Massive simulation of the universe investigates the mystery of ghostly neutrinos

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “Ask a Spaceman” and “Space Radio,” and author of “How to Die in Space.” Sutter contributed this article to’s Expert Voices: Op-Ed & Insights.

How do you test the theories of the universe? Building giant supercomputers and simulating the evolution of the cosmos.

A team of Japanese scientists has built the largest cosmic simulation in history to include tiny “ghost” particles called neutrinos. To explore one of the greatest unsolved mysteries in physics, the researchers used a whopping 7 million CPU cores to solve the evolution of 330 billion particles and a computational grid of 400 trillion units.

By far the most important form of matter in the universe is dark matter. We are not sure what it is or what it is made of, but we do know that there is a lot. It makes up about 80% of all matter. Baryonic matter, the matter that makes up stars, planets, and the rich variety of the entire periodic table, makes up only a small fraction of all matter in the universe.

Related: Where Did All The Baryons Go?

Dark matter forms the backbone of the cosmos. Billions of years ago, there were no structures in the universe. All matter, dark or otherwise, was distributed smoothly and without lumps. There just weren’t many density variations from place to place. Overall, it was a pretty boring universe.

But over time, the universe became more interesting. There were small differences in density, seeded from microscopic quantum fluctuations in the first seconds of the Big Bang. Places with a slightly higher density had a bit more gravity, and that’s where the dark matter started to accumulate. As those early structures flourished, they attracted even more material. Over billions of years, this process emptied vast regions of the cosmos, now known as cosmic voids, dragging all matter into an extensive network of clumps, walls, and filaments.

And then there are neutrinos, extremely tiny particles that have hardly any mass. In fact, they make up less than 0.1% of all the mass in the universe. But these tiny particles have a huge influence on the evolution of structures. They are fast, really fast, capable of traveling almost at the speed of light. This incredible speed dampens the formation of large structures, such as galaxies and clusters.

While dark matter wants to keep accumulating through gravity, neutrinos go too fast to settle in one place. And although neutrinos have very little mass, they still have some mass. They can use their gravity to weakly influence the behavior of dark matter, thus preventing it from clumping together as tightly as it normally would.

In other words, the universe is a little smoother than it would be without neutrinos.

Mysteries of the universe

Finding the masses of the three known neutrino “flavors” (electron neutrinos, muon neutrinos, and tau neutrinos) is a major unsolved problem in modern physics. But ironically, we can measure the masses of these tiny particles by mapping the largest structures in the universe.

To try to understand the nature of dark matter and the role of neutrinos in shaping cosmic evolution, cosmologists often turn to computer simulations. If you change the mass of neutrinos just a little bit in the simulations, you will change the way neutrinos influence the formation of structures over billions of years. So by measuring those same structures, you can understand the mass of neutrinos.

These simulations generally cover a small fraction of the real universe and start with a set of dark matter “particles”, with each particle representing a certain amount of dark matter, for example a single blob with a mass millions of times the mass. of the sun. The simulations then place these particles as they would be in the early universe. Simulations track how those particles evolve through their mutual gravity, giving rise to the giant structures we see today.

This is an approximation technique, because the true behavior of dark matter is represented by a limited number of particles, but it works very well for dark matter. Simulating neutrinos is much more difficult due to their ridiculous speed. It is difficult to follow their behavior within the simulation because they can move from one side of the simulation to the other in a short period of time. So the simulations can’t keep up with how neutrinos act and how they influence dark matter.

A matter of computing

So maybe we shouldn’t bother trying to approximate neutrino behavior. To correctly follow the evolution of neutrinos and account for their rapid behavior, it is necessary to solve an incredibly complex equation. However, solving this equation, called the Vlasov equation, after the Russian physicist Anatoly Vlasov, requires immense computational resources.

So a team of Japanese scientists did just that: They used 7 million processors in the Fugaku supercomputer to track the evolution of dark matter and the influence of neutrinos on the formation of structures. The researcher used 330 billion particles to represent dark matter and a computational grid of 400 trillion components to represent neutrinos, in the largest simulation of its kind.

And while it may not have solved the mystery of neutrino mass, the simulation paves the way for more of this type. In essence, this simulation was a proof of concept to show that we can now include neutrinos in simulations with greater precision than ever before. Armed with this new technology, future simulations will open a window into the role of neutrinos in the universe and perhaps even reveal a key to unlocking their mass.

The team’s document was recently posted on the arXiv prepress server, and you can view the simulation here.

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