An international research group including Yongjia Huang (Student Trainee, iTHEMS) and Shigehiro Nagataki (Deputy Program Director, iTHEMS) has performed numerical simulations based on general relativity for the merger of binary neutron stars and shown that the waveform of gravitational waves emitted after the merger provides detailed readings of the properties of ultra dense matter with a density of over 1 trillion kg per cm³. This research result is expected to contribute to the clarification of the internal structure of neutron stars and the properties of ultra-dense matter in gravitational wave astronomy.

For details, please see the related link.

The collaborative work on the quark matter and gravitational wave [1] led by our iTHEMS colleague, Yongjia Huang, together with Shigehiro Nagataki, Tetsuo Hatsuda, Kentaro Takami, Hajime Soltani and others, was featured in the Physics Magazine published by the American Physical Society with the title "Listening to Equation-of-State Changes" (see related links below). See how the general relativistic simulations of the postmerger gravitational waves from coalescing neutron stars could allow us to hear the phase transitions between exotic states of matter.

The RIKEN-Berkeley Center was featured in the Oct. 2022 issue of "News from RIKEN's Overseas Offices". The title of the article written by Adam Phillips in the RIKEN International Affairs Division is "What makes institutional international collaborations work?".
Enjoy the article!

On October 27th, Dr. Ebata talked about experiments and theoretical modeling of cell migration dynamics. In the introductory part, he explained the relation between fluctuations of the cell shape and migration dynamics, as well as the basics of durotaxis. After reviewing the experimental setups, he showed us the experimental results of cell migration dynamics on hydrogels with heterogeneous stiffness. He constructed models based on symmetry arguments and consistently explained the observed phenomena. We really enjoyed his clear talk with plenty of exciting movies on cell migration.

Reported by Kyosuke Adachi

In her talk, Dr. Elisa Ferreira exploited some of the intriguing models of dark matter (DM), one of the biggest mysteries in cosmology. While there are many different models to explain the nature of this elusive component, she presented a class of dark matter models: ultra-light dark matter (ULDM) or ultra-light axions (ULA). She explained that one of the most interesting features of this class of DM models is that it might condense in the interior of the halos of galaxies forming a Bose-Einstein condensate or superfluid. This interesting quantum phenomena on macroscopic scales, and the wave nature of ULDM leads to different and interesting astrophysical consequences that can be probed on small scales. She first reviewed the fuzzy dark matter model, one of the most well studied ULA models, and then introduced the DM superfluid model. Upon condensation in the interior of galaxies, DM dynamics in this model can represent that of so-called modified Newtonian dynamics on galactic scales. Dr. Ferreira showed the theoretical description of this model and its interesting phenomenology, especially on small scales.

Reported by Ryo Namba

The main focus of Dr. Filippo Anzuini's talk was on axions, which are pseudo-Goldstone bosons that provide a solution to the strong CP problem, and are prominent candidates for dark matter. In neutron stars, it has been shown recently that the potential of the QCD axion acquires finite density corrections that shift the axion field expectation value, which can be large compared to the vanishing expectation value in vacuum. Such a shift leaves an imprint on typical neutron star observables such as the redshifted thermal luminosity, which can be used to constrain the axion parameter space. In his talk, Dr. Anzuini discussed the coupling of axions with photons, which modifies Maxwell’s equations and alters the neutron star magnetic field. By performing state-of-the-art magneto-thermal simulations, he performed his calculations of the axion-induced perturbations to the neutron star’ magnetic field, and showed that they grow on relatively short time-scales. Intense electric currents then lead to enhanced ohmic dissipation, increasing the stars’ observable thermal luminosity. The activation of such mechanisms depends on the axion decay constant and the axion mass. His results indeed opened a new astrophysical avenue to constrain axions, extending significantly the parameter range that can be probed with direct axion searches.

Reported by Ryo Namba

In her talk, Prof. Yuko Urakawa discussed the generalized deltaN formalism of cosmological perturbations, which dramatically facilitates a computation of the primordial density perturbation and the primordial gravitational waves. In particular, she addressed a violation of the cosmological principle, namely a violation of the global isotropy in the Universe. A major implication of her study is that measuring the PGWs provides a powerful tool to explore a violation of the global isotropy. Such effects can potentially be searchable by the future space-based CMB polarization mission LiteBIRD.

Reported by Ryo Namba

I talked about my research progress about a mathematical model on how interspecific pollen transfer can affect coexistence and evolution of sex allocation.
First, analytical results without evolution showed that interspecific pollen transfer had negative impacts on coexistence by preventing fertilization in the other species. Also, asymmetry of sex allocation between species promoted extinction of either species.
Second, analytical results with evolution of sex allocation revealed that sex allocation always evolved into one regardless of strength of interspecific pollen transfer.
Although my model is very tough to understand, thank you for your listening and giving me comments!

Reported by Keiichi Morita

Title: Bump of sound velocity in dense 2-color QCD
Author: Etsuko Itou, Kei Iida
arXiv: http://arxiv.org/abs/2210.14385v1

Title: High-energy properties of the graviton scattering in quadratic gravity
Author: Yugo Abe, Takeo Inami, Keisuke Izumi
arXiv: http://arxiv.org/abs/2210.13666v1

Title: Double and Quadruple Flat Bands tuned by Alternative magnetic Fluxes in Twisted Bilayer Graphene
Author: Congcong Le, Qiang Zhang, Cui Fan, Xianxin Wu, Ching-Kai Chiu
arXiv: http://arxiv.org/abs/2210.13976v1

Title: Slowly Decaying Ringdown of a Rapidly Spinning Black Hole: Probing the No-Hair Theorem by Small Mass-Ratio Mergers with LISA
Author: Naritaka Oshita, Daichi Tsuna
arXiv: http://arxiv.org/abs/2210.14049v1

Title: Neutron star mass formula with nuclear saturation parameters for asymmetric nuclear matter
Author: Hajime Sotani, Shinsuke Ota
arXiv: http://arxiv.org/abs/2210.11651v1