A powerful new computer simulation is allowing scientists to explore one of the most mysterious periods in cosmic history, when the universe emerged from darkness and the first stars and galaxies began to transform the cosmos.
Developed by researchers at MIT, Harvard University, and other institutions, the simulation, called Lumina, recreates the evolution of the early universe across a region roughly 1.6 billion light-years wide. It follows 2 billion years of cosmic history, revealing how galaxies formed, how radiation spread through space, and how the universe gradually became transparent to light.
The research, published in the Monthly Notices of the Royal Astronomical Society (MNRAS), provides an unusually detailed picture of events that unfolded billions of years before our solar system existed.
How the Universe Entered Its Cosmic Dark Age
The story begins with the Big Bang, when the universe existed in an extremely hot, dense state filled with electrons and other subatomic particles. This primordial plasma, described as initially smaller than a single atom, expanded rapidly and cooled as the universe grew.
Eventually, particles slowed enough for electrons to combine with atomic nuclei, producing electrically neutral hydrogen. Around 380,000 years after the Big Bang, the universe entered a period known as the cosmic “dark age.”
Although the universe had become transparent to much of the radiation left over from the Big Bang, stars had not yet formed to illuminate the cosmos. Neutral hydrogen filled space and could absorb certain types of radiation, including the ultraviolet light that would later emerge from the first stars.
Lumina allows researchers to investigate how this early environment evolved, eventually giving rise to the galaxies, stars, and other structures that helped bring the dark age to an end.
How the First Stars and Black Holes Transformed the Universe
The simulation covers an immense region measuring 500 megaparsecs, equivalent to approximately 1.6 billion light-years. Within that volume, Lumina follows dark matter, gas, stars, and radiation (such as photons), capturing both the behavior of matter across enormous distances and the formation of individual galaxies.
One of its most important achievements is recreating two major changes in the early universe: hydrogen re-ionization and helium re-ionization.
Hydrogen re-ionization began as the first stars and galaxies produced increasing amounts of ultraviolet radiation. This energetic light removed electrons from neutral hydrogen atoms, a process called “re-ionizing” the gas. As more hydrogen became ionized, the universe became increasingly transparent to ultraviolet light, allowing radiation to travel across much greater distances.
This transformation helped clear the widespread neutral hydrogen that had dominated the early cosmos. Lumina reproduces the process in remarkable detail, showing how radiation from the first generations of stars gradually changed the conditions throughout space.
The simulation also follows helium re-ionization, which requires substantially more energetic radiation to strip electrons from helium.
The results point to different sources driving these two transformations. Early stars and galaxies appear to have been primarily responsible for ionizing hydrogen, while actively feeding supermassive black holes supplied the intense radiation needed to ionize helium.
This is the first simulation to capture both processes together, offering a more complete view of how the early universe changed.
“Lumina does everything from small-scale star and galaxy formation to large-scale reionization of hydrogen and helium, in a volume that is 100 times larger than previous best simulations,” says study author Mark Vogelsberger, a professor of physics at MIT.
The researchers are making Lumina available to other scientists, allowing its predictions about early cosmic structures to be compared with observations from existing and future telescopes. Such comparisons could help researchers better understand the events that shaped the first galaxies and stars.
Building a Virtual Universe With Half a Trillion Elements
Lumina gets its name from the Latin word meaning “lights.” Its underlying technology builds on IllustrisTNG, an earlier simulation developed by scientists at MIT and other institutions, including members of the Lumina research team.
IllustrisTNG models how galaxies form and evolve by following the interactions of dark matter, gas, and other physical processes from the Big Bang to the present day.
To create Lumina, researchers upgraded the existing software so it could reproduce galaxy formation in comparable detail across a much larger region of space. They also incorporated a radiation transport model that calculates how light, including ultraviolet radiation, travels through the universe and interacts with surrounding matter.
Together, these improvements allow the simulation to track both the formation of galaxies and the enormous changes that radiation produces across cosmic distances.
The resulting model contains approximately half a trillion individual elements, including particles of dark matter and cells representing gas. It provides roughly 30 times the resolution of IllustrisTNG while covering a region 100 times larger in volume than previous leading simulations.
Recreating billions of years of interactions across so many elements requires extraordinary computing resources. The researchers used Frontier, one of the world’s most powerful supercomputers, located at Oak Ridge National Laboratory in Tennessee.
“At peak scale, the calculation brings together the equivalent of more than 100,000 CPU cores and around 15,000 GPUs,” says Xuejian (Jacob) Shen, an MIT co-author of the study. “Making all of those processors communicate efficiently while continuously evolving trillions of interacting pieces of information is itself an extraordinary feat of computer engineering.”
Watching the First Galaxies Emerge From Darkness
Lumina begins approximately 45 million years after the Big Bang, during an era when neutral hydrogen dominated the universe.
Researchers initialized the simulation using conditions based on scientists’ understanding of that period. They then allowed the model to calculate how gas and dark matter interacted throughout its half a trillion elements as cosmic time progressed.
At the same time, Lumina tracked how radiation appeared, spread, and interacted with matter, including the ultraviolet light produced by emerging stars.
The result is a virtual universe that scientists can explore at dramatically different scales, from the arrangement of matter across enormous cosmic distances down to the development of a single galaxy.
On the project website, the researchers have released videos showing these processes in action. One offers a “zoom-in” from the entire simulated region to an individual disk-shaped galaxy. Another presents a time-lapse of the complete volume, showing how hydrogen re-ionization spreads across the universe over 2 billion years.
“On the one hand, it’s evolving dark matter as well as the gas in the universe, and on the other, it’s also tracking how radiation travels,” Zier explains. “We see dark matter and gas start to clump in these over-densities. And when we zoom in, we can see gas is falling into these dark matter haloes. Zooming in more, we see this is where the first galaxies form. And the first stars form in those first galaxies. Those stars then create radiation. The simulation then tracks that radiation, and we can zoom way out to see how that radiation travels out from stars, through the universe, until at some point it ionizes some neutral hydrogen.”
This ability to move between vastly different scales is one of Lumina’s defining strengths. Researchers can examine the formation of individual galaxies while also observing how their combined radiation affects the universe on a much larger scale.
“Other simulations cannot zoom in as far as this,” Vogelsberger notes. “The difference between the smallest and largest scales that we simulate is larger than in any other simulation.”
A New Way to Explore the Universe’s Earliest History
The research team plans to use Lumina to investigate the large-scale arrangement of matter in the early universe. The simulation could also help predict the patterns of gravitational waves generated by ancient galaxies and black holes.
Another major opportunity will come from comparing Lumina’s virtual universe with real observations. New and upcoming telescopes are expected to collect increasingly detailed information about enormous numbers of distant galaxies, providing scientists with fresh ways to test their understanding of the early cosmos.
Because Lumina simulates such a large volume of space, it can generate extensive populations of galaxies that can be directly compared with these observations.
“One of Lumina’s major advantages is its sheer size,” Shen says. “It produces enormous populations of simulated galaxies, finally allowing us to make comparisons with the very large galaxy samples that upcoming surveys will observe.”
By bringing together detailed galaxy formation, dark matter, gas, and the movement of radiation, Lumina offers researchers a powerful new tool for reconstructing the universe’s earliest chapters. It may help explain not only how the first galaxies took shape, but also how their stars and black holes transformed the surrounding cosmos.
Research Team and Support
The study’s MIT authors include Mark Vogelsberger, Xuejian (Jacob) Shen, and Teodora-Elena Bluichi. Other contributors include first author Oliver Zier (a former postdoc in the Vogelsberger Lab), Rongrong Liu, Sonja Koehler, and Lars Hernquist of the Harvard and Smithsonian Center for Astrophysics; Aaron Smith of the University of Texas at Dallas; Rahul Kannan of York University in Canada; and Volker Springel and Rüdiger Pakmor of the Max Planck Institute for Astrophysics in Germany.
The research received partial support from NASA and the National Science Foundation. It also used computing resources at the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Advanced Scientific Computing Research programs within the Office of Science at the U.S. Department of Energy.
