New Lumina simulation watches the universe emerge from the cosmic dark ages

In the beginning, there was no light to speak of. After the Big Bang, the universe existed as a hot, dense soup of electrons and subatomic particles, an ionized plasma that initially occupied a region smaller than a single atom before expanding and cooling at astonishing speed. As the temperature dropped, particles slowed enough to bind with electrons, forming electrically neutral hydrogen. That neutral hydrogen accumulated into an effective shroud, blocking light and other forms of radiation from traveling freely through space. By roughly 380,000 years after the Big Bang, the entire cosmos had been plunged into what astronomers call the cosmic dark age, a long stretch of history from which no freely propagating light emerged.

Now, a team of physicists from MIT, Harvard University, and partner institutions has built a computer simulation that reconstructs, in unprecedented detail, how the universe climbed out of that darkness. The project, named Lumina after the Latin word for lights, follows the early universe from the dark ages through the next 2 billion years of cosmic evolution, tracking dark matter, gas, stars, and radiation across a simulated volume far larger than any previous effort of comparable resolution. The results appear in the Monthly Notices of the Royal Astronomical Society, in a paper led by first author Oliver Zier, a former postdoc in the Vogelsberger Lab at MIT, with co-authors including Mark Vogelsberger, professor of physics at MIT, along with Xuejian (Jacob) Shen and Teodora-Elena Bluichi of MIT, 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.

Lumina covers a patch of the early universe spanning 500 megaparsecs, or about 1.6 billion light-years across. Within that enormous space, the simulation tracks the evolution of dark matter, gas, stars, and photons, capturing processes that operate on gigantic cosmic scales while simultaneously resolving the internal structure of individual developing galaxies. Over its 2-billion-year runtime, the simulation records the unfolding of two pivotal transformations in cosmic history: hydrogen reionization and helium reionization, the twin events that gradually stripped neutral atoms of their electrons and allowed light to travel unimpeded through intergalactic space.

Hydrogen reionization occurred when early stars poured out enough ultraviolet light to interact with neutral hydrogen and knock loose its electron. Once reionized, hydrogen no longer absorbed photons, and radiation could stream freely across the cosmos, effectively lifting the veil of neutral hydrogen that had shrouded the universe since the dark ages. Lumina captures this process in detail for the first time at such scale. The simulation also follows helium reionization, a related but distinct transformation in which much higher-energy light breaks apart neutral helium, a process that required far more energetic sources than its hydrogen counterpart.

One of the simulation’s most significant findings concerns the different engines behind these two epochs. Lumina revealed that hydrogen reionization was likely driven by ultraviolet light from early galaxies and their stars, while helium reionization was fueled by much more energetic sources: early supermassive black holes in their active, accreting phases, blasting out radiation as they devoured surrounding matter. According to the team, this represents the first time a simulation has captured both hydrogen and helium reionization together in a single, self-consistent model of cosmic evolution.

Lumina builds on IllustrisTNG, a large-scale simulation previously developed by scientists at MIT and elsewhere, including several members of the Lumina team, which tracks the interactions of dark matter, gas, and other processes involved in galaxy formation from the Big Bang to the present day. To create Lumina, the researchers refined IllustrisTNG’s code so the simulation could resolve galaxy formation with the same level of detail but across a much larger volume. They then combined this upgraded galaxy model with a radiation transport model, which simulates how different forms of radiation, such as ultraviolet light, interact at extremely large cosmic scales, enabling the code to capture reionization as it happens.

The numbers behind the project are staggering. The simulation encompasses roughly half a trillion individual elements, including dark matter particles and gas cells, about 30 times higher in resolution than IllustrisTNG, spread across a volume 100 times larger than previous best simulations. Running the calculations for every single cell over 2 billion years of cosmic time, and bridging scales from individual galaxies to cosmic structures hundreds of millions of light-years wide, demanded extraordinary computing power. The team ran Lumina on Frontier, one of the world’s most powerful supercomputers, based at Oak Ridge National Laboratory in Tennessee.

At peak scale, the calculation brought together the equivalent of more than 100,000 CPU cores and around 15,000 GPUs, according to Shen. Making all of those processors communicate efficiently while continuously evolving trillions of interacting pieces of information, he notes, is itself an extraordinary feat of computer engineering. The result is a simulation that bridges a wider range of spatial scales than any other: dark matter and gas begin to clump into over-densities, gas falls into dark matter haloes, the first galaxies form within those haloes, the first stars ignite inside those galaxies, and the radiation those stars produce travels outward through the universe until it ionizes neutral hydrogen along the way.

Lumina begins its simulation at around 45 million years after the Big Bang, in the midst of the dark ages, when neutral hydrogen dominated the early universe. The researchers set the simulation with initial conditions approximating what physicists theorize should have existed in such a dark cosmic period, then let the code calculate how matter in the form of gas and dark matter interacts across the simulation’s half-a-trillion cells. Simultaneously, the simulation calculates where and how radiation, such as ultraviolet light from stars, emerges and interacts with its surroundings. The Lumina team has made the simulation available to the scientific community, publishing videos on the project website that depict its various scales, time periods, physical processes, and layers of cosmic structure. Examples include a zoom-in from the full large-scale patch of the universe down to a single disk-shaped galaxy, and a time-lapse of the entire volume over 2 billion years showing the pattern of hydrogen as it is reionized over time. Other simulations, Vogelsberger notes, cannot zoom in as far as Lumina can; the difference between the smallest and largest scales the team simulates is larger than in any other simulation to date.

Looking ahead, the researchers plan to apply Lumina to map the large-scale structure of the early universe and to help predict the gravitational-wave patterns that early galaxies and black holes might produce. Perhaps most importantly, whatever Lumina simulates can soon be compared with actual observations from new and upcoming telescopes, allowing scientists to piece together the processes that shaped the first galaxies, stars, and structures in the universe. One of Lumina’s major advantages, Shen says, is its sheer size: it produces enormous populations of simulated galaxies, finally allowing direct comparisons with the very large galaxy samples that upcoming surveys will observe. The research was supported in part by NASA and the National Science Foundation, and used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Advanced Scientific Computing Research programs in the Office of Science of the U.S. Department of Energy.

Subject of Research: Large-volume radiation-hydrodynamic simulation of hydrogen and helium reionization in the early universe

Article Title: Watch the universe emerge from the dark ages, with Lumina

Article References: Watch the universe emerge from the dark ages, with Lumina. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: Lumina simulation, cosmic dark ages, hydrogen reionization, helium reionization, early universe, dark matter, galaxy formation, supermassive black holes, IllustrisTNG, Frontier supercomputer, radiation transport, MIT