LeCosPA Cosmology and Particle Astrophysics Seminar

Next seminar:

  • Note: There will be 1 on-site talk next week at 11:00 Dec 18 (Mon) UTC+8.

  • Location: Room 7S1, Chee-Chun Leung Cosmology Hall, NTU  國立臺灣大學次震宇宙館7樓7S1會議室

Talk #1:

  • Date and time [YYYY-MM-DD hh:mm]: 2023-12-18 11:00-12:00 (UTC+8)

  • Speaker: Prof. Misao Sasaki ((YITP, Kyoto University / IPMU, Tokyo University / LeCosPA)

  • Title: Prospects in Inflation

  • Abstract:

Thanks to the rapid progress in observational cosmology, inflation has now become part of the standard model of the Universe. In this seminar, I will review the theory of inflation and discuss future prospects.

General Information:

  • Regular time: 11:00-12:00 (UTC+8), every Monday (* may be different for some special talks)

  • Location: Room 7S1, Chee-Chun Leung Cosmology Hall, NTU 國立臺灣大學次震宇宙館7樓7S1會議室 (see the map below)

  • Organizers :

  • Dr. Justin Feng (contact: jcfeng@ntu.edu.tw)
    Dr. Shih-Hao Wang (contact: wsh4180@gmail.com)



List of Seminar Speakers and Talk Titles

2023 Fall (also available in our Wiki)

Date [YYYY/MM/DD] Name (Affiliation) Title and Abstract
2023/10/06 Dr. Jakub Řípa
(Masaryk University, Czech)
GRB-detecting nano-satellites and a small UV space telescope for transient follow-up
  I will present the results of the GRBAlpha and VZLUSAT-2 nanosatellites, which carry onboard gamma-ray detectors for monitoring transients. GRBAlpha, a 1U CubeSat launched in March 2021 has, so far, detected 70 gamma-ray transients, such as gamma-ray bursts (GRBs), flashes from the soft-gamma repeater SGR 1935+2154, and solar flares. It detected the extraordinarily bright GRB 221009A, which was the most intense GRB ever recorded in the 55-year history of GRB science, and was able to provide an unsaturated measurement of its peak flux. More than two years after the launch, the detector performance is good and the SiPM photon counters’ degradation remains acceptable. The same detector system, but double in size, was launched in January 2022 on the VZLUSAT-2 3U CubeSat. So far, it has detected 57 gamma-ray transients, including GRBs, the activity of SGR 1935+2154 and SGR 1806-20, as well as solar flares. CubeSats have detected the exceptional GRB 230307A (2nd brightest, long-duration but with detected kilonovae pointing to a merger scenario).
I will also introduce the UV space telescope on a 110 kg SmallSat with a moderately fast repointing capability and a real-time alert communication system that is being developed. The Quick Ultra-VIolet Kilonova surveyor (QUVIK) mission is currently at the end of the B1phase of a Czech-sponsored study under the auspices of ESA. It shall measure the brightness evolution of kilonovae, resulting from mergers of neutron stars in the UV band, and thus, it shall distinguish between different explosion scenarios. The satellite, carrying a GRB detector with a degree scale localization capability, will also greatly benefit the study of GRBs and their afterglows. The mission has been recently selected to be funded.
slides: LeCosPAseminar-JRipa-20231006
2023/10/18 Dr. Mariam Bouhmadi-López
(Ikerbasque and the University of the Baque Country, Spain)
Dark energy and some of its potential gravitational effects
 In the current talk, we present several dark energy models ranging from perfect fluid, imperfect fluid, scalar fields to 3-forms. We present observational constraints on them. For those models that show a phantom or a phantom-like behavior, i.e., some energy condition violations, it can be shown their harmlessness from a quantum point of view, i.e., the singularities or abrupt events they might induce are cured at the quantum level. In addition, some of those models can support regular blackholes as well as wormholes that do not require matter with a wrong kinetic term.
2023/11/13 Prof. Matt Visser
(Victoria University of Wellington, NZ)
 Defect wormholes are defective
“Defect wormholes” have recently attracted considerable attention — especially in view of the fact that the simplest example, the so-called “vacuum defect wormhole”, was claimed to be an everywhere-vacuum everywhere-Ricci-flat exact solution to the Einstein equations.  We take a careful look at to exactly what goes wrong. 
2023/11/20 Prof. Kuo-Chuan Pan
 Core-Collapse Supernova Explosion and its multi-messenger signals
  Core-collapse supernovae (CCSNe) are among the most explosive events in the Universe and birthplaces of neutron stars and stellar-mass black holes in extreme conditions. CCSNe are also ideal multi-messenger sources that are expected to connect
In this talk, I will present the latest results of our multi-dimensional supernova simulations with self-consistent neutrino transport. In particular, I will discuss the explosion engine and how our simulations aid future searches of neutrino and gravitational wave emissions from core-collapse supernovae.
Prof. Yuki Inoue
 Improvement of calibration uncertainty estimation for worldwide gravitational wave observation 4
The first direct detection of gravitational waves was achieved by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. LIGO, comprising two detectors located in the United States, employs laser interferometry to detect gravitational waves. The initial observation successfully detected gravitational waves emitted during the merger of two black holes. Subsequently, LIGO and Virgo have observed 90 gravitational wave events in previous observations.
In May 2023, LIGO, Virgo, and KAGRA initiated the worldwide gravitational wave observation 4. At the beginning of observation 4, LIGO achieved an averaged sensitivity of approximately 150 Mpc for the detection of binary neutron star signals. To minimize systematic uncertainty, we have contributed to the calibration and reconstruction study in this observation. This presentation aims to provide an overview of the current status of the calibration study and discuss future in LIGO.
Dr. Reggie Bernardo
(APCTP, Korea)
Pulsar timing array gravitational physics frontier
The pulsar timing arrays’ compelling observation of the stochastic gravitational wave background opens up a new observational window to constrain fundamental gravity. During this talk, we briefly review theoretical aspects that played a significant part in reaching this astronomical feat. Then, we lay down the road for potential rigorous tests of gravity that is now possible due the milestone detection of the quadrupolar timing residual correlation characteristic of gravitational waves. We compare tests utilizing inter-pulsar correlation observations and direct measurements of the correlation power spectrum, and emphasize the invaluable role of the cosmic variance for physically interpreting the results. If there’s time, we also discuss a prospect with an anisotropic polarized stochastic gravitational wave background and open up an interesting way to test the Gaussianity hypothesis.
2023/12/11 Prof. Steven Prohira
(University of Kansas, US)
 Detecting the highest energy neutrinos...with radar?
Detection of ultrahigh energy (UHE) neutrinos is key to identifying the most energetic objects and processes in the universe. These are the sources of UHE cosmic rays that have been detected at earth with energies exceeding 1 Joule per nucleon (roughly the kinetic energy of a bird in flight). As UHE cosmic messengers, neutrinos are unparalleled for their ability to travel from their sources to the Earth, interacting only weakly with matter, and therefore able to traverse great distances unimpeded. UHE neutrinos can also provide a powerful handle on physics beyond the standard model. However, their flux—the number of neutrinos arriving at Earth per unit time—decreases significantly with increasing energy. This, coupled with their disinclination for interaction, makes them very difficult to detect at the highest energies.
In this talk, I will discuss these challenges and the extensive experimental work that has been done so far to meet them. I will start with a general overview of the experimental landscape, and then focus on our new experimental effort, the Radar Echo Telescope (RET), which uses well-known radar technology to attempt detection of the cascade produced by these UHE neutrinos as they interact in polar ice. I will discuss the theory and storied history of the radar echo method, and recent experimental work including our summer 2023 deployment of a prototype detector. I will conclude with a discussion of the UHE neutrino landscape in the near future.
2023/12/18  Prof. Misao Sasaki
(YITP, Kyoto University/IPMU, Tokyo University/LeCosPA)
 Prospects in Inflation
Thanks to the rapid progress in observational cosmology, inflation has now become part of the standard model of the Universe. In this seminar, I will review the theory of inflation and discuss future prospects.


Previous Talks

click the title to expand the abstract

Date [YYYY/MM/DD] Name (Affiliation) Title and Abstract
2023/03/06 Dr. Shih-Hao Wang
(National Taiwan University)
Detecting Ultra-High Energy Particles with TAROGE-M Radio Array on Antarctic High Mountain
Detecting ultra-high energy (UHE, beyond 1017 eV) cosmic rays and neutrinos is crucial for unraveling their yet elusive origin and testing fundamental physics at energies and distances not achievable by accelerator experiments. Particularly, UHE neutrino, expected to exist but not yet detected, is the unique cosmic messenger bringing information deep in the universe and the most energetic stellar objects. UHE particles can be efficiently detected by coherent radio pulses emitted by air showers induced by their interaction with the atmosphere. One unexpected discovery of this technique is the so-called ANITA anomalous events (AAEs), having feature of upward-going UHE air showers but cannot be explained by tau neutrinos.TAROGE-M is a radio antenna station operating at 180-450 MHz atop 2.7 km-high Mt. Melbourne in Antarctica for detecting UHE air showers in near-horizontal directions. Besides the detection of cosmic rays and Earth-skimming tau neutrinos, its primary goal is to verify the origin of AAEs. The detection concept and advantages, the first result on cosmic ray detection, and the current status of TAROGE-M will be reviewed in this talk.
2023/03/13 Prof. Dong-Ham Yeom
(Pusan National University)
 Euclidean quantum gravity and information loss paradox: past and future
I first summarize recent progresses on the Euclidean path integral approach as a solution to the information loss paradox. We may extend the topics for the ultimate understanding of the information recovery of evaporating black holes. I will carefully suggest possible future research topics.
2023/03/14  11:20 Prof. Misao Sasaki
(YITP, Kyoto University/IPMU, Tokyo University/LeCosPA, National Taiwan University)
Primordial Black Holes from Inflation
The rapid progress in gravitational wave astrophysics/cosmology has made projections of detecting signatures of PBHs quite feasible. Parallel to it, various models of inflation that produce PBHs have been proposed. In this talk I first overview the PBH formation from inflation, and introduce some recently-found novel mechanisms that lead to it.
2023/03/20 Prof. Masahiro Hotta
(Tohoku University)
Expanding Edges of Quantum Hall Systems in a Cosmology Language - Hawking Radiation from de Sitter Horizon in Edge Modes
Expanding edge experiments are promising to open new physics windows of quantum Hall systems. In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to analyze the edge dynamics. We propose methods to address the following questions using edge waveforms from the expanding region: Does the conformal symmetry survive? Is the nonlinear interaction of the edge excitations induced by edge expansion? Do the edge excitations interact with the bulk excitations? We additionally show that the expanding edges can be regarded as expanding universe simulators of two-dimensional dilaton-gravity models, including the Jackiw-Teitelboim gravity model. As an application, we point out that our theoretical setup might simulate emission of analog Hawking radiation with the Gibbons-Hawking temperature from the future de Sitter horizon formed in the expanding edge region.
2023/03/27 Dr. Che-Yu Chen
(Academia Sinica -> RIKEN, Japan)
Testing black hole eikonal correspondence

Adopting geometric-optics approximations in black hole spacetimes enables the construction of a mapping between black hole images and eikonal black hole quasinormal modes (QNMs). More explicitly, the real part and imaginary part of the QNM frequencies correspond to the ring size and the detailed ring structure of the image, respectively. This correspondence may be violated when going beyond general relativity. In this talk, I'll discuss the possibility of testing this mapping using real black hole observations. We propose a novel method to test the eikonal correspondence via the comparison of two sets of observables from a nonrotating black hole, one extracted from QNM spectra and the other from the lensed photon rings on the image plane. In particular, I'll demonstrate that the photon ring observables robustly capture the information of the black hole spacetime itself regardless of the surrounding emission models. Therefore, the proposed test of eikonal correspondence can be validated in quite broad scenarios.
2023/04/10 Prof. Keiju Murata
(Nihon University)
Creating stars orbiting in AdS

We propose a method to create a star orbiting in an asymptotically AdS spacetime using the AdS/CFT correspondence. We demonstrate that by applying an appropriate source in the quantum field theory defined on a 2-sphere, the localized star gradually appears in the dual asymptotically AdS geometry. Once the star is created, the angular position can be observed from the response function. The relationship between the parameters of the created star and those of the source is studied. We show that information regarding the bulk geometry can be extracted from the observation of stellar motion in the bulk geometry.
2023/04/11 Prof. Machael Good (Nazarbayev University) Electron Acceleration Temperature

Thermal radiation from a moving point charge is found. The derivation is completely classical but has an immediate connection to quantum theory.
2023/04/17 17:00-18:00 Dr. Jason Kristiano
(RESCEU, Tokyo University)
Primordial black holes from single-field inflation?

The most widely studied formation mechanism of a primordial black hole (PBH) is collapse of large-amplitude perturbation on small scales generated in single-field inflation. In this talk, we will present the one-loop correction to the large-scale power spectrum in such a model. We find models producing appreciable amount of PBHs generically induce too large one-loop correction on large scale probed by cosmic microwave background radiation. We therefore conclude that PBH formation from single-field inflation is ruled out.
2023/04/24 Prof. Tetsuya Hashimoto
(National Chung Hsing University)
Towards uncovering the origin of fast radio bursts and future prospects in Taiwan

This talk will include an overview of recent observational research progress on fast radio bursts (FRBs) and future prospects of FRB science in Taiwan. FRBs are mysterious millisecond pulses in radio, most of which originate from distant galaxies. Despite more than 50 theoretical models of FRB progenitors, their origin is still unknown. Therefore, revealing the origin of FRBs is becoming central in astronomy and astrophysics. After presenting some basic knowledge of FRBs, I will summarize the current observational constraints on the FRB progenitors, including our approaches using the FRB number density, host galaxies, machine learning etc. I will also introduce a new FRB telescope project in Taiwan: Bustling Universe Radio Survey Telescope in Taiwan (BURSTT). Our paper predicts that BURSTT would detect more than two orders of magnitude more FRB-like events in the Milky Way compared with the other wide field-of-view FRB telescopes, STARE2 and GReX. BURSTT would also be useful to detect gravitationally lensed FRBs to constrain the primordial black hole scenario as the origin of the dark matter.
2023/05/08 Prof. I-Non Chiu
(National Cheng Kung University)
First Cosmology from galaxy clusters observed by eROSITA

We present the first cosmological constraints using the cluster abundance of a sample of eROSITA clusters, which were identified in the eROSITA Final Equatorial Depth Survey (eFEDS). In a joint selection on X-ray and optical observables, the sample contains 455 clusters within a redshift range of 0.1 < z < 1.2, of which 177 systems are covered by the public data from the Hyper Suprime-Cam (HSC) survey that enables a uniform weak-lensing mass calibration. In a framework of empirical modelling and blind analysis, we simultaneously model the cosmology, the X-ray selection, and the observable-to-mass-and-redshift relations with the observables including the X-ray count rate, the optical richness, and the weak-lensing mass. As a result, we deliver cosmological constraints that are in excellent agreement (at a level of < 1 sigma) with the results from the Planck mission, the galaxy-galaxy lensing and clustering analysis of the Dark Energy Survey, and the cluster abundance analysis of the SPT-SZ survey. With the empirical modelling, this work presents the first fully self-consistent cosmological constraints based on a synergy between wide-field X-ray and weak lensing surveys.
2023/05/15 Dr. Karan Fernandes
(National Taiwan Normal University)
Nonlinear memory effect from a subleading eikonal dressing
The early inspiral phase in binary mergers can be derived using gravitationally interacting point particles considered within the eikonal approximation to scattering amplitudes. In particular, at third and higher post-Minkowski orders, the scattering process is inelastic and leads to an imaginary contribution to the eikonal phase. The corresponding eikonal operator describes a coherent state constructed from the leading Weinberg soft factor. These states have been recently used to derive gravitational wave observables in the finite and soft (vanishing frequency) limits of the emitted gravitational radiation. In this talk, I will discuss how next-to-eikonal corrections can be used to derive a generalized coherent state. We discuss key properties of this operator and use it to derive the nonlinear memory effect in the soft limit. The talk is based on an ongoing project with Prof. Feng-Li Lin (NTNU).
2023/05/22 Prof. Naritaka Oshita
(YITP, Japan)

Overtones, Greybody Factors and Thermal Ringdown of Black Holes

A gravitational wave from a binary black hole merger is an important probe to test gravity. Especially, the observation of ringdown may allow us to perform a robust test of gravity as it is a superposition of excited quasi-normal (QN) modes of a Kerr black hole. The excitation factor is an important quantity that quantifies the excitability of QN modes and is independent of the initial data of black hole mergers. Also, the reflectivity of the black hole geometry, quantified by the greybody factor, could be imprinted on the ringdown spectrum. The greybody factor is another no-hair quantity of a black hole and is useful to test gravity in strong gravity regimes.In this talk, I will discuss the modeling of ringdown with QN modes and with the greybody factors. I will show that the 5th overtone has the highest excitability of ringdown (i.e., has the highest value of the excitation factor) among the tones for l=m=2 mode with the spin parameters of ~ 0.7. Also, I will show that the greybody factor for the near extremal cases is very similar to the Fermi-Dirac distribution of the Hawking frequency. By virtue of the mysterious nature of spinning black holes, the ringdown spectrum of a small mass ratio merger involving a massive and rapidly spinning black hole can be modeled by the thermal Fermi-Dirac distribution characterized by the black hole surface gravity.
2023/05/29 Dr. Junsup Shim
Identifying Cosmic Voids as Massive Cluster Counterparts

We develop a method to identify cosmic voids from matter/galaxy density fields by adopting a physically-motivated void-cluster correspondence theory that voids are the counterpart of massive clusters. To prove the concept we use a pair of LCDM simulations, a reference and its initial density-inverted mirror simulation, and study the relation between the effective size of voids and the mass of corresponding clusters. We study the voids corresponding to the halos more massive than 10^{13}Msol/h and find a power-law scaling relation between the void size and the corresponding cluster mass. We also find that the density profile of the identified voids follows a universal functional form. Based on these findings, we propose a method to identify cluster-counterpart voids directly from the matter/galaxy density field without their mirror information. The completeness and reliability we achieve in recovering voids corresponding to clusters more massive than 3x10^{14}Msol/h are about 70-74 % for matter density fields and 60-67% for galaxy density fields. Our results demonstrate that we can apply this method to the galaxy redshift survey data to identify cosmic voids corresponding statistically to the galaxy clusters in a given mass range.
2023/06/01 Prof. Jun'ichi Yokoyama (RESCEU, Tokyo University) Generation of neutrino dark matter, baryon asymmetry and radiation after quintessential inflation

We construct a model explaining dark matter, baryon asymmetry and reheating in quintessential inflation model. Three generations of right-handed neutrinos having hierarchical masses. Three generations of right handed neutrino explain dark matter and baryon asymmetry, while reheating is realized by spinodal instabilities of the Standard Model Higgs field induced by the non-minimal coupling to the scalar curvature, which can solve overproduction of gravitons and curvature perturbation created by the Higgs condensation.
2023/08/02 Prof. Jeremie Szeftel (Sorbonne Université) Nonlinear stability of Kerr for small angular momentum

I will introduce the celebrated black hole stability conjecture according to which the Kerr family of metrics are stable as solutions to the Einstein vacuum equations of general relativity. I will then discuss the history of this problem, including a recent work on the resolution of the black hole stability conjecture for small angular momentum.
2023/08/03 Prof. Jeremie Szeftel (Sorbonne Université) Nonlinear stability of Kerr for small angular momentum

In the first hour, I will introduce the Einstein equations and the corresponding evolution problem. I will then review some of the technics used in the stability of Minkowski. I will end the first hour by presenting Kerr black holes and the Kerr stability conjecture. In the second hour, I will recall the history of the Kerr stability conjecture. I will then focus on a recent work on the resolution of the black hole stability conjecture for small angular momentum.
Date  Name (Affiliation) Title and Abstract

Dr. Shu-Yu Ho
(Korea Institute for Advanced Study, Korea)

 Asymmetric SIMP Dark Matter

In this talk, I will show for the first time an asymmetric strongly interacting massive particles (SIMP) dark matter (DM) model, where a new vector-like fermion and a new complex scalar both having nonzero chemical potentials can be asymmetric DM particles. After the spontaneous breaking of a U(1)_D dark gauge symmetry, these two particles can have accidental Z_4 charges making them stable. By adding one more complex scalar as a mediator between the SIMP DM, the relic density of DM is determined by 3 to 2 and two-loop induced 2 to 2 annihilations in this model. On the other hand, the SIMP DM can maintain kinetic equilibrium with the thermal bath until the DM freeze-out temperature via the new gauge interaction. Interestingly, this model can have a bouncing effect on DM, whereby the DM number density rises after the chemical freeze-out of DM. With this effect, the prediction of the DM self-interacting cross section in this model can be consistent with astrophysical observations, and the ratio of the DM energy density to the baryonic matter energy density can be explained by primordial asymmetries. We also predict the DM-electron elastic scattering cross section that can be used to test this model in future projected experiments.




Prof. Po-Feng Wu
(National Taiwan University, Taiwan)

  Extragalactic archeology: deciphering the evolution of galaxies with their fossil records


One of the fundamental constraints on studying galaxy evolution is that we are not able to monitor individual galaxies throughout their lifetime to track the evolution. The stars in galaxies provide fossil records on the build-up processes of galaxies.

I will demonstrate that with current observing facilities, we are able to track the evolution of individual galaxies in the distant Universe from their stellar populations. The archeological method can date the ages of galaxies, pinpoint where they are in their evolutionary process, and identify galaxies in the key phase of galaxy evolution for follow-up studies. The high precision allows quantitative comparison to cosmological hydro-dynamical simulations and validate current models of galaxy formation.

With the coming survey projects and observing facilities, the archeological method will be able to track the evolution of galaxies in even earlier Universe, and with better statistics and precision. 



Dr. Jen-Tsung Hsiang
(National Central University, Taiwan)

  Can the Universe get decohered all by itself?


One of the common beliefs in cosmology is that cosmic expansion (or more dramatically inflation) is all we need to make the quantum to classical transition of the cosmological perturbations. We try to debunk this myth.




Dr. Jakub Ripa
(Faculty of Science, Department of Theoretical Physics and Astrophysics, Masaryk University)

  Early results from GRBAlpha and VZLUSAT-2 CubeSats with gamma-ray burst detectors


Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. Some of them, the short GRBs, are important sources of gravitational waves originating in mergers of neutron stars or possibly also in mergers of neutron stars with black holes. I will present the detector performance and early science results from GRBAlpha, a 1U CubeSat mission, which is a technological pathfinder to a future constellation of nanosatellites monitoring and localazing GRBs. The localization can be achieved by measuring the time difference between the arrival of the signal at different satellites (synchronized by GPS). GRBAlpha was launched in March 2021 and has been operating already about a year on a 550 km altitude sun-synchronous orbit. The onboard gamma-ray burst detector consists of a 75×75×5mm CsI(Tl) scintillator, read out by a dual-channel multi-pixel (SiPM) photon counter (MPPC) setup. It is sensitive in the ~30-900 keV range. The main goal of GRBAlpha is the in-orbit demonstration of the detector concept, verification of the detector's lifetime, and measurement of the background level on low-Earth orbit, including polar regions and in the South Atlantic Anomaly. GRBAlpha has already detected five GRBs and was even able to detect two GRBs within 8 hours, proving that nanosatellites can be used for routine detection of gamma-ray transients. For one GRB, we were able to obtain a high resolution spectrum and compare it with measurements from the Swift satellite. We find that, due to the variable background, about half of the low-Earth polar orbit is suitable for gamma-ray burst detection. One year after launch, the detector performance is good and the degradation of the SiPM photon counters remains at an acceptable level. The same detector system, but double in size, was launched in January 2022 on VZLUSAT-2 (3U CubeSat). The GRB detectors perform as expected and I will present the early measurements from this mission as well. Our ultimate aim is to develop and launch a constellation of nanosatellites monitoring GRBs and these precursor missions help us to converge to this ambitious goal. 




Prof. Vitor Cardoso
(Center for Astrophysics and Gravitation, Instituto Superior Técnico, Portugal

Niels Bohr Institute, Denmark)

  Testing GR with GWs


One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes, leaving behind a geometry with light rings, ergoregions and horizons. These peculiarities are responsible for uniqueness properties and energy extraction mechanisms that turn black holes into ideal laboratories of strong gravity, of particle physics (yes!) and of possible quantum-gravity effects. I will review the status of black holes, in light of gravitational-wave and electromagnetic observations in the last few years. 


2022/01/24 16:00

Prof. Eiichiro Komatsu
(Max-Planck-Institut für Astrophysik, Germany)

 Hunting for Parity-violating Physics in Polarisation of the Cosmic Microwave Background


Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics. In this presentation we report on a measurement of parity violation from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy. We also report on a new measurement from the latest Planck public data release 4.  


Date  Name (Affiliation) Title and Abstract

Prof. Masahiro Hotta
(Tohoku University, Japan)

Generalized partners in quantum field theory
Black hole evaporation causes the information loss problem. Nobody knows what  carries initial quantum information after the last burst of black holes even if unitarity is preserved. After the evaporation, the density of Hawking radiation decreases in time. In the regime, quantum gravity and matter interaction may be neglected. If so, low-energy free quantum fields carry whole of the initial quantum information under the unitary evolution. In this perspective, it is quite cruicial to explore what kinds of information storage are possible in free quantum field theory.  In this talk, it will be shown that new information storages like generalized partners and quantum information capsules are able to share complete quantum information. I will also provide application of the generalized partners to expanding universes and moving mirror models. 

Prof. Daniel Baumann
(University of Amsterdam, Nederland)

Cosmological Collider Physics
Inflation is likely to have occurred at an energy far exceeding the highest energies accessible in terrestrial experiments. Very massive particles can then be produced by quantum fluctuations. The decay of these particles creates distinct higher-order correlations in the density after inflation which can be looked for in future large-scale structure observations. In this talk, I will describe a new approach to compute these effects called the cosmological bootstrap.  I will describe the conceptual and practical advantages of the bootstrap method and explain how it is connected to the S-matrix bootstrap in particle physics. 

Dr. Che-Yu Chen
(Institute of Physics, Academia Sinica, Taiwan)

Black Hole Quasinormal Modes in General Relativity and Beyond
Quasinormal modes (QNMs) of perturbed black holes have recently gained much interest because of their tight relations with the gravitational wave signals emitted during the post-merger phase of a binary black hole coalescence. One of the intriguing features of these modes is that they respect the no-hair theorem, and hence, they can be used to test black hole spacetimes and the underlying gravitational theory. In this talk, I will exhibit three different aspects of how black hole QNMs could be altered in theories beyond Einstein’s general relativity (GR). These aspects are (i) the direct alterations of QNM spectra as compared with those in GR, (ii) the violation of the geometric correspondence between the high-frequency QNMs and the photon geodesics around the black hole, and (iii) the breaking of the isospectrality between the axial and polar gravitational perturbations. Several examples will be provided in each individual case. A theory-agnostic approach to address this issue will also be discussed. 

Prof. Yuan-Hann Chang
(Institute of Physics, Academia Sinica and National Central University, Taiwan)

Update of Dark Matter search with the AMS experiment and the TASEH project
Understanding the nature of the Dark Matter is one of the most important issues in physics. It is also a very challenging task because we have so far not found any clue of its composition experimentally. Particle-natured cold Dark matter is currently the prevailing theory. However, there is no information about the masses and other properties of these hypothetic particles. The spectrum of possibilities is wide, and many search schemes, with very different technologies, are being conducted. We have been involved in the search for Dark matter in two projects. In the AMS experiment, we look for anomalous excess of antiparticles in the cosmic rays, which may come from the annihilation of Dark matter particles in the interstellar space. The most recent result of AMS do indicate a significant excess of positrons and antiprotons at a few hundred GeV/c2 energy. In parallel, we initiated a local haloscope experiment, TASEH, to search for eV/c2 Axion Dark matter. TASEH is preparing for its first data taking now. In the presentation, I will update the latest status of Dark matter search with the AMS experiment and the TASEH experiment, and discuss the prospects of these experiments in the near future. 

Prof. Feng-Li Lin
(National Taiwan Normal University, Taiwan)

Weak Cosmic Censorship for Higher Derivative Gravity Theories
In this talk I will discuss how the weak cosmic censorship conjecture (WCCC) constrains the higher derivative gravity theories based on Wald's gedanken experiment by throwing the matter into the extremal or near-extremal black holes. We find that even though the WCCC still holds for the extremal black hole for the extremal black holes of quartic gravity theories, it can be violated for the near-extremal cases. 
2021/11/29 10:00

Dr. Geoff Chih-Fan Chen 
(University of California, Los Angeles, United States)

H0 measurement from time-delay cosmography
The Hubble constant is one of the most important parameters in cosmology. Its value directly sets the age, the size, and the critical density of the Universe. Despite the success of the flat LCDM model, the derived Hubble constant from Planck data under the assumption of a flat LCDM model has 4.4-sigma tension with the direct measurements. If this tension is not due to the systematics, it may indicate the new physics beyond the standard cosmological model. H_0 from time-delay lensing is a powerful independent tool for addressing the H_0 tension since it is independent of both Planck and the distance ladder. One way to do this is to increase the number of high-quality lens systems since this allows us to look for correlations and other effects due to systematics, and to do hierarchical approaches to assess known systematic effects. Keck AO data is not only the key component to increase the precision of H0 measurement but also provides systematic checks with the H_0 results based on HST imaging. In this talk, I will present the current H_0 measurement, the systematic checks, and the future prospective of H0LiCOW/TDCOSMO collaboration. 

Prof. Yen Chin Ong
(Yangzhou University, China)

Musings on Springy Black Holes
I will discuss a suitably defined "Hookean force law" F=kx for a rotating black hole, which seems to tie in with thermodynamical properties such as Emparan-Myers fragmentation in higher dimensions, and the maximum force conjecture in general relativity. I also discuss possible connections to the microstructures of black holes via the Ruppeiner approach of thermodynamic geometry.
2021/12/13 17:15

Prof. Hans Kristian Kamfjord Eriksen
(Institute of Theoretical Astrophysics, University of Oslo, Norway)

BeyondPlanck: Optimal end-to-end Bayesian analysis of Planck LFI
Observations of the cosmic microwave background (CMB) have played a critical role in establishing the current cosmological concordance model. Next generation CMB experiments promise to go even further, and image the very birth of the universe by observing primordial gravitational waves created during the Big Bang. However, observing this signal will be a tremendous challenge, as the expected signal can easy be contaminated by systematic uncertainties both from confusing radiation from the Milky Way and from instrumental imperfections. In this talk, I will describe the BeyondPlanck project, which developed the world's first end-to-end Bayesian CMB analysis code, accounting for both instrumental and astrophysical uncertainties, and applied this to the Planck LFI data set. I will argue that this approach not only sets a new standard for CMB analysis, but also that the main ideas are applicable to any experiment for which systematic error propagation is the main limiting factor.

2020 Spring: cancelled due to CoVID-19 pandemic

Date  Name (Affiliation) Title and Abstract
2020/10/19 Dr. Shubham Maheshwari
(University of Groningen, Netherland)
  Stable, ghost-free solutions in UV non-local gravity
I consider higher derivative, UV modifications to GR. In particular, I will focus on a specific kind of string theory-inspired higher derivative gravity where one includes derivatives to all orders in the action. First, I will discuss how such a non-local theory of gravity admits stable, non-singular bouncing solutions in the absence of matter. Moreover, around this bouncing background, there exists only one propagating (and ghost-free) scalar mode, and no vector or tensor modes. Next, I will discuss the general analysis of scalar-vector-tensor perturbations in non-local gravity - in particular, I will show how non-local gravity is ghost-free around (A)dS and certain non-maximally symmetric backgrounds, and how certain (A)dS backgrounds have special physical spectra in that the propagating degrees of freedom are different from usual expectations. 
2020/11/30 Prof. Benjamin L'Huillier
(Yonsei University, Korea)
Constraining Cosmology with the Large-Scale Structures
  Despite great predictive power and its successes in the last decades, the concordance LCDM cosmological model suffers from both observational (H0 tension, ....) and theoretical issues (nature of dark energy, dark matter, inflation,…). Therefore, it is important to further test the model and its underlying hypotheses. In this talk, I will discuss how the study of the large scale structures can help shed light on some fundamental questions such as the nature of dark energy, gravity, or the early Universe, in the context of a new generation of survey such as Euclid, DESI, or LSST. I will focus on two different aspects: (i) modeling the nonlinear regime of structure formation through N-body simulations, in particular beyond LCDM, and (ii) applying advanced statistics, in particular model-independent methods, to state-of-the-art cosmological data to test different aspects of the concordance such as the metric, gravity, or the nature of dark energy.  
2020/12/07 Dr. Yen-Yung Chang
(Caltech/ NASA JPL)
  Quantum information science for next generation dark matter search
Dark matter (DM) is one of the few outstanding questions in fundamental physics that are well-defined without theoretical aesthetics. Unambiguous observations in direct detection experiments, or an efficient parameter space removal, are the keys to theoretical development as well as motivating future large-scale experiments. I will begin by discussing the rising interests in sub-GeV DM models, which are well motivated but relatively underexplored due to previous technological limitations. I will then use our latest results in SuperCDMS beyond-SNOLAB R&D to explain the foreseeable challenges to an ultimate experiment for probing the entire particle DM mass spectrum. Motivated by these challenges, as well as developments in cutting-edge quantum sensing techniques, I will introduce our work in applying superconducting micro-resonators (Kinetic Inductance Detector) as an promising approach for future large-scale ultra-low-threshold experiments. I will also share the ongoing R&D for quantum-noise-limited parametric amplifiers at JPL, cryogenic radiation control for quantum devices, and target materials beyond semiconductors. I will conclude the talk with the implications, future prospects, and our plan toward probing the complete particle DM parameter space with quantum sensing technology.
2020/12/21 Dr. Hsu-Wen Chiang
(LeCosPA, NTU)
  Modification to the Hawking temperature of a dynamical black hole by a flow-induced supertranslation, or could microsoft save the day?
One interesting proposal to solve the black hole information loss paradox without modifying either general relativity or quantum field theory, is the soft hair, a diffeomorphism charge that records the anisotropic radiation in the asymptotic region. This proposal, however, has been challenged, given that away from the source the soft hair behaves as a coordinate transformation that forms an Abelian group, thus unable to store any information. To maintain the spirit of the soft hair but circumvent these obstacles, we consider Hawking radiation as a probe sensitive to the entire history of the black hole evaporation, where the soft hairs on the horizon are induced by the absorption of a null anisotropic flow, generalizing the shock wave considered by Hawking, Perry and Strominger. To do so we introduce two different time-dependent extensions of the diffeomorphism associated with the soft hair, where one is the backreaction of the anisotropic null flow, and the other is a coordinate transformation that produces the Unruh effect and a Doppler shift to the Hawking spectrum. Together, they form an exact BMS charge generator on the entire manifold (a realization of the interpolation between the BMS group on the null asymptote to the BMS group on the horizon) that allows the nonperturbative analysis of the black hole horizon, whose surface gravity, i.e. the Hawking temperature, is found to be modified. The modification depends on an exponential average of the anisotropy of the null flow with a decay rate of 4M, suggesting the emergence of a new 2-D degree of freedom on the horizon, which could be a way out of the information loss paradox.
2020/12/28 Dr. Hsu-Wen Chiang
(LeCosPA, NTU)
  Modification to the Hawking temperature of a dynamical black hole by a flow-induced supertranslation, or could microsoft save the day? Part 2
One interesting proposal to solve the black hole information loss paradox without modifying either general relativity or quantum field theory, is the soft hair, a diffeomorphism charge that records the anisotropic radiation in the asymptotic region. This proposal, however, has been challenged, given that away from the source the soft hair behaves as a coordinate transformation that forms an Abelian group, thus unable to store any information. To maintain the spirit of the soft hair but circumvent these obstacles, we consider Hawking radiation as a probe sensitive to the entire history of the black hole evaporation, where the soft hairs on the horizon are induced by the absorption of a null anisotropic flow, generalizing the shock wave considered by Hawking, Perry and Strominger. To do so we introduce two different time-dependent extensions of the diffeomorphism associated with the soft hair, where one is the backreaction of the anisotropic null flow, and the other is a coordinate transformation that produces the Unruh effect and a Doppler shift to the Hawking spectrum. Together, they form an exact BMS charge generator on the entire manifold (a realization of the interpolation between the BMS group on the null asymptote to the BMS group on the horizon) that allows the nonperturbative analysis of the black hole horizon, whose surface gravity, i.e. the Hawking temperature, is found to be modified. The modification depends on an exponential average of the anisotropy of the null flow with a decay rate of 4M, suggesting the emergence of a new 2-D degree of freedom on the horizon, which could be a way out of the information loss paradox.
Date Name (Affiliation) Title and Abstract
2019/01/07 Dr. Akira Matsumura
(Nagoya University, Japan)
 Detectability of Bell-CHSH nonlocality by two spin detectors with optimal local filters
The detection problem of quantum non-local correlation by two spin detectors is investigated. First we review the detection model of quantum entanglement of the Minokowski vacuum. It is shown that the entanglement between two spacelike points for some initial conditions of detectors is observed. Next we consider the detection of Bell nonlocaliy which is known as an important quantum correlation. Unlike the detection of entanglement, we need a local filtering operation to observe the Bell nonlocality. The local filtering operation is a probabilistic process performed by local observers and hence the detection of Bell nonlocality is probabilistic. Through the analysis of the Bell nonlocality detection, we find the relation between the detection probability and the detectable region of Bell nonlocality.
2019/01/21 Dr. Yen-Yung Chang
SuperCDMS SNOLAB and Future Low-Mass Dark Matter Searches
In recent years, development in dark matter (DM) phenomenology below weak/WIMP scale has been booming. I will begin by discussing the rich phenomenology and rising interest in low-mass DM candidates, e.g. dark photon. I will then introduce SuperCDMS SNOLAB, one of the leading next generation experiment for DM direct detection. It will search for DM mass in 0.5-10 GeV range with a projected sensitivity down to 10^-43 cm^2 DM-nucleon scattering cross section. It utilizes cryogenic Ge/Si crystal with superconducting Transition Edge Sensor (TES) for phonon-mediated recoil detection, and it will begin data taking in 2020. The second half of the talk will be focused on the ongoing R&D and proposed concepts beyond SuperCDMS SNOLAB. I will introduce Kinetic Inductance Detector (KID) as a promising alternative to TES for future larger scale, high resolution, MeV-scale DM searches. Finally, I will briefly discuss recent proposals for new cryogenic target materials and conclude the talk with known/potential limitations and other future prospects toward the complete search for low-mass DM.  
 2019/02/26 Dr. Ippei Obata
(ICRR, Tokyo University, Japan)
 Axion Dark Matter Search with Optical Cavity Experiment
The outline of this talk is mainly based on our recent paper 10.1103/PhysRevLett.121.161301. In this talk, I will present a novel experimental approach to search for axion dark matter which does not need a conventional strong magnetic field but use an optical cavity. This new method aims to measure the difference of phase velocity between two circular-polarized photons which is caused by the coupling to axion dark matter. The experimental sensitivity is in principle only limited by quantum noise which enables us to probe tiny axion-photon coupling g_{aγ} <~ 10^{−11}GeV^{-1} with axion mass range m_a <~ 10^{-10}eV, which is competitive with other experimental proposals.
 2019/03/07 Prof. Pisin Chen
(LeCosPA, NTU)
 Tabletop analog black holes to investigate the information loss paradox
 2019/03/14 Dr. Aniket Agrawal
 Tensor Non-Gaussianity from Axion-Gauge-Fields Dynamics
I will talk about an inflation model in which a spectator axion field is coupled to an SU(2) gauge field and produces a large three-point function (bispectrum) of primordial gravitational waves, on the scales relevant to the cosmic microwave background experiments. The amplitude of the bispectrum at the equilateral configuration is characterized by $B_{h}/P_h^2=mathcal{O}(10)times Omega_A^{-1}$, where $Omega_A$ is a fraction of the energy density in the gauge field and $P_h$ is the power spectrum of gravitational waves produced by the gauge field. These gravitational waves are chiral, and can have a scale-independent power spectrum over a large range of wavenumbers, necessitating the measurement of TB and EB cross correlations along with measurement of non-Gaussianity to claim quantum fluctuations of the metric.
 2019/03/21 Dr. Shohei Aoyama
 Dust Hrains in Cosmological simulations
The distribution of dark matter is one of the important issues of cosmology. However, because dark matter cannot be observed directly by electromagnetic observatory, galaxies are used to trace the dark matter distribution. Hence the optical property of galaxies is important for cosmology. In galaxies, dust grains play an important role in the star formation and the modification of intrinsic spectral energy distribution. For former, dust grains are an efficient catalyst of molecular hydrogen formation. For latter, dust grains absorb ultraviolet photons, which originates young and massive stars, and reemit the infrared light. Because the absorption efficiency highly depends on the grain size distribution and spatial distribution of dust grains, we have to predict both of them theoretically. In order to realize the prediction, hydro-dynamical simulations are a powerful tool. In order to complete the simulation within reasonable timescale, we implemented two-size model (Hirashira 2015) into GADGET3-Osaka code. By considering the size distribution, we have been able to treat grain-grain interactions such as coagulation and shattering, which are growth and destruction process of dust grains, respectively. We obtained grain size distribution at every spatial point and radial distribution of dust grain around massive galaxies, which is comparable to that in observation. In addition to the spatial distribution of dust grains, we calculated the radiation transfer by Mie theory, we obtained the luminosity function of infrared luminous galaxies. At z=0, we roughly realized observed luminosity function taken by Herschel satellite.
 2019/03/28 Dr. Yen-Wei Liu
(NCU, Taiwan)
 Acausality in Transverse-Traceless Spin-2 Gravitational Waves
The transverse-traceless (TT) portion of the metric perturbations has been widely used to study the physical observables in gravitational wave physics. However, through an explicit calculation of the relevant Green's functions, we show that, in both Minkowski background and spatially-flat cosmologies with constant equation-of-state, the TT gravitational waves receive contributions from the source(s) outside the null cone of the observer, which therefore prevents the TT perturbation from being a standalone observable. In this talk, I will demonstrate this acausal character and discuss how the other gauge-invariant variables must be incorporated to ensure strictly causal gravitational tidal forces, and, being intimately related, the violation of micro-causality for the quantum operators associated with free massless spin-1 and spin-2 states will also be pointed out.
 2019/04/11 Prof. Kai-Feng Chen
 Explore the universe with big Data at LHC
 2019/04/25 Prof. Laurent Baulieu
(CNRS and UNESCO. LPTHE Sorbonne Université Paris)
 On the stochastic quantisation of quantum gravity, and its physical predictions for the early cosmology
 2019/05/02 Prof. Rampei Kimura
(Waseda University, Japan)
 Are redshift-space distortions actually a probe of growth of structure?
Although multiple cosmological observations indicate the existence of dark matter and dark energy, cosmological tests of interactions between them have not yet been established. We point out that, in the presence of a coupling between dark matter and dark energy, a peculiar velocity of total matter field is determined not only by a logarithmic time-derivative of its density perturbation but also by density perturbations for both dark matter and baryonic matter, leading to a large modification of the physical interpretation of observed data obtained by measurements of redshift-space distortions. We reformulate a galaxy two-point correlation function in the redshift space based on the modified continuity and Euler equations. We conclude from the resultant formula that redshift space distortions provide us information on the coupling between dark matter and the scalar field by combining weak lensing measurements. We will also discuss future prospects of constraining specific models using future surveys. 
 2019/05/06 Prof. Yi Yang (NCKU, Taiwan)  The AMS experiment and it's latest results
Alpha Magnetic Spectrometer (AMS-02), led by the Nobel Prize winner Professor Samuel C. C. Ting, is an international collaborative research project in experimental high energy physics. AMS-02 is a general purpose particle detector mounted on the International Space Station (ISS) since May 19 2011, and is expected to operate with the ISS in taking high energy cosmic ray data. The main purpose of the AMS-02 experiment is to collect and to measure the relevant high energy particles originating from deep space, such as electrons, positrons, protons, anti-protons, and so on. To this end, we might be able to explain some of the most important mysteries of Universe, namely, the asymmetry in the ratio of matter and anti-matter and the origin of dark matter. I will introduce the AMS-02 experiment and present it's latest results. 
 2019/05/27 Prof. Daniel R. Terno  (Macquarie University, Australia) A self-consistent analysis of black hole horizons
Conceptual problems caused by quantum effects, including the information loss paradox, possible role of quantum gravity, and uncertainty regarding the end result of the gravitational collapse have been motivating investigations of exotic compact objects that are the hypothetical products of collapse that does not lead to formation of an event horizon and/or singularity. Recent developments make these studies observationally relevant. Investigation of exotic models and their observable features requires understanding of the benchmark that is based on the minimal number of assumptions. Taking existence of a trapped region as the defining property of a black hole we assume its formation at finite time according to a distance observer. In addition, we require regularity of the apparent horizon as expressed by finite values of the curvature scalars. A self-consistent approach within the framework of semiclassical gravity that is applied to a spherically-symmetric collapse allows to obtain the limiting form of the total energy-momentum tensor and metric in the vicinity of the apparent horizon. We explore the consequences of these results for possibility of horizon avoidance, existence of firewalls and stability of black holes.
2019/10/07 Prof. Mariam Bouhmadi-López (University of Basque Country, Spain) Cosmological constraints of phantom dark energy models
On this talk, we will address three genuine phantom dark energy models where each of them induces the particular future events known as Big Rip, Little Rip and Little Sibling of the Big Rip. The background models are fully determined by a given dark energy equation of state. We first observationally constrain the corresponding model parameters that characterise each paradigm using the available data of supernova type Ia, Cosmic Microwave Background and Baryonic Acoustic Oscillations by using a Markov Chain Monte Carlo method. The obtained fits are used to solve numerically the first order cosmological perturbations. We compute the evolution of the density contrast of (dark) matter and DE, from the radiation dominated era till a totally DE dominated universe. Then, the obtained results are compared with respect to ΛCDM. We obtain the predicted current matter power spectrum and the evolution of fσ8 given by the models studied in this work. Finally, the models are tested by computing the reduced χ2 for the `Gold2017' fσ8 dataset. We will as well address some theoretical issues inherent to the phantom dark energy scenario on its largest scope.
2019/10/18 14:20 Prof. Kazunori Kohri (KEK/Sokendai/Kavli IPMU, Japan)   Axion-Like Particles and Recent Observations of the Cosmic Infrared Background Radiation and the GeV-TeV Gamma-rays
The CIBER collaboration released their first observational data of the Cosmic IR background (CIB) radiation, which has significant excesses at around the wavelength ∼ 1 μm compared to theoretically inferred values. The amount of the CIB radiation has a significant influence on the opaqueness of the Universe for TeV gamma-rays emitted from distant sources such as AGNs. With the value of CIB radiation reported by the CIBER experiment, through the reaction of such TeV gamma-rays with the CIB photons, the TeV gamma-rays should be significantly absorbed during propagation, which would lead to energy spectra in disagreement with current observations of TeV gamma ray sources. In this talk, I discuss a possible resolution of this tension between the TeV gamma-ray observations and the CIB data in terms of axion [or Axion-Like Particles (ALPs)] that can increase the transparency of the Universe by the anomaly-induced photon-axion mixing. We find a region in the parameter space of the axion mass, m_a ∼ 7.E−10 eV − 5.E−8 eV, and the axion-photon coupling constant, 1.2E−11 GeV^−1 < g_aγγ < 8.8E−10 GeV^−1 that solves this problem.
2019/10/21 Prof. Yu-Ting Huang (NTU)   Kerr black holes/ Taub-NUT geometry from on-shell S-matrix of higher spin particles/dyons
The no-hair theorem states that black holes are completely characterized by its mass, charge and spin. The same set of quantum numbers also characterize elementary particles. In this talk, we give the precise statement of this correspondence in terms of on-shell observables. In particular, we demonstrate that the classical potential, as well as the scattering angle of rotating blackholes is given by that of minimally coupled higher spin particles. Similarly the solution for Taub-Nut space time is mapped to dyons. This map also teaches us interesting properties of these classical solutions. In particular, the Kerr and Taub-Nut solution is simply a complex shift of Schwarzschild black holes. 
2019/12/16 Dr. Naoki Watamura (Shanghai University, China)   On the time development of Entanglement Entropy in massive scalar field theory
We discuss the time evolution of (R’enyi) Entanglement Entropy ((R)EE) in 4 dimensional flat spacetime, for free massive scalar field theory. We divide the space into two subspaces, i.e. the positive and negative part of the $x^1$ axis, respectively, and study the REE of an excited state which is defined by acting with a spacetime local operator on the vacuum state. The insertion of this operator causes a change in REE, and by using the replica method, we are able to follow the time evolution of this (R)EE. We found that the propagation of REE respects the causality, and also in a special configuration, special relativity.

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