The effect that nearly massless, subatomic particles called neutrinos have on the formation of galaxies has long been a cosmological puzzle – one that physicists have tried to measure since the particles were discovered in 1956.
An international research team, including the Kavli Institute for Physics and Mathematics of the Universe (Kavli IPMU), Principal Investigator Naoki Yoshida, who is also a professor in the Physics Department at Tokyo University, has created cosmological simulations showing the role of neutrinos in 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 on the team, says the work marks a milestone in simulating the formation of the structure of the universe. Saito is also a visiting scholar at the Kavli IPMU.
The team used a system of differential equations known as Vlasov-Poisson equations to explain how neutrinos move around the universe with different values associated with their mass.
Figure 1: Density distribution of neutrinos (left) and dark matter (right) in the cosmic structure on a large scale. While the neutrinos move quickly and look diffuse, the distribution of dark matter forms cosmic networks like the filament structure. Photo credit: KAVLI IPMU
The technology precisely represented the speed distribution function of the neutrinos and followed their development over time. The researchers then studied the effects of neutrinos on galaxy formation and evolution.
Their results showed that neutrinos suppress the accumulation of dark matter – the undefined mass in the universe – and thus of galaxies. They found that neutrino-rich regions correlate strongly with massive galaxy clusters and that the effective temperature of the neutrinos varies significantly depending on the mass of the neutrino.
The researchers say that the most rigorous experiments to estimate neutrino mass are cosmological observations, but they can only be relied on if the simulation predictions are accurate.
Figure 2: The researchers’ Vlasov-Poisson simulation (left) predicts a smoother and less noisy density distribution of neutrinos compared to a conventional N-body particle simulation of Newton’s gravitational interaction (right). Photo credit: KAVLI IPMU
“Overall, our results agree with both theoretical predictions and the results of previous simulations,” says Dr. Kohji Yoshikawa of the Center for Computational Sciences at Tsukuba University and lead author of the study. “It is reassuring that the results of completely different simulation approaches agree with one another.”
“Our simulations are important because they limit the unknown amount of neutrino mass,” says Saito from Missouri S&T. “Neutrinos are the lightest particles we know. We recently learned that neutrinos have mass from the 2015 Nobel Prize in Physics discovery. ”
The award was given to two scientists, including Kavli IPMU Principal Investigator Takaaki Kajita, who is also director of the Institute for Cosmic Radiation Research at the University of Tokyo, for their separate discoveries that one type of neutrino can transform into another, which showed that Neutrinos this have mass.
“Our work could ultimately lead to a robust determination of neutrino mass,” says Saito.
Dr. Satoshi Tanaka, postdoctoral fellow at the Yukawa Institute for Theoretical Physics at Kyoto University, was the fourth member of the study entitled “Cosmological Vlasov-Poisson Simulations of Structure Formation with Relic Neutrinos: Nonlinear Clustering and Neutrino Mass”.
Reference: “Cosmological Vlasov-Poisson simulations of structure formation with relic neutrinos: Nonlinear cluster formation and neutrino mass” by Kohji Yoshikawa, Satoshi Tanaka, Naoki Yoshida and Shun Saito, November 30, 2020, The astrophysical journal.
DOI: 10.3847 / 1538-4357 / abbd46
