Supercomputers are giving astronomers a spectacular route into parts of the universe that cannot yet be observed in sufficient detail. A new simulation now carries researchers into the birth of the first stars and galaxies, when the cosmic dark ages came to an end.

Called Lumina, the simulation concentrates on the universe’s crucial formative period. It covers a region 1.6 billion light years wide and incorporates dark matter, gas, and radiation, allowing those ingredients to evolve as galaxies and their components emerge.

“Lumina simulates the first 2 billion years of the universe – the epoch of reionization – self-consistently. This means it follows how galaxies and black holes grow and how the radiation they release strips electrons off the gas between galaxies, creating the ionized plasma astronomers are familiar with today,” Oliver Zier, an astronomer at the Center for Astrophysics | Harvard and Smithsonian, told IFLScience.

The simulation therefore follows not only the growth of galaxies and black holes, but also the transformation of the gas stretching between galaxies. That sweeping transition produced the ionized plasma familiar to astronomers studying the universe today.

“Lumina combines an enormous simulated volume of the universe with a detailed treatment of radiation, allowing us to follow hydrogen and helium reionization together and connect these processes to the galaxies and black holes that drive them.”

Researchers at Harvard, MIT, and other institutions created the simulation. By its conclusion, Lumina contained 200 million galaxies that formed and grew according to the best theoretical understanding of the universe and galaxy evolution.

The enormous digital universe gives the team a way to transform its understanding of physics into predictions. Those predictions can then be placed beside observations from telescopes, revealing where physical models agree with what astronomers see.

“Simulations let us work out what our physical models predict and compare those predictions with what the telescopes actually see,” Zier told IFLScience.

Lumina is already helping researchers test observations against theory. Its predictions about the gas between galaxies are consistent with observations from JWST, but the telescope has surprisingly detected more massive galaxies than the simulation produces.

Much of the current understanding of galaxy evolution comes from the local universe, where galaxies can be observed in greater numbers and detail. Astronomers still do not know everything about this region, and 2,000 more galaxies were recently found in the closest cluster.

Conditions in the early universe may have been different, even significantly so. Lumina allows astronomers to examine the profound period when cold, neutral gas gave way to a universe containing galaxies, growing black holes, and hot, ionized gas between them.

“This was when the universe changed from being filled mostly with cold, neutral gas to containing galaxies, growing black holes and hot, ionized gas between them. Studying this transition helps us understand how the universe we see today developed,” Zier told IFLScience.

JWST will continue studying the most distant objects and pushing observational limits. The Nancy Grace Roman Telescope, currently on its way to its final orbit, will capture a wide area of the early universe and help reveal how many massive, bright galaxies should exist.

Lumina can help astronomers distinguish among alternative scenarios when observations and simulations do not line up. For Zier, those moments of disagreement are among the most exciting aspects of the work because they can reveal where models succeed or where something may be absent.

“For me, one of the most exciting parts of this work is what happens when simulations and observations disagree. Simulations turn our understanding of physics into predictions. Comparing those predictions with the real universe helps us see where our models work and where something may be missing,” Zier told IFLScience.

Understanding galaxy evolution is also central to understanding why humans are here. The elements that made us came either directly from the Big Bang or were forged in stars that disappeared long ago.

The early universe belongs to an extremely remote past, but its consequences are not confined to the distant sky. Lumina offers astronomers a powerful new tool for exploring that history across 2 billion years, 1.6 billion light years, and 200 million naturally formed galaxies.

The study is published in the Monthly Notices of the Royal Astronomical Society. Its immense simulated universe now gives researchers a detailed arena for comparing physical predictions with observations of cosmic dawn.

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