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Giant virtual universe.

A VAST simulation of the universe is digitally recreating the lives of stars, black holes and galaxies. Approximately 1 billion light years across,it is modelling tens of thousands of galaxies.
Richard Bower at Durham University in the UK and his colleagues started the simulation last week. It will runnon-stop for 50 days across 30,000 computer processors in both Durham and Paris developed by tech firm Intel.
The simulation is 30 times larger than one the team ran in 2015, which led to predictions about the mergers of supermassive black holes.
The new simulation includes the physics of allthe “normal” matter suchas the atoms and molecules that make up humans and Earth, as well as the mysterious dark matter that forms around 85 percent of the universe but which we haven’t yet beenable to directly observe.
It also incorporates the physics of star and blackhole formation, and the conditions at the start of the universe. “We set the initial conditions, represented by hundreds of billions of particles in play, and then we let the universe go,” says Bower. The team will look at galaxies by breaking them up into blocks about 3000 light years across. One objective is to try to understand rare objects in the universe, such as galaxies that are so distant from Earth that they are invisible to many telescopes. By simulating what the universe looked like at different points in time – at half or a quarter of its present age, say – the team can test theories about how galaxies are related to the growth of black holes, and what happens when they die.
Simulating individual galaxies insufficient detailis a challenge. “If you try to do a calculation where you have less detail in the galaxy, then you really can’t understand the rate at which stars are going to form,” says Bower.
Another difficulty in simulating the internal structure of galaxies, which containgas, dust and
billions of stars, is that there are huge uncertainties about their underlying physics.
“The physics is so complicated that any small mistake could lead to a verywrong prediction,” says Romain Teyssier at the University of Zurich in Switzerland. He was part of a team behind a massive universe simulation in 2017 that contained 25 billion galaxies, focusing on dark matter.
The paradox is that although we don’t know what dark matter is,its physics is simple to model.
That is because it doesn’t seem to interact with anything or it self except through gravity, which is why we don’t see it emitting light or other radiation.
In contrast, we don’t know very accurately how stars form or how much energy is released when a star goes supernova, says Bower. To check the accuracy of the virtual universe,the team will compare simulated features to the observed universe to check for discrepancies. Teyssier says this is like weather forecasting, in which actual observations are used to refine predictions.

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