iTHEMS Theoretical Physics Seminar
55 events

Seminar
Gravitational Lensing in Black Hole Spacetimes of the PlebanskiDemianski Class
December 6 (Wed) at 16:00  17:30, 2023
Torben Christian Frost (Postdoctoral Researcher, Kavli Institute for Astronomy and Astrophysics, Peking University, China)
Einstein's field equations allow various different black hole solutions. Among these solutions, the most famous are most likely the Schwarzschild and the Kerr spacetimes, which are both special cases of the socalled PlebanskiDemianski spacetime. Besides the Schwarzschild and Kerr spacetimes, the PlebanskiDemianski spacetime also includes other solutions as special cases, among them the Cmetric and the NUT metric. They describe a linearly accelerating black hole and a black hole with gravitomagnetic charge, respectively. The question is now how we can determine if an astrophysical black hole can be described by one of these spacetimes. We will address this question using gravitational lensing for the three spacetimes with the most salient lensing features, namely the Cmetric, the NUT metric, and the Kerr metric. For this purpose, we will first outline how to solve the equations of motion analytically using elementary and Jacobi's elliptic functions as well as Legendre's elliptic integrals. Then we will fix an observer in the domain of outer communication and relate the constants of motion of the lightlike geodesics to latitudelongitude coordinates on the observer's celestial sphere. We will use the analytic solutions to write down the lens equations, calculate the redshift, and the travel time. Finally, we will discuss and compare the results and comment on how we can use them to place constraints on the spin parameter, the acceleration parameter, and the gravitomagnetic charge of a black hole.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Rotating discs on the Kerr black hole background
December 5 (Tue) at 15:00  16:30, 2023
David Kofroň (Postdoctoral Researcher, Institute of Theoretical Physics, Charles University, Czechia)
Analytical solution of a rotating black hole surrounded by accretion disc in full GR is, so far, unknown. The Ernst equation is nonlinear. In this talk, we will provide a framework in which the solutions of linearised Ernst equations can be obtained from the linear perturbations of Kerr black hole treated in the formalism of the Debye potentials. In this way, we recover all the metric perturbations in term of a single complex scalar function (which solves the Laplace equation).
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Gravity of Accretion Discs and Black Holes
December 5 (Tue) at 14:00  15:00, 2023
Petr Kotlařík (Ph.D. Student, Institute of Theoretical Physics, Charles University, Czechia)
The typical black hole solutions describe only isolated black holes. However, in astrophysics, such a condition is never strictly satisfied. As matter accretes onto the black hole, disc structures are often formed. In this talk, I will summarize our recent attempts to find the gravitational field of such a nonisolated black hole. We start from the simplest case of static and axially symmetric metric. Although it is a somewhat "rough" approximation in the astrophysical context, this idealization may already help us to understand some interesting implications of the disc's gravity. Moreover, with such a simplification, we can obtain exact analytical "superpositions" of the Schwarzchild black hole and a disc. When some rotation is present, dragging effects complicate the situation dramatically. Then, one typically has to resort to numerical relativity or some approximate methods, e.g., perturbations. In the talk, I also address the stationary case and demonstrate what we can do on the level of the direct metric perturbation.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Breaking down the magnonic WiedemannFranz law in the hydrodynamic regime
December 4 (Mon) at 15:00  16:30, 2023
Ryotaro Sano (Ph.D. Student, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
Quantum transport has attracted a profound growth of interest owing to its fundamental importance and many applications in condensed matter physics. Recent significant developments in experimental techniques have further boosted the study of quantum transport. Notably in ultraclean systems, strong interactions between quasiparticles drastically affect the transport properties, resulting in an emergent hydrodynamic behavior. Recent experiments on ultrapure ferromagnetic insulators have opened up new pathways for magnon hydrodynamics. Hydrodynamic magnon transport implies exhibiting extraordinary features and has a potential for innovative functionalities beyond the conventional noninteracting magnon picture. However, the direct observation of magnon fluids remains an open issue due to the lack of probes to access the time and length scales characteristics of this regime. In this work, we derive a set of coupled hydrodynamic equations for a magnon fluid and study the spin and thermal conductivities by focusing on the most dominant time scales [1]. As a hallmark of the hydrodynamic regime, we reveal that the ratio between the two conductivities shows a large deviation from the socalled magnonic WF law. We also identify an origin of the drastic breakdown of the magnonic WF law as the difference in relaxation processes between spin and heat currents, which is unique to the hydrodynamic regime. Therefore, our results will become key evidence for an emergent hydrodynamic magnon behavior and lead to the direct observation of magnon fluids.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Compact Star Solutions Beyond General Relativity
November 7 (Tue) at 13:30  15:00, 2023
Kota Numajiri (Ph.D. Student, Graduate School of Science, Nagoya University)
The neutron star solutions have been gathering attention. Their high compactness enables us to observationally access the information about extreme regimes of hadron physics. On the other hand, their strong gravity features bring up another possibility, gravity beyond general relativity (GR). Although GR has been a great success until now, the present scenario for our universe still has several problems, such as dark sectors and the quantum description of gravity. To tackle these problems, the modified gravity theories have been discussed for decades. Their modifications are expected to become noticeable in strong gravity regimes like compact stars. In this talk, I will discuss the configuration of the compact star solution under the F(R) gravity, one of the most popular and simplest modifications of GR. The background hydrostatic solutions are calculated with some F(R) models, which show nontrivial influences from the additional scalar DOF in this theory. The tidal deformation phenomenon is also considered to focus on another observable, tidal deformability. I will comment on how to utilize obtained observables to determine the gravity theory and the unknown equation of state simultaneously.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Topological Aspect of Adsorption Site Selectivity on Metal Surfaces
October 24 (Tue) at 13:30  15:00, 2023
Yuta Tsuji (Associate Professor, Faculty of Engineering Sciences, Kyushu University)
In this talk, the presenter will discuss which adsorption structure is preferred in the adsorption of atoms and molecules on metal surfaces based on the topology of the adsorption structures. The method of moments is used to analyze the electronic density of states of the surface. The thirdorder moment, which characterizes the skewness of the distribution of the electronic density of states, is related to the topology of the triangles at the adsorption interface. By further relating this to the change in energy of the system with the change in electron occupancy of the states, it is shown that it is possible to discuss the relationship between the type of metal and the topological features of the energetically stable adsorption structure.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
First Xray polarimetry of neutron stars with strong magnetic fields
September 25 (Mon) at 13:30  15:00, 2023
Toru Tamagawa (Chief Scientist, High Energy Astrophysics Laboratory, RIKEN Cluster for Pioneering Research (CPR))
We launched the world's first highly sensitive Xray polarimetry satellite, IXPE, in December 2021. IXPE began observations in January 2022 and detected significant Xpolarization from objects in all categories. The IXPE observations have opened a new window in astrophysics. In this talk, I will introduce IXPE and present the results of Xray polarimetry observations of neutron stars with strong magnetic fields (magnetars and neutron star binaries). The neutron star observations show results quite different from our prior expectations and await further theoretical interpretation.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Induced gravitational waves from inflaton oscillons
September 22 (Fri) at 14:00  15:30, 2023
Kaloian Dimitrov Lozanov (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
We present a new way to study cosmic inflation with gravitational waves. The gravitational signal is generated thanks to nonlinear structures in the inflaton field, called oscillons. This novel probe allows us to test models of inflation which are challenging to test with CMB experiments.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Dark matter heating vs vortex creep heating in old neutron stars
August 7 (Mon) at 13:30  15:00, 2023
Motoko Fujiwara (Postdoctoral Researcher, Theoretical Particle Physics Group, Technical University of Munich, Germany)
Old isolated neutron stars have been gathering attention as targets to probe Dark Matter (DM) through temperature observations. DM will anomalously heat neutron stars through its gravitational capture and annihilation process, which predicts surface temperature as T_s ~ (1 − 3) × 10^3 K for t > 10^6 years. We may put constraints on DMnucleon scattering cross section by finding even colder neutron stars. This story, however, assumed that there is no relevant heating source for old neutron stars. In this talk, we discuss the creep motion of vortex lines in the neutron superfluid of the inner crust as the heating mechanism. This creep mechanism is inherent in the structure of neutron stars. The heating luminosity is proportional to the time derivative of the angular velocity of the pulsar rotation, and the proportional constant J has an approximately universal value for each neutron star. If this vortex creep heating is quantitatively relevant against DM heating, this mechanism may cause a serious background to probe DM. The J parameter can be determined from the temperature observation of old neutron stars because the heating luminosity is balanced with the photon emission in the late time. We study the latest data of neutron star temperature observation and find that these data indeed give similar values of J, in favor of the assumption that these neutron stars are heated by the frictional motion of vortex lines. Besides, these values turn out to be consistent with the theoretical calculations of the vortexnuclear interaction. Integarting all the results, we evaluate the vortex creep heating and conclude its significance against DM heating.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Maximum Force Conjecture and Black Hole Thermodynamics
June 30 (Fri) at 14:00  15:30, 2023
Yen Chin Ong (Professor, Center for Gravitation and Cosmology, College of Physical Science and Technology, Yangzhou University, China)
I review the current controversial status of the socalled "maximum force conjecture" in general relativity, whose validity has recently been debated. Then I will discuss how maximum force conjecture can nevertheless be relevant for black hole thermodynamics.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
The classical equations of motion of quantised gauge theories
June 23 (Fri) at 13:30  15:00, 2023
Tom Melia (Associate Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
The Einstein and Maxwell equations are the jewels in the crown of classical physics. But classical physics is only an approximation to nature, arising as a limit of the underlying quantum mechanical description. And in the case of both general relativity and electromagnetism, owing to their gauge theory nature, the full set of classical equations of motion are not guaranteed to follow from the quantum theory. The timetime and timespace components of the Einstein equations in GR and Gauss’ law in EM are enforced ‘by hand' in the quantisation procedure—a choice so as to make the classicallike states behave as per our classical belief. But what if our universe was actually described by another classicallike state? For GR, the resulting modification of the Einstein equations can be packaged as the inclusion of an auxiliary energymomentum tensor describing a ’shadow’ matter that adds no additional degrees of freedom to the theory. The homogeneous and isotropic background piece of this auxiliary matter contributes to expansion of the universe identical to cold dark matter, and the inhomogeneous components source curvature perturbations that grow linearly at linear order.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
A Spin on Wave Dark Matter
June 15 (Thu) at 16:00  17:30, 2023
Mustafa Amin (Associate Professor, Department of Physics and Astronomy, Rice University, USA)
What can we learn about the intrinsic spin of ultralight dark matter field from astrophysical observations? That is, is dark matter is a scalar (spin 0), (spin 1) or tensor (spin 2) field? Using analytic calculations and 3+1 dimensional simulations, I will argue that the imprint of spin can be seen via (i) the initial density power spectrum, (ii) interference patterns in the density field inside dark matter halos, and through (iii) (polarized) solitons with macroscopic intrinsic spin. Based on features in the initial power spectrum, I will provide a bound on the dark matter mass > 10^(18) eV for postinflationary production. With increasing intrinsic spin, interference patterns in halos are reduced (and the inner shapes of halos modified) — which can be probed by lensing and dynamical heating of stars. Finally, after introducing polarized solitons, I will show that the timescale of emergence of solitons (within halos) increases with increasing spin, and briefly discuss electromagnetic and gravitational wave signatures from such polarized solitons. Timepermitting, I will also mention connections to “spinor" BoseEinstein condensates in the laboratory.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Termination of Superradiance from a Binary Companion
June 6 (Tue) at 13:30  15:00, 2023
Huiyu Zhu (Ph.D. Student, Department of Physics, Hong Kong University of Science and Technology, Hong Kong)
We study the impact of a binary companion on black hole superradiance at orbital frequencies away from the gravitationalcolliderphysics (GCP) resonance bands. A superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. Below a critical binary separation, this superradiance rate becomes negative, and the boson cloud gets absorbed by the black hole. This critical binary separation leads to tight constraints on GCP. Especially, a companion with mass ratio q > 10^−3 invalidates all GCP fine structure transitions, as well as almost all Bohr transitions except those from the 211> state. Meanwhile, the backreaction on the companion manifests itself as a torque acting on the binary, producing floating/sinking orbits that can be verified via pulsar timing. In addition, the possible termination of cloud growth may help to alleviate the current bounds on the ultralight boson mass from various null detections.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Spectral correlations and scrambling dynamics in SachdevYeKitaev type models
May 30 (Tue) at 13:30  15:00, 2023
Masaki Tezuka (Assistant Professor, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
Note: Due to unexpected trouble, we have made the decision to postpone the seminar scheduled for February 21 to May 30. Sorry for the trouble. Abstract: The SachdevYeKitaev (SYK) model, proposed in 2015, is a quantum mechanical model of N Majorana or complex fermions with alltoall random fourbody interactions. The model has attracted significant attention over the years due to its features such as the existence of the largeN solution with maximally chaotic behavior at low temperatures and holographic correspondence to lowdimensional gravity. The sparse version of the SYK model reproduces essential features of the original model for reduced numbers of disorder parameters. We recently proposed [1] a further simplification, where we set the nonzero couplings to be +1 or 1 rather than sampling from a continuous distribution such as Gaussian. This binarycoupling model exhibits strong correlations in the spectrum, as observed in the spectral form factor, more efficiently in terms of the number of nonzero terms than in the Gaussian distribution case. We also discuss the scrambling dynamics with the binarycoupling sparse SYK model, comparing the model with the original model as well as the SYK model with random twobody terms [2], where the localization of the manybody eigenstates in the Fock space has been quantitatively studied [3,4].
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Statistical methods to probe binary stellar evolution with ZTF and LISA data
May 23 (Tue) at 13:30  15:00, 2023
Lucy McNeill (JSPS Fellow, Department of Physics, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
The LaserInterferometerSpaceAntenna (LISA) will be capable of detecting all galactic double neutron star binaries (DNSBs) with orbital periods < 20 minutes, as well as 10100’s of thousands of double white dwarf binaries. I will present our method to use LISA detections to constrain the formation frequencies of galactic DNSBs, which are determined by supernova physics and various mass exchange processes. Next, I summarise the key differences in DNSBs and white dwarf binaries (WDBs) in the context of future LISA observations, as well as the current Zwicky Transient Facility (ZTF) for the latter. Then we will examine the current catalogue of short period white dwarf binaries (orbital periods < 1 hour) detected by ZTF so far, including the first ever measurement (Burdge et al. 2023) of the temperature of a mass transferring white dwarf in a binary. Here I will make the case that taken together, these observations are in conflict with the theoretical picture commonly used in Galactic modelling specifically related to cooling and mass transfer leading to WDB mergers. However, heating from tidal interactions may explain and mediate this inconsistency. Finally I will put these results into context regarding preparing for the unprecedented data set of galactic white dwarf binaries from LISA in the 2030s.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Towards Smatrix theory of unstable particles
March 15 (Wed) at 13:30  15:00, 2023
Katsuki Aoki (Research Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
The Smatrix is one of the central objects in quantum field theory and gains renewed interest recently to understand the possible structures of lowenergy effective field theories and quantum gravity. However, most of the particles have finite decay widths and thus do not appear in asymptotic states. Therefore, the standard Smatrix arguments may not be directly applied to scatterings of such unstable particles and we need to formulate “the Smatrix theory of unstable particles” to properly understand the availability of the Smatrix arguments in realistic systems. In this talk, I will talk about the first steps towards this goal. In particular, I will discuss nonperturbative consequences of unitarity in a scattering amplitude of unstable particles and its analytic properties.
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English

Seminar
Boltzmann or Bogoliubov? A Case of Gravitational Particle Production
February 9 (Thu) at 13:30  15:00, 2023
Kunio Kaneta (Lecturer, Graduate School of Science, Tokyo Woman's Christian University)
Despite its weakness, gravity is the primordial source of particle production in the early Universe. All the particles, including dark matter, can inevitably be created after the end of inflation through gravity. To study this production channel, two different approaches have commonly been considered, one of which is based on the Boltzmann equation, and the other is based on the Bogoliubov transformation. The former approach has widely been used in phenomenological studies of dark matter, while the latter has been developed to describe particle production in curved spacetime. I will discuss when these two approaches are equivalent and when they are not by considering the pure gravitational production of a scalar particle.
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English

Seminar
Tetraneutron system studied by RIbeam experiments
January 17 (Tue) at 13:30  15:00, 2023
Susumu Shimoura (Research Scientist, Spin isospin Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Multineutron systems have attracted a longstanding attention in nuclear physics. In several decades, experimental attempts have been made with a particular focus on the tetraneutron system. Among them, the two different experiments, the doublecharge exchange reaction on 4He and the alphaparticle knockout reaction from the 8He, show a sharp peak just above the threshold in the fourneutron spectra, which could be a signature of a "resonant state", separate from a broad bump structure at higher excitation energy regions. Both the experiments have been realized by using the 8He beam above 150 A MeV at the RIKEN RI Beam Factory. Details of the two experiments including basic idea, experimental techniques, and analysis are presented as well as a historical review of previous experimental attempts. Emphasis is made for the experimental conditions for populating a kinematically isolated tetraneutron system with very small momentum transfer. The spectral shape is discussed by means of reaction processes and correlations in the final tetraneutron system with several recent theoretical studies.
Venue: Common Room #246248 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Chiral effects on lepton transport in corecollapse supernovae
December 13 (Tue) at 13:30  15:00, 2022
DiLun Yang (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
Dynamics of leptons such as electrons and neutrinos play an important role in the evolution of corecollapse supernovae (CCSN). Nevertheless, chirality as one of fundamental microscopic properties that could affect lepton transport, through e.g. weak interaction, has been mostly overlooked. In this talk, I will discuss how chiral effects such as the renowned chiral magnetic effect (CME), generating an electric charge current along magnetic fields with chirality imbalance, could result in the unstable modes of magnetic fields and inverse cascade, which potentially influence the matter evolution in CCSN and pulsar kicks. I will also show how an effective CME could be realized via the backreaction from neutrino radiation even in the absence of an axial charge characterizing an unequal number of right and lefthanded electrons.
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English

Seminar
Cosmic Birefringence: how our universe violates leftright symmetry
December 6 (Tue) at 13:30  15:00, 2022
Tomohiro Fujita (Assistant Professor, Waseda Institute for Advanced Study, Waseda University)
Our universe is lefty: recent observations imply that the polarization plane of light that has traveled through cosmic space for 13.8 billion years rotates about 0.3 degrees to the left. A similar phenomenon is known to occur in materials such as crystals, and is called birefringence. But why does birefringence occur even in the outer space, which is supposed to be a vacuum? Dark energy, the unknown energy that fills the universe, may be responsible for it. In this seminar, I will review observations and theories of cosmic birefringence and discuss future prospects.
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English
55 events
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