Charlotte Mason, an astrophysicist at the University of Copenhagen, had modest expectations 9 months ago, when she and her collaborators began to use JWST, the giant new space telescope, to look back in time for the universe’s first galaxies. Modeling suggested the patch of sky they were examining would hold just 0.2 galaxies—none, in other words, unless they got lucky. Yet out of the images popped not one, but two bright galaxies. “That was the biggest surprise to me,” she says.
The surprises have kept coming. JWST astronomers have found more than 15 galaxies shining within the first half-billion years of the 13.7-billion-year-old universe—far too many according to theorists’ models of galaxy formation. Initial estimates of the galaxies’ age and distance come from their brightness at particular wavelengths, but astronomers are now applying the gold standard method: analyzing the galaxies’ spectra in detail to see how much their light has been stretched by the expansion of the universe. The spectra have confirmed nine of the early galaxies, including two added to the roster this week following JWST observations on 24 March. “This is the most exciting period of my recent life,” Casey Papovich of Texas A&M University, College Station, told astronomers last week at a meeting at the Kavli Institute for Cosmology in Cambridge, England.
The discoveries are leaving theorists scratching their heads. The standard theory of cosmology, lambda-cold dark matter (LCDM), says clouds of dark matter—the mysterious stuff making up 85% of the universe’s mass—began to clump up into halos soon after the big bang. The halos’ strong gravity sucked in gas, which collapsed to form stars. LCDM cannot account for the excess galaxies astronomers are seeing, but few astronomers are ready to tear it up. “Let’s get a bigger population,” says Alice Shapley of the University of California, Los Angeles. “Then it will be time to look at theories.”
Instead, cosmologists wonder whether the excess of galaxies in the newborn universe is more apparent than real. It could be that surveys so far have, by chance, zoomed in on areas dense with galaxies. The apparent excess could also arise if the galaxies are merely overly bright and stuffed with stars, so more of them poke above the threshold that JWST can see. But that creates a new theoretical problem: Why are they so bright and full of stars? “There’s no convincing explanation yet,” says Richard Ellis of University College London.
In young galaxies closer to Earth, feedback limits the rate of star formation. Theorists believe baby stars emit stellar winds: streams of particles that slow the process by blowing gas out of the galaxy. Adding to the effect are supernovae, which occur when fast-burning stars run out of fuel, collapsing and triggering explosions that blow away gas and surround the galaxy with dust, scattering its light and giving it a reddish hue. The galaxy’s gravity draws some of the gas back in, but star formation efficiency, a measure of stars formed per unit of gas, typically sticks below 10%.
Avishai Dekel of the Hebrew University of Jerusalem argues that star formation must have been more efficient in the early universe, which was physically much smaller. The gas from which stars form would have been 1000 times denser than it is after billions of years of expansion, making star formation easier. Moreover, that primordial gas was not yet enriched with the heavier elements and dust forged by supernovae. As a result, the stellar winds of these first stars would have been less intense than today—and a weaker brake on star formation. For 1 million years or so, Dekel says, these galaxies could churn out stars with a formation efficiency of nearly 100%. “All galaxies at this epoch should make a feedback-free starburst if they are massive enough.” What’s more, the lack of dust would have allowed the stars to shine more brightly than comparable stars today, and at the bluer wavelengths seen by JWST.
Andrea Ferrara of the Scuola Normale Superiore in Pisa, Italy, takes a different tack. He says the dense galaxies of the early universe would ramp up star formation in cycles that repeat every 100 million years. During the star-forming phases, the radiation pressure from the stars would blast out dust, making the galaxies appear bright and blue.
Ferrara finds some evidence to support the model: The JWST spectrum of one distant galaxy, GNz-11, had one spectral line—for hydrogen gas—shifted out of place as if the gas was moving at 300 kilometers per second. “We see clear signs of outflowing material,” with radiation pressure sweeping out both hydrogen and dust, he says. JWST has also spied a galaxy without signs of star formation 700 million years after the big bang, which Ferrara suggests could be in a quiet phase between star-forming bursts.
Another possible explanation for the galaxies’ surprising brightness is that it was driven not by stars, but massive black holes at their hearts. Hot disks of dust and gas swirling down the gravitational drains of monster black holes are what drive quasars, some of the brightest objects in the universe. But astronomers have not seen quasars any earlier than about 650 million years after the big bang, and they struggle to explain how their black holes could have grown big enough to blaze brightly much earlier. Nonetheless, at the Kavli conference Papovich showed the JWST spectrum of a galaxy from when the universe was 550 million
years old. It showed a hint of light being both stretched and squeezed—a telltale sign of swirling gases around a black hole. Ellis still isn’t convinced giant black holes could form early enough. “The black hole idea is the most extreme,” he says.
Few want to countenance an even more extreme option: that the LCDM model is at fault. It could be tweaked to produce more dark matter halos or larger ones able to concentrate gas more quickly into bigger galaxies. But theorists are loath to tinker with it because it explains so many things so well: the observed distribution of galaxies, the abundances of primordial gases, and the accelerating expansion of the universe. “We’d be at risk of screwing everything else up,” Ferrara says. “You’d need to be pretty desperate.”