Cosmic clues from Webb’s first light
When the James Webb Space Telescope launched in 2021, scientists repeatedly praised its sharp vision and sensitivity. It enabled viewing the glow of the universe’s first galaxies, forming merely a few hundred million years after the Big Bang. By aiming its mirror at the farthest, oldest objects whose light has taken over 12 billion years to reach Earth, Webb effectively lets researchers look back in time. This direct glimpse into the early universe marks a key advance over prior instruments.
Cosmological model under scrutiny
The initial images stunned astronomers. The earliest galaxies looked unusually mature and massive, as if they had accumulated countless stars over billions of years.
Mass estimates based on total luminosity suggested these galaxies were about 50 times heavier than what standard models predicted, approaching the mass of the Milky Way, despite forming in the first few hundred million years. Since luminosity alone is not a reliable measure of mass, this raised questions about the current cosmological framework.
One view is that after the Big Bang matter spread as gas in all directions, gravity stitched denser pockets of cooling gas into stars and black holes within the first 500 million years. Those stars then attracted one another, slowly building up galaxies that merged and grew over cosmic time.
Researchers have proposed several explanations for the puzzling brightness. Some argued that the first stars were metal-poor and exotic enough that traditional stellar evolution models don’t fully apply. Others called for a major overhaul of cosmology. Yet many think the anomaly may have a more conventional answer that does not upend established theory.
Unusual brightness in early galaxies
Guochao Sun, a scientist born in China and working at Northwestern University in the United States, and colleagues used the FIRE-2 simulator to explore the formation and evolution of the first galaxies. The simulation models how interstellar gas behaves and how newborn stars affect that gas, including the impact of supernova explosions which eject material into intergalactic space.
The study found that the observed brightness can be explained by the overall mass being large enough. It suggests that star formation in these galaxies can occur in bursts, with intervals of low activity followed by rapid, simultaneous star production. This pattern creates brief periods of unusually high luminosity.
Claude-André Faucher-Giguère, a Canadian-born researcher involved in the work, explained that most of a galaxy’s light comes from its brightest stars. Since these giants burn quickly, their short lifetimes tie a galaxy’s brightness closely to recent star formation rather than the total stellar mass accumulated over its lifetime.
The proposed cycle begins when gas conditions trigger a burst of star formation. After a few million years, many new stars explode as supernovae, ejecting gas and enabling new stars to form in interactions with gas from other galaxies. The cycle repeats until the galaxy grows so large that supernovae can no longer eject gas efficiently. At that point, star formation settles to a rate similar to what we observe in the present era, and peak brightness gradually wanes.
The simulation offered a precise match to the brightness levels seen in ancient galaxies captured by Webb. The number of galaxies at their peak luminosity agreed with the observed counts, lending strong support to the bursting scenario.
Webb’s precursor and the path ahead
Although the idea of bursty star formation has surfaced before, this work demonstrates its feasibility with modern models. The proposed framework helps explain why some extremely old-looking galaxies appear so luminous without requiring a wholesale rewrite of astrophysical theory.
The Hubble Space Telescope, launched decades earlier, already prompted revisions in cosmology by revealing that the universe’s expansion is accelerating. That insight brought dark energy into physics, a mysterious component driving cosmic acceleration. While the true nature of dark energy remains elusive, it continues to shape how scientists describe the cosmos mathematically.
Webb is viewed as a forward step similar to Hubble’s impact. If any of Webb’s surprising observations resist straightforward explanations, researchers may refine the cosmological model rather than replace it entirely. The ongoing exploration aims to deepen understanding of how the earliest galaxies formed and evolved in a universe that still holds many secrets.