Astrophysicists currently have three star-based theories that could help explain the JWST early-galaxy data. The first of these accounts for the possibility that the properties of the early-Universe’s stars were very different from those we see today. In the Milky Way and its closest galaxies, the majority of stars have masses close to that of the Sun, a property that astrophysicists have built into early-Universe models. But the predicted spectral profiles of toddler galaxies that contain mostly suns don’t fit with the JWST observations, which instead more closely match the predicted spectra for newly-formed galaxies that contain a higher number of more massive stars.
The more massive a star, the more ultraviolent light it emits, which, after redshifting, translates to JWST-detectable infrared light. The redshifted light from Sun-like stars is beyond the JWST’s detection range. More massive stars also emit more light. “In the nearby Universe, we almost never see stars bigger than about 100 times the mass of our Sun,” Somerville says. “But if at early times extremely massive stars were more common, that would help explain why these early galaxies are so bright.”
According to the second theory, the high brightness could arise if early stars appeared in bursts . Sun and his colleagues have shown that in a model where the star-formation rate can rapidly change, fledgling galaxies can intermittently shine at the intensities detected by the JWST. To match the data, the rate needs to increase by 10 times over a period of about 100 million years. While short on cosmic timescales, this time span matches the lifetimes of more massive stars, which Sun says makes bursty behavior on these timescales physically reasonable.
Sun notes that the bursty behavior could be shut off by the stellar explosions (supernovae) that mark the ends of massive stars’ lives. Such an explosion can purge star-forming gas from a star’s surroundings, and if many stars explode simultaneously, star formation in a galaxy could halt. “If our simulations are accurate, then burstiness can explain the JWST observations,” he says, though he notes that his model has many assumptions that he isn’t yet certain are justified.
Another possibility—the third theory—is that star formation was significantly more efficient in the early Universe than it is today. Currently, only a few percent of the gas in a galaxy turns into stars, which implies that the process is highly inefficient. This low efficiency stems from interactions of the gas with radiation from starlight, from stellar winds, from supernovae, and from debris around black holes. These interactions heat interstellar gas, making it harder for the gas to condense and form stars.
Simulations of molecular clouds (stellar nurseries) show that these heating effects decrease with increasing gas density, which leads to an increased star-formation efficiency. That trend plays out in the nearby Universe, where abnormally dense regions have higher star-forming efficiencies. Given the higher densities in the early Universe—before the billions of years of expansion—it’s likely that star formation was more efficient, Finkelstein says. But astrophysical models of the early Universe haven’t typically accounted for that possibility. That’s because large-scale simulations can’t resolve the detailed processes of single stars and so must assume a value for the star-formation efficiency rate. In their early-Universe simulations, Somerville and her colleagues have replaced the commonly used value of 1% with one in the 50%–80% range—which fits with early-Universe density predictions. “In making that simple change, we can reproduce the JWST data,” she says. But that fix—like the other starry ideas—comes at a cost, a very expensive, dusty one.