The James Webb Space Telescope has been observing the sky for only a few weeks, and already it has made a startling discovery: dozens, hundreds, perhaps even 1,000 times more bright galaxies in the early universe than astronomers expected.
“Nobody expected anything like this,” says Michael Boylan-Kolchin of the University of Texas at Austin. “Galaxies are bursting out of the tree,” says Rachel Somerville of the Flatiron Institute.
Models of galaxy formation may now need to be revised, as current ones suggest that gas clouds should coalesce into stars and galaxies much more slowly than Webb’s galaxy-rich picture of the early universe suggests, less than 500 million years after the Big Bang. “This is beyond what the models predicted,” says Garth Illingworth of the University of California (UC), Santa Cruz.
Webb, an orbiting observatory led by NASA with participation from the European and Canadian Space Agencies, began observing in late June from its vantage point 1.5 million kilometers from Earth. Most of his time has now been devoted to projects aimed at demonstrating its capabilities, such as the Cosmic Evolution Early Release Survey (CEERS). Webb is designed to probe deeper into cosmic history than its predecessor, the Hubble Space Telescope. Its 6.5-meter mirror, six times the area of Hubble’s mirror, can capture more light from distant sources and, unlike Hubble, operates at infrared wavelengths, making Webb more sensitive to those distant sources whose light is stretched to longer, redder wavelengths. cosmic expansion.
A few days after Webb began observing, he spotted a candidate galaxy that appeared to have shone brightly when the universe was only 230 million years old, 1.7% of its current age, which would make it the most distant of all. ever seen. Studies since then have shown that the object is just one of a staggering number of early galaxies, each smaller by today’s standards but brighter than astronomers expected.
Some researchers warn that abundance based on images of a small patch of heaven may be an illusion. Boylan-Colchin wonders if Webb just got “luckier” and looked at a huge cluster of galaxies denser than the rest of the early universe. This question will be resolved when CEERS expands its scope later this year and receives results from other large-scale surveys.
It is also possible that astronomers mistakenly identify galaxies from a slightly later time as very early ones. Spectra are the gold standard for determining the age of a galaxy because they allow accurate measurements of the reddening of its light. But collecting spectra from many galaxies takes time. Instead, Webb’s surveys have so far estimated the age 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 bands, giving astronomers a rough measure of color; redder means more distant. But dust surrounding a galaxy can mislead observers because it can absorb starlight and re-emit it at a longer wavelength, making the galaxy appear redder.
Webb’s early science groups have already identified several such masked 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 the prevailing cosmology.
Looking at nearby galaxies, the researchers concluded that the heat in the gas clouds slows the rate at which gravity would otherwise condense matter into stars, making star formation take about 100 times longer than if gravity were alone. When the first stars in the protogalaxy begin to shine, they inject more heat into the gas, inhibiting further star formation. And the first stars are short-lived giants; when they explode as supernovae, they heat the gas clouds even more or tear them out of the forming galaxy entirely.
Hubble’s research has shown that the rate of star formation was relatively constant for about 600 million years after the Big Bang, says Charlotte Mason of the Niels Bohr Institute. But Webb’s results suggest that the pace used to be much faster — as fast, Somerville suggests, as if gas clouds were collapsing freely, without any inhibitions from heat or supernovae.
Indeed, UCLA’s Tommaso Treu, who runs another Webb survey called GLASS, says his team is watching 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 could have accelerated star formation. Another theory is that the first stars could have formed more quickly because they formed only from the primordial matter left over from the Big Bang—hydrogen and helium—without the heavier elements created by later generations of stars.
Or there may be something wrong with the current understanding of how the universe evolves. The prevailing theory of cosmology, known as lambda-CDM (for cold dark matter), describes how, shortly after the big bang, the invisible dark matter that makes up most of the universe coalesced under its own gravity into a “nimbus.” These haloes then pulled in ordinary matter and created the conditions for its condensation 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 a lot of wiggle room,” says Boylan-Kolchin.
Somerville says it will be possible to tweak the lambda-CDM to create something closer to what Webb sees. Or, she says, cosmologists may be forced to reevaluate the earliest moments of the Big Bang itself: the era of inflation, a period of rapid growth when quantum fluctuations grew into regions of higher or lower density of matter—the seeds of later haloes. “If inflation is wrong, it can be very fundamental,” she says. “But I wouldn’t dwell on it.”
By unraveling the early problem of galaxies, Webb may provide the data needed to solve it. So far, Webb sees only young, hot, bright stars in new, early galaxies. Further longer-wavelength observations of these galaxies with the Webb Mid-Range 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 provide 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.”