose light is stretched to longer, redder wavelengths by cosmic expansion.
Within days after Webb began observations, it spotted a candidate galaxy that appears to have been shining brightly when the universe was just 230 million years old, 1.7% of its current age, which would make it the most distant ever seen. Surveys since then have shown that object is just one of a stunning profusion of early galaxies, each small by today’s standards, but more luminous than astronomers had expected.
Some researchers caution that the abundance, based on images of a small patch of sky, may be an illusion. Boylan-Kolchin wonders whether Webb just got “extra lucky” and stared into a huge clump of galaxies, denser than the rest of the early universe. That question will be resolved when CEERS broadens its scope later this year and results come in from other wide-ranging surveys.
It is also possible that astronomers are misidentifying galaxies from slightly more recent times as very early ones. Spectra are the gold standard for gauging a galaxy’s age because they allow the reddening of its light to be measured precisely. But gathering spectra from many galaxies takes time. Instead, Webb surveys so far have estimated galaxy ages from the color they appear in images—a relatively crude method. Webb’s near-infrared camera filters their light into a few wide wavelength bins, giving astronomers a rough measurement of color; redder equals 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 look redder.
Webb’s early science teams have already identified a few such masquerading galaxies, as they report in several recent preprints. But if the profusion of early galaxies is real, astronomers may have to fundamentally rethink galaxy formation or the reigning cosmology.
Viewing nearby galaxies, researchers have concluded that heat within gas clouds slows how quickly gravity would otherwise condense the matter into stars—making star formation take about 100 times longer than if gravity alone was in charge. 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 up gas clouds even more or blast them out of a forming galaxy completely.
Studies with Hubble have shown that the rate of star formation has been relatively constant as far back as about 600 million years after the big bang, says Charlotte Mason of the Niels Bohr Institute. But the Webb results imply that at earlier times its pace was much more rapid—as fast, Somerville suggests, as if gas clouds were collapsing freely, without any braking from heat or supernovae.
Indeed, Tommaso Treu of UC Los Angeles, 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 don’t know if the higher density of matter and higher temperatures of the early universe might have sped star formation. Another theory is that the first stars could have formed faster because they took shape from just the primordial matter leftover from the big bang—hydrogen and helium—without the heavier elements forged by later generations of stars.
Or something may be wrong in the current understanding of how the universe evolves. The prevailing theory of cosmology, known as lambda-CDM (referring to cold dark matter), describes how, soon after the big bang, the unseen dark matter that makes up most of the stuff of the universe clumped together under its own gravity into “halos.” These halos then drew in normal matter and created the conditions for it to condense into galaxies. Lambda-CDM predicts the number and size of halos that should exist in the early universe, and hence the number of galaxies. “There’s not much wiggle room,” Boylan-Kolchin says.
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 reassess the first moments of the big bang itself: the era of inflation, a period of rapid growth when quantum fluctuations grew into areas of higher or lower matter density—the seeds of later halos. “If inflation is wrong that could be very fundamental,” she says. “But I wouldn’t bet on it being that.”
Having revealed the early galaxies problem, Webb may provide the data needed to answer it. So far Webb is only seeing young, hot, bright stars in the newfound early galaxies. Follow-up observations of these galaxies at longer wavelengths with Webb’s midinfrared instrument or ground-based radio telescopes sensitive to submillimeter waves could reveal the gas clouds actively building stars. Those observations might help astronomers confirm that early galaxies were unusually prodigious star factories—and hold clues to how they did so.
“In 6 months we’ll have a much better picture of all this,” Boylan-Kolchin says. “It’s a very exciting time.”
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