When the universe was just 1.2 billion years old, galaxies were already being shaped by their cosmic address—a discovery that rewrites our understanding of how the universe's largest structures evolved.
Using the James Webb Space Telescope, an international team of astronomers examined the Loktak Protocluster, a massive congregation of galaxies that existed 12.6 billion years ago, and found that where galaxies lived mattered profoundly, even in the universe's infancy. The finding, published in The Astrophysical Journal Letters, reveals that environmental effects—the way a galaxy's surroundings influence its growth—emerged far earlier than scientists had previously thought possible.
Lead author Ronaldo Laishram of the National Astronomical Observatory of Japan and his colleagues first spotted the Loktak Protocluster using the Subaru Telescope's wide-field camera, Hyper Suprime-Cam, during a large sky survey. They identified the structure by detecting Lyman-alpha emission, a distinctive light signature produced when radiation from young, hot stars excites hydrogen gas. This light is a natural tracer of galaxies undergoing vigorous star formation, making them ideal markers for mapping the universe's large-scale structure during its first billion years.
The team named their discovery after Loktak Lake in Manipur, India, because the protocluster's architecture mirrors the lake's distinctive floating islands—four separate galaxy concentrations linked together into a single, coherent system. This clearly organized arrangement offered a rare opportunity to study environmental effects in the young universe.
When Laishram's team compared galaxies inside the densely packed protocluster with galaxies in typical environments at the same cosmic epoch using JWST infrared images, they uncovered a striking asymmetry. In ultraviolet light, which traces regions actively forming stars, the two populations looked nearly identical in size. But in optical light, which maps the distribution of older stars already formed, galaxies in the protocluster were on average about 1.4 times larger than their counterparts in normal environments.
This difference tells a crucial story about how galaxies grew. Although the star-forming cores proceeded similarly regardless of environment, galaxies in dense regions built up their outer stellar structures earlier and more rapidly. Gravity in crowded neighborhoods appears to have accelerated the assembly of stars in the galactic outskirts, a process that would have taken much longer in isolation.
Today's universe demonstrates environmental effects clearly: galaxies in massive clusters are typically more massive, struggle to form new stars, and often display rounder shapes—a stark contrast to isolated galaxies. But the mechanism driving these differences, and when it began, remained a mystery. The Loktak Protocluster now demonstrates that these environmental influences were already operating when the universe was in its infancy, suggesting that the cosmic processes shaping galaxies have remained fundamentally unchanged across more than 12 billion years of history.
As Laishram noted, protoclusters are "the construction sites of the most massive structures in the present-day universe." Understanding when and how environment began to shape galactic growth brings astronomers closer to solving one of cosmology's deepest questions: how did the universe's largest structures form?