Massive simulation of the universe probes mystery of ghostly neutrinos

 

An artist's depiction of a galaxy with a quasar at its center. (Image credit: NASA, ESA and J. Olmsted (STScI))

How do you put universe hypotheses to the test? By constructing massive supercomputers and simulating the evolution of the universe.

A group of Japanese researchers has created the world's largest cosmic simulation, which includes microscopic "ghost" particles known as neutrinos. The researchers employed a remarkable 7 million CPU cores to solve for the evolution of 330 billion particles and a computational grid of 400 trillion units to investigate one of physics' largest unresolved puzzles.

Dark matter is by far the most important type of matter in the cosmos. We have no idea what it is or what it is composed of, but there is a lot of it. It accounts for around 80% of all matter. Baryonic matter, which makes up stars, planets, and the entire periodic table's rich variety, makes up a tiny percentage of all the matter in the universe.

The universe' backbone is made up of dark matter. There were no structures in the universe billions of years ago. All of the substance, black or light, was evenly dispersed and not lumpy in the least. There were few differences in density from one location to the next. In general, it was a rather dull universe.

However, as time passed, the universe grew more fascinating. There were minuscule density discrepancies in the early seconds of the Big Bang, which were seeded by microscopic quantum fluctuations. Dark matter began to gather together in places with slightly higher density and slightly more gravity. As those early buildings grew, additional material was attracted to them. This process emptied enormous parts of the cosmos — now known as cosmic voids — over billions of years, pushing all matter into a vast network of clusters, walls, and filaments.

Then there are neutrinos, which are extremely small particles with almost no mass. Indeed, they account for less than 0.1 percent of the universe's total mass. These microscopic particles, on the other hand, have a huge impact on the evolution of structures. They're quick - incredibly fast — and can travel at speeds approaching those of light. The creation of massive structures like galaxies and clusters is slowed by this extraordinary speed.

Neutrinos are too quick to settle down in one place, whereas dark matter wants to keep stacking up through gravity. Despite the fact that neutrinos have extremely little mass, they do have some. They can use gravity to gently control dark matter's behaviour, preventing it from clustering as tightly as it might otherwise.

To put it another way, the cosmos is a little smoother than it would be if neutrinos didn't exist.

Mysteries of the universe

A key unsolved problem in current physics is determining the masses of the three known neutrino "flavours" - electron neutrinos, muon neutrinos, and tau neutrinos. Ironically, we can determine the masses of these tiny particles by studying the universe's greatest structures.

Cosmologists frequently use computer simulations to try to understand the nature of dark matter and the role of neutrinos in determining cosmic evolution. If the neutrino mass is changed slightly in the simulations, the neutrinos' influence on the creation of structures over billions of years will alter. So you can figure out how much neutrino mass there is by measuring those identical structures.

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