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The effect that almost massless subatomic particles called neutrinos have on the formation of galaxies has long been a cosmological mystery – a mystery physicists have sought to measure since the particles were discovered in 1956.
But an international research team including the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Principal Investigator Naoki Yoshida, who is also a professor in the Department of Physics at the University of Tokyo, has created cosmological simulations that describe with precision 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 researcher with the team, says this work is an important step in the process of simulating the formation of the structure of the universe. Saito is also a Visiting Associate Fellow at IPMU Kavli.
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 quickly and appear diffuse, the distribution of dark matter makes up cosmic webs such as filament structure. Credit: KAVLI IPMU
The technique accurately represented the neutrino velocity distribution function and followed its evolution over time. The researchers then looked at the effects of neutrinos on the formation and evolution of galaxies.
Their results showed that neutrinos suppress the aggregation of dark matter – the undefined mass in the universe – and, therefore, galaxies. They found that regions rich in neutrinos are strongly correlated with massive clusters of galaxies and that the effective temperature of neutrinos varies considerably with the mass of the neutrino.
The researchers say the most rigorous experiments used to estimate the mass of neutrinos are cosmological observations, but these can only be used if the simulation predictions are correct.
Figure 2: The researchers’ Vlasov-Poisson simulation (left) predicts a smoother and quieter neutrino density distribution compared to a traditional simulation of Newtonian gravitational N-body particles (right). Credit: KAVLI IPMU
“Overall, our results are consistent with theoretical predictions and the results of previous simulations,” says Dr. Kohji Yoshikawa of the Center for Computational Sciences at the University of Tsukuba and lead author of the study. “It is reassuring to see that the results of totally different simulation approaches are consistent.”
“Our simulations are important because they place constraints on the unknown amount of neutrino mass,” says Saito of Missouri S&T. “Neutrinos are the lightest particles we know of. We only recently learned that neutrinos have mass thanks to the discovery presented in the 2015 Nobel Prize in Physics.
This award was given to two scientists, including Kavli IPMU principal investigator Takaaki Kajita, who is also the director of the Cosmic Ray Research Institute at the University of Tokyo, for their distinct findings that one type of neutrino can transform into another, which showed that neutrinos have mass.
“Our work could ultimately lead to a robust determination of the neutrino mass,” Saito says.
Dr Satoshi Tanaka, postdoctoral fellow at the Yukawa Institute for Theoretical Physics, Kyoto University, was the fourth member of the study, titled “Vlasov-Poisson Cosmological Simulations of Structure Formation with Relic Neutrinos: Clustering No linear 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, November 30, 2020, The astrophysical journal.
DOI: 10.3847 / 1538-4357 / abbd46
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