Yoichi Kazama, Rikkyo University

Issues in Quantum Gravity

##### Among the important questions that remain after the success of the standard model, by far the most fundamental would be the understanding of quantum gravity, which is not only of utmost academic interest but also might play the key role in the observational cosmology in an unexpectedly near future.

Although many studies are made on this subject, more often than not, papers on “quantum gravity” actually discuss quantum behavior of matter fields in a (semi-)classical gravity background. Clearly, what is required, however, is a bona fide understanding of the quantization of gravity itself.

In this talk, I wish to make an attempt to

(i) clarify what the essential issues of quantum gravity are,

(ii) survey the present status of our understanding, and

(iii) look for promissing directions of attack,

Yuichiro Tada, Nagoya University

Aspects of primordial black hole in the light of gravitational wave

##### Since Carr and Hawking proposed it more than 4 decades ago, primordial black hole (PBH) has attracted people and is increasing its importance more recently. It can explain ubiquitous massive BHs for LIGO/Virgo events, the main component of dark matters, etc., etc. The key point is that, once abundant PBHs are required, it might change the “naturalness” of inflation: the small scale perturbations are free from the CMB scale and might have a large amplitudes. In such a case, stochastic gravitational waves (GWs) induced by the large density perturbations get important. They are not just an evidence of large perturbations but also can carry the information of phase transitions in the universe such as QCD and/or EW. In this talk, I will introduce the astrophysical motivations of PBH, its implication to theories of inflation, and future possibilities in the light of stochastic GWs.

Rodrigo Alonso De Pablo, Kavli IPMU, University of Tokyo

On resonances, amplitudes and the UV completion of gravity

##### We will construct, making use of the on-shell amplitude methods, a possible ultraviolet completion of gravity following a “bottom-up” approach. The assumptions of locality, unitarity and causality i) require an infinite tower of resonances with increasing spin and quantized mass, ii) introduce a duality relation among crossed scattering channels, and iii) dress all gravitational amplitudes in the Standard Model with a form factor that closely resembles either the Veneziano or the Virasoro-Shapiro amplitude in string theory. As a consequence of unitarity, the theory predicts leading order deviations from General Relativity in the coupling of gravity to fermions that can be explained if space-time has torsion in addition to curvature.

Jonathan Miller, OIST

[10th KEK joint colloquium] Psychophysics of Cephalopod Camouflage: What is the input/output response function of a Cuttlefish？カモフラージュの精神物理学：コウイカは何を「考えて」身体模様を変化させるのか？

##### The physicist and philosopher Hermann von Helmholtz, whose foundational contributions to physics you all know very well, also invented “psychophysics,” a paradigm for phenomenology by

– inferring minimal predictive quantitative models;

– combining physical calculations and behavioral studies to establish that human color vision is trichromatic.

Detailed mechanisms were confirmed a century later by electro/neurophysiological, genetic/genomic and biochemical methods. Now phenomenology can drive development of novel AI/machine learning technology by elucidating the neuroscience, principles and computational algorithms of cognitive processing in humans and animals. Think of, for example, generalized adversarial networks [GAN], the state of the art in deconstructing human visual processing.

Cephalopods (octopus, cuttlefish, and squid) modulate their skin color and texture to match their marine backgrounds on millisecond timescales; in this sense, they “report” to us their perceptions. I will describe how biologists in my unit at OIST have developed a flexible model organism ideal for deconstructing this process. Living organisms are wetware, and as such one role of science is to establish their device characteristics. My unit is pursuing the development of customized physical hardware, algorithms, and software to quantitatively probe the i/o response function of this model organism, and to reverse engineer the biophysics and neural algorithms of its cognition and consciousness.

Yoshinobu Kuramashi, University of Tsukuba

Application of Tensor Renormalization Group to Particle Physics

##### Tensor renormalization group, which is a numerical algorithm in tensor network scheme, has fascinating features: (i) no sign problem, (ii) logarithmic volume dependence of computational cost, (iii) direct treatment of Grassmann variables, (iv) direct measurement of partition function itself. These inspires the interest across various research fields. Application of tensor renormalization group to particle physics has been led by our group. I will explain the recent progress of our research.

Ping Yeh, Google

[IPNS Physics/Theory Seminar] Google's quantum computer and pursuit of quantum supremacy

##### I’ll describe the hardware of Google’s superconducting qubit system and the approach we’re taking towards demonstration of quantum supremacy, which means that a quantum computer can super-polynomially outperform classical computers on a given problem.

Han Yan, OIST

Hyperbolic fracton model, subsystem symmetry, and holography

##### We propose that the fracton models with subsystem symmetry can be a class of toy models for the holographic principle. The discovery of the anti–de Sitter/conformal field theory correspondence as a concrete construction of holography and the subsequent developments including the subregion duality and Ryu-Takayanagi formula of entanglement entropy have revolutionized our understanding of quantum gravity and provided powerful tool sets for solving various strongly coupled quantum field theory problems. To resolve many mysteries of holography, toy models can be very helpful. One example is the holographic tensor networks, which illuminate the quantum-error-correcting properties of gravity in the anti–de Sitter space. In this work we discuss a classical toy model featuring subsystem symmetries and immobile fracton excitations. We show that such a model defined on the hyperbolic lattice satisfies some key properties of the holographic correspondence. The correct subregion duality and Ryu-Takayanagi formula for mutual information are established for a connected boundary region. A naively defined black hole’s entropy scales as its horizon area. We also present discussions on corrections for more complicated boundary subregions, the possible generalizations of the model, and a comparison with the holographic tensor networks.

Kenji Toma, Frontier Research Institute for Interdisciplinary Sciences, Tohoku University

Theoretical Interpretation of the M87 black hole shadow imaged by EHT

##### The Event Horizon Telescope (EHT) observed the central compact radio source of the elliptical galaxy M87 at 1.3mm with unprecedented angular resolution, and on April 10, 2019, the EHT Collaboration reported the asymmetric ring-like image reconstructed from the data. We generated a theoretical model image library of magnetohydrodynamic simulations and photon ray tracing calculations in the steady rotating black hole spacetime with broad ranges of parameter values to compare the observed data with the models. As a result, we confirmed that the asymmetric ring is consistent with strong gravitational lensing and Doppler beaming of synchrotron emission from a hot plasma orbiting near the supermassive black hole event horizon. In this seminar, we mainly review our theoretical interpretation, shown in the EHT Collaboration paper v. This time EHT did not have sufficient number of stations for detecting extended emission component such as the jet and accretion flow. We try to discuss how to identify the driving mechanism of the jet in M87 with next EHT observations.

Hiroshi Isono, Department of Physics, Faculty of Science, Chulalongkorn University

A microscopic model for inflation from supersymmetry breaking

##### In this talk, I first introduce our recent proposal of a class of small-field inflation models driven by supersymmetry breaking. The inflaton is a superpartner of the goldstino of the supersymmetry breaking, and charged under a gauged U(1) R transformation. The models in this class has a linear superpotential, leading to avoiding the eta problem. Furthermore, the gauged U(1) R invariance makes the pseudo-scalar companion of the inflaton absorbed into the U(1) R gauge field, and also allows for a global minimum with tuneable cosmological constant. I introduce a concrete model in this class as an effective field theory, and propose a generalisation of Fayet-Iliopoulos model in supergravity as a microscopic model leading to this effective theory.

Hisashi Okui, Niigata University

Tomography by neutrino pair beam