What did the Universe look like when the first stars and galaxies appeared? Researchers have attempted to travel back in time using a vast computer simulation.
Their project, called Lumina, tells the story of the first two billion years of the Universe. The calculation begins about 45 million years after the Big Bang, at a time when stars had not yet illuminated space. It then follows the formation of the first galaxies and their evolution.
Visualization of the structure of matter and the ionized regions of the Universe in the Lumina simulation.
Credit: Lumina collaboration.
The scale is dizzying. The region studied measures about 1.6 billion light-years across. In this gigantic space, the computer tracks the movements of gas, the effects of dark matter, the birth of stars, and the light they produce. In total, the researchers used nearly 500 billion small computational elements.
At first, matter is distributed almost uniformly. Gradually, gravity brings gas and dark matter together. Denser regions form, eventually giving birth to galaxies. The simulation makes it possible to observe this evolution on a large scale or to zoom in on a particular galaxy.
The researchers also wanted to understand how the Universe became transparent to a large part of light. After the formation of the first atoms, space was filled with hydrogen that absorbed certain types of light. The first stars emitted enough radiation to gradually transform this gas and allow light to travel more freely.
Map of variations in ordinary matter and dark matter under the initial conditions of the Lumina simulation. The white lines show the relative movements of the two components.
Credit: Lumina collaboration.
A similar phenomenon later affected helium. According to the calculations, it was primarily active supermassive black holes, located at the centers of some galaxies, that provided the highly energetic light needed. Lumina makes it possible to study these two major transformations in a single simulation.
To carry out this work, the scientists used Frontier, one of the most powerful supercomputers in the world. They also published videos showing the formation of galaxies and the evolution of gas across space.
These images represent what the laws of physics and current knowledge make it possible to calculate. The researchers can now compare their results with observations from the James Webb Telescope and other instruments to see whether their reconstruction matches the real Universe.