The James Webb Space Telescope has only been watching the sky for a few weeks, and already it has yielded startling discoveries: tens, hundreds, maybe 1,000 times more luminous galaxies in the early universe than astronomers thought.
“No one expected this,” says Michael Boylan-Kolchin of the University of Texas, Austin. “Galaxies are bursting out of the woodwork,” says Rachel Somerville of the Flatiron Institute.
Galaxy formation models may now need revision, as recent data suggest that gas clouds must have been too slow to coalesce into stars and galaxies, as Webb’s galaxy-rich images suggest in the early universe, 500 million years after the Big Bang. “It’s way outside the box of what the models predict,” says Garth Illingworth of the University of California (UC), Santa Cruz.
Webb, an orbiting observatory led by NASA with contributions from the European and Canadian space agencies, began observing from a vantage point 1.5 million kilometers from Earth in late June. Most of its time so far has been devoted to projects to demonstrate 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 capture more light from distant sources, and unlike Hubble it operates at infrared wavelengths, making the web more sensitive to those distant sources, whose light is spread in longer, redder wavelengths. Universe expansion.
Within days of Webb starting observing, it spotted a candidate galaxy that appeared to be glowing brightly when the universe was only 230 million years old, 1.7% of its current age, making it the most distant yet seen. Surveys since then have shown that the object is just one of a surprising abundance of early galaxies, each small by today’s standards, but brighter than astronomers expected.
Some researchers warn that abundance, based on images of small patches of sky, may be an illusion. Boylan-Kolchin wonders if Webb just got “extra lucky” and happened to be looking at a large cluster of galaxies, denser than the rest of the early universe. That question will be resolved later this year when CEERS broadens its scope and results come in from other comprehensive surveys.
It’s also possible that astronomers are misidentifying galaxies from very early and slightly recent times. Spectra are the gold standard for measuring a galaxy’s age because they allow the reddening of its light to be accurately measured. But collecting spectra from many galaxies takes time. Instead, Webb surveys estimate galaxy ages from the color they appear in the images—a relatively crude method. Webb’s near-infrared camera filters their light into a few broad wavelength bins, giving astronomers a rough measure of color. redder equals more distance. But the dust surrounding a galaxy can fool observers, as it can absorb starlight and re-emit it at longer wavelengths, making the galaxy appear redder.
Early science teams on the web have already identified some such masquerading galaxies, as they report in several recent publications. But if the abundance of early galaxies is real, astronomers may have to fundamentally rethink galaxy formation or the governing cosmology.
By looking at nearby galaxies, the researchers deduced that the heat inside the gas cloud slows how quickly gravity condenses matter into stars—star formation takes about 100 times longer than if gravity were the only charge. As the first stars in the 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 monsters; When they explode as supernovae they heat the gas clouds even more or eject them from the forming galaxy altogether.
Hubble’s study showed that the rate of star formation remained relatively constant for about 600 million years after the Big Bang, says Charlotte Mason of the Niels Bohr Institute. But Webb’s results indicate that its speed was much faster at an earlier time — as fast, Somerville suggests, as if the gas clouds had fallen freely without any break from heat or supernova.
In fact, says Tommaso Treu of UC Los Angeles, who leads another Webb survey called GLASS, his team is seeing “stars like crazy” in these early galaxies. They look, he adds, “like giant balls of star formation and nothing else.”
Theorists do not know whether the high density of matter and high temperature of the early universe may have accelerated star formation. Another theory is that the first stars could have formed so quickly because they formed from the primordial matter left over from the Big Bang – hydrogen and helium – without the heavier elements made by later generations of stars.
Or there may be something wrong with the current understanding of how the universe evolved. The prevailing theory of cosmology, known as lambda-CDM (referring to cold dark matter), describes how, shortly after the Big Bang, most of the stuff in the universe was composed of invisible dark matter “halos” under its own gravity. These haloes then pull in the normal matter and create the conditions for it to condense into galaxies. Lambda-CDM predicts the number and size of haloes present in the early Universe, and hence the number of galaxies. “There’s not a lot of wiggle room,” Boylan-Colchin says.
Somerville says it may be possible to tweak lambda-CDM to create something closer to what the web is seeing. Or, she says, cosmologists may be forced to reevaluate the first moments of the Big Bang: the era of inflation, the period of rapid growth when quantum fluctuations grew in regions of high or low matter density—the seeds of later halos. “If inflation is wrong it can be very fundamental,” she says. “But I can’t bet on it happening.”
After the initial galaxy problem was revealed, the web could provide the data needed to answer it. So far, Webb is seeing only young, hot, bright stars in newly discovered early galaxies. Follow-up observations of these galaxies at longer wavelengths with Webb’s midinfrared instrument or a ground-based radio telescope sensitive to submillimeter waves may reveal that the gas clouds are actively forming stars. Those observations could help astronomers confirm that early galaxies were unusually eccentric star factories—and hold clues to how they did so.
“We’ll have a much better picture of all this in six months,” says Boylan-Kolchin. “It’s a very exciting time.”