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The effect that nearly massless subatomic particles called neutrinos have on the formation of galaxies has long been a cosmological mystery, one that physicists have tried to measure since they discovered the particles in 1956.
But an international research team that includes the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), Principal Investigator Naoki Yoshida, who is also a professor in the physics department at the University of Tokyo, has created cosmological simulations that describe accurately the role of neutrinos in the evolution of the universe. Their study was recently published in The Astrophysical Journal.
Missouri University of Science and Technology (Missouri S&T) cosmologist Dr. Shun Saito, assistant professor of physics and team researcher, says the work is a milestone in the process of simulating the formation of the structure of the universe. Saito is also an associate scientist visiting the Kavli IPMU.
The team used a system of differential equations known as the Vlasov-Poisson equations to explain how neutrinos move through the universe with different values assigned to their mass.
Figure 1: Density distribution of neutrinos (left) and dark matter (right) in the large-scale cosmic structure. While neutrinos move fast and appear to be scattered, the distribution of dark matter composes cosmic webs like filamentous structures. Credit: KAVLI IPMU
The technique accurately represented the neutrino velocity distribution function and followed its evolution over time. The researchers then examined the effects of neutrinos on the formation and evolution of galaxies.
Their results showed that neutrinos suppress the cluster of dark matter – the undefined mass in the universe – and, in turn, galaxies. They found that neutrino-rich regions are strongly correlated with massive clusters of galaxies and that the effective temperature of neutrinos varies substantially depending on the neutrino mass.
The researchers say the most rigorous experiments used to estimate the mass of neutrinos are cosmological observations, but these can only be relied upon if the simulation predictions are accurate.
Figure 2: The researchers’ Vlasov-Poisson simulation (left) predicts a smoother and less noisy neutrino density distribution than a traditional N-body particle simulation of Newtonian gravitational interaction (right). Credit: KAVLI IPMU
“Overall, our results are consistent with both theoretical predictions and the results of previous simulations,” says Dr. Kohji Yoshikawa of the Tsukuba University Center for Computational Sciences and lead author of the study. “It is reassuring that the results of completely different simulation approaches agree with each other.”
“Our simulations are important because they establish constraints on the unknown amount of the neutrino mass,” says Saito of Missouri S&T. “Neutrinos are the lightest particles we know of. We have only recently learned that neutrinos have mass since the discovery presented in the 2015 Nobel Prize in Physics. ”
That award awarded two scientists, including lead researcher Kavli IPMU Takaaki Kajita, who is also the director of the Institute for Cosmic Ray Research, University of Tokyo, for their separate discoveries that one type of neutrino can morph into another, which proved that neutrinos have mass.
“Our work could ultimately lead to a robust determination of the neutrino mass,” says Saito.
Dr Satoshi Tanaka, a postdoctoral fellow at the Yukawa Institute of Theoretical Physics at Kyoto University, was the fourth member of the study, titled “Cosmological simulations of Vlasov-Poisson structure formation with relict neutrinos: nonlinear grouping and mass of neutrinos “.
Reference: “Cosmological Vlasov-Poisson Simulations of Structure Formation with Relic Neutrinos: Nonlinear Clustering and the Neutrino Mass” by Kohji Yoshikawa, Satoshi Tanaka, Naoki Yoshida and Shun Saito, 30 November 2020, The Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / abbd46
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