Recent topics



Time flies very fast. I went to Osaka Prefectural Yao High School to give a "delivery lecture" yesterday. The content of the lecture was "Thought Experiment and the Universe". Starting with Galileo's thought experiment, I discussed Einstein's thought experiment, time and space, black holes, and emergent space-time. The students were energetic so I enjoyed it. I also went to Taiwan the week before to attend this Taiwan string workshop. It was hot in Taiwan. While I am very happy that most of the papers I published this year were accepted by JHEP, I am now in the process of finishing my next work.
(in Japanese)


I have been very busy recently, visiting the Kavli Institute in Beijing at the end of August and organizing a workshop with various local people in Beijing. There were various interesting talks related to thermalization, SYK, Black Holes, etc and I enjoyed very much many of the contents, the physics, and the discussions there! China was vibrant and very nice, and I would like to go back in the future!
As soon as I returned, from this week I had another visitor from India, Prof. Sandip Trivedi, who has been my friend since I was a post-doctoral fellow in India a long time ago. It was very fruitful to discuss various physics (IP model, SYK model, JT gravity, etc.) with him and Sunil, who is currently in Japan, and to discuss the direction of my research.
I am going to Kyoto next week for this workshop, which I am one of the organizers. I will talk about the Krylov complexity of the IP matrix model with Dr. Nishida on the 11th of September, so if you are interested, please come or see my slides.


Last week I attended a very fruitful workshop in APCTP, Korea, Quantum Black holes, Quantum Information and Quantum Strings. I had a very useful discussion with various researchers from around the world. I thank the organizer for the wonderful workshop and invitation! At the meeting, I talked about this recent paper on arXiv arXiv:2306.04805 which is in collaboration with Dr. Mitsuhiro Nishida, a former grad student at Osaka U. I have also been very busy recently and have not had time to upload my website, but I will be posting various papers that I have recently completed here shortly.


Today I am excited to announce our new paper appearing in arXiv arXiv:2305.07505! This is the work that I have done with my wonderful collaborators in TIFR, India(=where I did a postdoc a long long time ago), Arka, Sandip, Sunil(=now Sunil is a postdoc here in Osaka) and with my student Takanori. It investigates the relationship between sparse random matrices and gravity. You may have heard of the SYK model (=Sachdev-Ye-Kitaev model). This model is made up of N Majorana fermions. The important thing is that the coupling constants in this model are random numbers, and because of that, this model allows a new large N expansion, which is similar to the famous 't Hooft's work. Anyway, the fact that the coupling constants are random numbers implies that the SYK model itself is sparse random matrices. Here sparse implies that some elements of the random matrices are removed. In fact, in the SYK model, we have q fermion couplings and if we sum over all q, and if all the variance of the coupling constants are the same, it is a random matrix called GUE. Given this, we have studied the vast sparse random matrix "landscapes", which includes the SYK model and also the "local" SYK model. The local SYK model is the model where the q couplings are allowed if and only if they are next to each other. Our diagnostic tools to study this vast sparse random matrix landscape include density of states, thermodynamical quantities, normal correlators and also out-of-time-ordered correlators, nearest neighbor level spacing, etc. It is more than 80 pages and contains not only many beautiful figures but also a very nice appendix such as sparse SYK model analysis. So please have a look at it!


Our new paper arXiv:2304.14620 appears today! We studied holographic complexity in the accelerating expanding universe under the presence of small and early perturbations. Very early perturbation becomes a shock wave, traveling through space and time and we found that under this shock wave, the hyperfast property of de Sitter complexity is always delayed. It turns out the other group in Israel and Canada also studied the same problem and obtained similar conclusions (but with much more detailed) in arXiv:2304.15008. I also received several interesting comments from experts. de Sitter spacetime is interesting since it is related to our universe and we hope we can understand it better in the future!


The international conference in Kyoto, `Quantum Physics of Black Holes 2023' was a success! There were so many interesting talks and we thank all the speakers and participants, especially speakers from abroad!


I gave an informal talk on de Sitter spacetime and complexity at Nagoya U. today. Thank you so much for all the interesting questions!


Another new paper arXiv:2303.10314 appears today! We study the late-time behavior of the spectrum form factors in topological gravity, which includes JT gravity. Never heard about topological gravity? Me too! Indeed, I had never understood that either, until last fall, when Prof. Sakai kindly explained to me all about it very clearly at the 1st study group of black holes and quantum gravity (the website is in Japanese). We thank Sakai san and Okuyama san for their wonderful collaborations. This project was very educational for me!


Our new paper arXiv:2303.05025 appears today! A simple message in this paper is that, unlike Einstein gravity in higher dimensions, in 2D gravity with dilaton (=a scalar field), a vacuum solution is characterized by both dilaton and metric. Therefore by ignoring the dilaton, if you determine the quantum complexity only from the metric i.e., complexity = volume conjecture, it is not the right prescription. You should take into account the dilaton as well, especially for de Sitter spacetime (=expanding universe).


Today we have exciting news. Takanori Anegawa (姉川尊徳) received `Outstanding Research Award, Graduate School of Science, Osaka University'. It seems that only one person from the Department of Physics in grad school received this each year. Congratulations Takanori! We are very proud of you.
(in Japanese)
大阪大学大学院理学研究科優秀研究賞: 姉川 尊徳君(大阪大学理学研究科)が大阪大学大学院理学研究科優秀研究賞を受賞しました!こちらも参照) 姉川君、おめでとうございます!


We will have an on-site international workshop "Quantum Physics of Black Holes 2023" at the Yukawa Institute for Theoretical Physics on April 3 - 7. Workshop website is here. We will have lots of exciting speakers from abroad including Prof. Dan Kabat, my friend, and my former PhD mentor at Columbia. There are also short talk slots. If you wish to give a short talk, please apply for it at the time of registration. In case of applying for a short talk, the deadline for registration is March 20th. We are looking forward to your participation at this very interesting workshop!


Our new postdoc, Dr. Nicolò Zenoni from Italy arrived in Japan! He recently received a double PhD from UCSC, Brescia and Leuven U. Let's work hard and write an excellent paper together!


We had the 2nd ExU Annual meeting during Dec.26-28 @Kobe Convention Center. On the 26th, I gave a group activity report on this year and also gave a brief description of our hourglass prescription of entanglement entropy. In addition to meeting my old friends, I met several new faces who showed interest in our work and I enjoyed this meeting a lot. Some of the contents are available online. Program are available here.


Our new postdoc, Dr. Sunil Kumar Sake from TIFR arrived in Japan! He is Sandip's excellent student and from this November he will start his new postdoc career. We are already collaborating and preparing a paper together. Welcome to Japan, Sunil! Let's work hard and write an excellent paper together!


Our paper with Dan and Takanori about 4-d Maxwell theory entanglement entropy by hourglass prescription is accepted for PRD!


This weekend, we had a the 1st black hole and quantum gravity workshop here at Osaka university Nambu hall. It was a very productive 2 days and I enjoyed it. Many experts in our field gave a talk on recent stuff, including Prof. Tezuka's (from Kyoto) recent development on SYK, Prof. Tamaoka's (from Nihon U.) half-wormholes, Prof. Sakai's (from Meiji-Gakuin) a big lecture series on Multi-boundary/multi-brane amplitudes in JT gravity, Prof. Ishibashi's (from Kindai) lecture on de Sitter entropy, and Prof. Maeda's (from Shibaura U.) lecture on singularity on extremal black holes in Anti-de Sitter space. I learned quite a lot and hopefully, this leads us to productive collaborations for future work. Certainly, we will do the next one in the future.


Today I gave an online zoom seminar talk for the TIFR string theory group people. If you are interested, the video link of the talk is here. The topic is our recent hourglass prescription to calculate entanglement entropy, with Dan and Takanori. It was full of discussion and extremely productive 2 hours I spent, and I enjoyed talking to them and learned a lot today.


Our paper is accepted for JHEP.


Today I gave an online zoom NITEP colloquium talk for Osaka Metropolitan University Joint seminar of mathematical physics and particle physics. Lots of excellent questions and I enjoyed it a lot!


Today, I visited Nagoya University and gave a seminar talk on our recent entanglement entropy calculation using the hourglass prescription. Thank you Sugishita-kun for the kind invitation.


In the last 3 days, I gave a series of intensive Zoom lectures on "Introduction to AdS/CFT correspondence" for advanced grad students and postdocs. Unfortunately, the talk is in Japanese but at least Japanese students interested in this topic can enjoy it. If you are interested, the video link of the talk is here. Hope you found these useful! These lectures are parts of the Japanese school of our MEXT-KAKENHI-Grant-in-Aid for Transformative Research Areas (A) "Extreme Universe" activity.


Today our new paper appears on arXiv. A long time back, we found an attractor mechanism in non-supersymmetric but extremal black holes in here. In this paper, with my great collaborator Prof Ishibashi-san (from Kindai) and Prof. Maeda-san (from Shibaura U.), we added deformation to these attractor extremal black holes and found that some of them flow to p.p. singular black holes!


Today we had the 7th Extreme Universe Online Colloquium, the speaker is our friend Prof. Sandip Trivedi from TIFR. His talk is excellent so I highly recommend it. Youtube video are available. The slide is also available at the extreme universe homepage here.


Our new paper appears on arXiv today. The results are cute, I will explain the details later but in short, we evaluate the entanglement entropy for 4-dimensional Maxwell theory in a spherical entangling surface by using our hourglass prescription and obtained the right answer − 16/90. This is quite nontrivial to me. Thank you very much, Dan and Takanori for the great collaborations.


Here in 「Recent topics/最近の話題」, I will update latest news./「Recent topics/最近の話題」では主に、最近arXivに出した私達の論文の軽い背景説明や、参加した研究会等の話題や、また研究関連のニュース記事などを定期的に書いていこうと思います。




My brand new homepage was released/ホームページを公開しました。

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