There is a lake in Manipur that floats. Loktak Lake — India’s largest freshwater lake, cradled in the hills of the northeast — is famous for its phumdi: organic islands of soil, vegetation, and biomass that drift across the water’s surface yet remain invisibly tethered to one another, connected by the lake’s own submerged ecology. They move. They breathe. They belong together.
Twelve-point-six billion light-years away, and 12.6 billion years back in time, an international team of astronomers has found something that behaves, at a cosmic scale, remarkably like those floating islands — four dense concentrations of young galaxies, linked by filaments of matter across the early universe, drifting yet interconnected. They have named it the LoktakProtocluster.
The name is not incidental. It is the considered choice of Dr. Ronaldo Laishram — a Manipuri astrophysicist, now a researcher at the National Astronomical Observatory of Japan (NAOJ) — who led the international team that discovered the structure. The naming is simultaneously a scientific metaphor of precision, a cultural act of profound visibility, and an assertion that Indian science is now operating at the very frontier of humanity’s oldest question: how did the universe build itself?
The Discovery: Time-Travelling to the Infant Universe
The universe is approximately 13.8 billion years old. The LoktakProtocluster is seen as it existed 12.6 billion years ago — meaning the light captured by telescopes left this structure when the cosmos was barely 1.2 billion years old, barely out of what cosmologists call the Epoch of Reionisation, still in the throes of its earliest structural assembly.
What Laishram and his colleagues found, in technical terms, is a protocluster at redshift z≈4.9 — a cosmological redshift so high it places the object firmly in the universe’s adolescence, not its maturity. A protocluster is precisely what the name implies: a galaxy cluster in the act of forming, a gravitational gathering-in-progress that, over the subsequent billions of years, will continue to collapse and virialise into a structure comparable to the great mature galaxy clusters we observe in the nearby universe today, such as the Coma Cluster or the Virgo Cluster.
Loktak is not one dense knot of galaxies. It is four — four distinct galaxy concentrations, or overdensities, linked across the cosmic web into a single evolving system. “City of galaxies” is the description astronomers have reached for, and it earns its grandeur: this is a metropolis of young galaxies already beginning to organise themselves into neighbourhoods, connected by the invisible scaffolding of dark matter filaments, 1.2 billion years after the Big Bang.
How Two of the World’s Greatest Telescopes Found Loktak
The discovery required the complementary architecture of two flagship observatories — one built for panoramic census work, one for exquisite fine-scale detail.
The search began with Japan’s Subaru Telescope on Maunakea, Hawaii, and its instrument Hyper Suprime-Cam (HSC) — one of the widest-field imaging cameras in existence, capable of surveying enormous swathes of sky in a single pointing. The team used HSC to search for distant galaxies via their Lyman-alpha emission — a specific ultraviolet spectral signature produced when hydrogen gas is excited by intense star formation in young galaxies. At cosmological distances, this emission shifts into visible wavelengths, allowing it to be detected as a statistical fingerprint of early, vigorous galactic activity.
In the HSC data, Laishram and colleagues identified a statistically significant overdensity of Lyman-alpha emitting galaxies at redshift z≈4.9 — a concentration far above what random distribution would predict, flagging a probable protocluster. But overdensity alone does not tell you what is happening inside those galaxies, or how their structures compare to galaxies in less crowded regions of the same epoch.
That is where the James Webb Space Telescope (JWST) entered. NASA’s flagship space observatory was targeted at the Loktak region to obtain near-infrared imaging of extraordinary sharpness and sensitivity — imaging capable of resolving the internal structure, sizes, and morphologies of individual galaxies in the protocluster to a degree impossible from the ground. Combining Subaru’s wide-field census of the large-scale environment with JWST’s fine-scale structural dissection of individual galaxies allowed the team to do something genuinely novel: connect the cosmic geography of these galaxies to the details of their growth.
The study has been published as “Discovery of a z≃4.9 Lyα Emitter Protocluster: Wavelength-dependent Environmental Effects on Galaxy Structure” in The Astrophysical Journal Letters — one of the premier venues in observational astronomy.
Environment Was Already at Work
The most consequential finding of the Loktak study is not the discovery of the protocluster itself — remarkable as that is — but what the protoclusterreveals about how galaxies grow.
When the team compared galaxies inside the LoktakProtocluster with “field” galaxies — those in less crowded regions at the same cosmic epoch — they found a striking, wavelength-dependent difference in size.
In ultraviolet light, which traces the compact, actively star-forming cores of galaxies where new stars are currently igniting, cluster and field galaxies looked essentially similar in size. But in optical light — wavelengths sensitive to the older stellar populations that were formed in earlier epochs — galaxies inside the LoktakProtocluster were, on average, approximately 1.4 times larger than their field counterparts.
What this size difference tells cosmologists is profound. The optical-light envelopes of galaxies — their extended outer stellar bodies — had already been built up more substantially in the dense cluster environment. In other words, the dense gravitational neighbourhood of a protocluster was accelerating the outward growth of galaxies, enriching their outer stellar envelopes, at an epoch when the universe was barely one billion years old.
This is the early-universe equivalent of observing that city-dwellers develop differently from rural inhabitants — not because of their intrinsic nature, but because of their environment. And the Loktak data confirms this environmental sculpting was already well underway a billion years after the Big Bang.
Dark Matter: The Invisible Architect
Underlying all of this structural organisation — the four galaxy concentrations, the cosmic-web filaments, the accelerated stellar growth — is the gravitational scaffolding of dark matter. Protoclusters like Loktak sit at the densest nodes of the cosmic web, regions where dark matter has pooled into deep gravitational wells that funnel baryonic matter — gas, dust, young stars, entire galaxies — toward common centres of mass.
Mapping the distribution and growth of visible galaxies in a structure like Loktak allows astronomers to trace the invisible dark matter architecture underneath, because the galaxies serve as luminous tracers of where dark matter has concentrated. Comparing Loktak’s properties with predictions from cosmological N-body simulations — the large-scale computational models that simulate how dark matter structures form and grow across billions of years — allows researchers to test whether current theoretical frameworks correctly reproduce the existence of such massive, early overdensities.
Loktak is especially valuable for this test precisely because it is both massive and young. A large, coherent overdensity already present at z≈4.9 sets a stringent benchmark: cosmological models must be able to produce such structures at such epochs, or require revision. The LoktakProtocluster thus functions simultaneously as an observed cosmic object and as a diagnostic probe of our fundamental models of structure formation.
Why ‘Loktak’? The Metaphysics of a Name
It is worth pausing on the naming — because it is not merely a sentimental gesture.
Loktak Lake’s phumdi islands are the world’s only floating biomass islands, forming a networked ecosystem of drifting organic matter across the lake’s surface. They are distinct concentrations — separate, individual, identifiable — yet connected through the lake’s own underwater ecology, its currents, its chemistry. They move independently but belong to one system.
The four galaxy overdensities of the LoktakProtocluster have exactly this character: distinct yet connected, separated in space yet bound gravitationally and by dark matter filaments into one evolving megastructure. The metaphor is precise at the level of topology: a network of dense nodes linked by an invisible web, drifting through cosmic time toward inevitable merger.
For Dr. Laishram — a scientist from Manipur now working at NAOJ in Japan, publishing in The Astrophysical Journal Letters, operating JWST and Subaru telescope time on one of the most significant early-universe structures found in recent years — the name is also an act of cultural geography. Manipur, and Loktak Lake within it, are now inscribed permanently on the map of the observable universe. The lake that floats in the hills of India’s northeast now has a cosmic namesake floating at the edge of observable time.
What Loktak Means for India’s Astronomy Future
The Loktak discovery arrives at a moment of genuine momentum for Indian-led astronomy. The country’s growing presence in international telescope collaborations, the emergence of researchers like Laishram operating at the very frontier of observational cosmology, and the demonstrated ability to lead multi-facility, multi-instrument campaigns using the world’s most capable telescopes — these are not isolated events. They are indicators of a maturing scientific infrastructure and an increasingly confident generation of researchers.
For India’s astrophysics community, the LoktakProtocluster is likely to become both a cultural landmark and a continuing scientific laboratory. Deeper JWST integrations targeting individual galaxies in the system, follow-up spectroscopy to measure precise galaxy velocities and confirm cluster membership, observations from future observatories — all of this will make Loktak a recurring subject of investigation for years, possibly decades.
The cosmic web extends across the entire observable universe. But it was a lake in Manipur, and a scientist from its shores, that gave us one of its most ancient, most luminous, and most scientifically pregnant nodes.
–Vijaya Yandrapalli
