Forum Schedule Spring 2023

Fridays 3:45pm - 4:45pm BPB-217

Date Speaker Topic (click down-arrow to see abstract)
Jan 17 Tuesday BinBin Zhang
Nanjing University
host: Bing Zhang
Gamma-ray Bursts with Peculiar Origins

I will present our recent research on GRBs that deviate from standard GRB classifications or progenitor models. Those GRBs include the discoveries of a peculiar GRB from a magnetar giant flare, a genuinely short GRB not originating from a compact star merger, and a genuinely long GRB from a compact binary star merger. Those novel phenomena suggest that our understanding of GRBs is still far from complete, and a thorough investigation of those special events is needed to comprehend the diverse mechanisms of GRBs.

Jan 20

Jan 27

Feb 3 Len Zane
host: Tao Pang
Do Moving Rockets REALLY Shrink?

Special Relativity (SR) was developed in 1905 by Albert Einstein as a kinematic theory that compared measurements made in two inertial reference frames moving with respect to one another. In the usual presentation of SR, local observers note the time and position of events that happen in their immediate vicinity. Those individual observations are sent to a Command Center. The people in the Command Center collate and analyze the data to construct an overall picture of what is happening in spacetime. Their analysis implies that moving meter sticks shrink and moving clocks run slow. This talk aims to test the validity of those conclusions.

Webex meeting link:
meeting number:
2623 639 2807
Feb 10

Feb 17
William P. Comaskey
Florida State University
host: Tao Pang
The Genesis of a Comprehensive Catalogue of Topological 2D Materials and Hetero-structures

Topological materials have emerged as a promising area of condensed matter physics, because they allow for the realization of many exotic phases. One of these areas is for their use in quantum computing, due to their electronic properties being resistant to temperature fluctuations, and other external physical stimuli. Topological materials have many promising applications such as in spin- tronic devices, in nearly dissipation-less electronic devices by means of the quantum hall effect and the quantum anomalous hall effect and in free space optical communication. However, since their experimental discovery in 2007, there has been an increasing number of calculated and experimentally observed topological materials as the field has boomed in popularity and the mechanisms which cause them to occur have become better understood.

Here, we are developing a general approach to predict new topological materials while trying to avoid the previous shortcomings by combining quantum mechanics, big data, and machine learning. We first approach this problem from the perspective of big data by creating a large database of 2D- materials to identify topological materials which will enable us to scale machine learning methods as the database takes shape. This database will grow from thousands of structures from our 2D calculations to potentially many millions of structures upon the combination of materials into multi- layer hetero-structures allowing for a future scalable machine learning framework to be developed.

Condensed matter physics has made significant use of theoretical quantum calculations to further understand topological insulators, critical materials, quantum architectures, and other exotic materials. We have applied new methods for the analysis of topological systems which accurately treat the physics necessary for describing topological materials. This ab initio formulation of a two-component spin- current density functional theory (SCDFT) will be used to model topological features such as Weyl nodes, Dirac nodes and gapped Dirac-like nodes characteristic of quantum spin Hall insulators[1]. Here, we will investigate this field of topological materials through the lens of mono-layer or 2D materials and finally the combination of these materials to form novel hetero-structures with potentially tunable properties.

Feb 21 Tuesday Salvatore Vitale
host: Carl Haster
Astrophysics with gravitational-wave sources: today and tomorrow.

In the first 7 years of operation, ground-based gravitational-wave detectors LIGO and Virgo have detected nearly 100 compact binaries, most of them black hole binaries. In this talk I will describe what can be learned about the formation channels of black holes using the gravitational waves they emit, and what was actually learned with the current dataset. I will highlight "expected" results, surprising findings, and associated caveats. I will then describe the scientific potential of next-generation gravitational-wave detectors, which could be online in the 2030s. Whereas current observatories will detect at most hundreds of compact binaries, at redshift below ~2, next-generation detectors will discover hundreds of thousands of compact binaries per year, at redshifts as high as ~100. They will thus explore the high-redshift universe and the formation of the first stars and the black holes they left behind in a totally new way, and search for stellar-mass primordial black holes early in the history of the universe.

Feb 24

Mar 3 Zhilei Xu
host: Bing Zhang
Precision Cosmology: from CMB to 21cm

Over the past decades, observations of the cosmic microwave background (CMB) have established the standard cosmological model. In the future, 21cm signals from neutral hydrogen have great potential for cosmological and astrophysical studies. In this talk, I will briefly review the previous CMB observations. Then I will discuss the instrumentation and on-sky calibration of the Cosmology Large Angular Scale Surveyor (CLASS) and the Simons Observatory (SO). At the end, I will discuss how we can use the 21cm observation as a powerful probe to study cosmology,and introduce our novel mapping method for 21cm interferometric data.

Mar 9 Thursday 2:30 Zi-Kui Liu
Penn State University
host: Qiang Zhu
Quantitative Predictive Theories Through Integrating Quantum, Statistical, and Irreversible Thermodynamics

Thermodynamics is a science concerning the state of a system, whether it is stable, metastable, or unstable. The combined law of thermodynamics derived by Gibbs laid the foundation of thermodynamics though only applicable to equilibrium or freezing-in systems. Gibbs further derived the classical statistical thermodynamics in terms of the probability of configurations in a system, which was extended to quantum mechanics-based statistical thermodynamics by Landau, while the irreversible thermodynamics was derived by Onsager and Prigogine. The development of density function theory (DFT) by Kohn enabled the quantitative prediction of properties of the ground state of a system from quantum mechanics. In this presentation, we will present our theories that integrate quantum, statistical, and irreversible thermodynamics in a coherent framework by utilizing the predicative capability of DFT to revise the statistical thermodynamics (zentropy theory) and by keeping the entropy production due to irreversible processes in the combine law of thermodynamics to derive flux equations (theory of cross phenomena). It will be demonstrated that the zentropy theory is capable of predicting the free energy landscape as a function of internal degrees of freedom of a system including singularity and instability at critical point and emergent positive and negative divergences of properties, while the theory of cross phenomena can predict the coefficients of internal processes between conjugate variables (direct phenomena) and non-conjugate variables (cross phenomena) in the combined law of thermodynamics, both with inputs from DFT-based calculations only and without fitting parameters, as shown in this publication.

Mar 10

Mar 17 Spring Break

Mar 20 Monday Kentaro Nagamine
Osaka University
host: Bing Zhang
Probing feedback via IGM tomography and Galaxy Formation

It is widely believed that feedback by supernovae and supermassive black holes strongly affects galaxy formation and evolution. There are many pieces of evidence for their impact, and in particular, I will describe our theoretical work on intergalactic medium (IGM) tomography using cosmological hydrodynamic simulations and upcoming PFS project on the Subaru telescope. I will also touch on the new status of the "missing satellites problem" as a probe of dark matter and feedback. The main message is that the cold dark matter (CDM) model is still going strong, and that understanding the baryonic physics on small scales is important to make further progress even for cosmology.

Mar 23

Mar 24

Mar 31

Apr 7

Apr 14

Apr 21 Kedron Silsbee
host: Rebecca Martin
Cosmic ray ionization in molecular clouds

The cosmic ray ionization rate is one of the more uncertain parameters entering models of star formation. In UV-extinguished regions, cosmic rays determine the heating and ionization of the gas. The ionization state of the gas affects not only the chemical evolution, but also the gas dynamics, as ion-neutral collisions determine the spatial scale at which the matter and the magnetic field decouple. While the higher energy (roughly GeV and above) cosmic rays are homogeneously distributed throughout the local part of the Galaxy, the lower energy ones that dominate the ionization rate display significant spatial variability, as well as suffering substantial attenuation by the dense gas in molecular clouds. For this reason, ionization rates measured in nearby molecular clouds vary by more than an order of magnitude. I will discuss different techniques to estimate the ionization rate in molecular clouds, as well as some recent efforts to model cosmic ray propagation. In particular I will highlight the oft-misunderstood role of magnetic mirroring and focusing, as well as how to observationally distinguish between diffusive vs. ballistic propagation.

Apr 28 Katerina Chatziioannou
host: Carl Haster
Studying the neutron star equation of state with gravitational waves

Detections of neutron stars in binaries through gravitational waves offer a novel way to probe the properties of extremely dense matter. In this talk I will describe the properties of the signals we have observed, what they have already taught us, and what we expect to learn in the future. I will also discuss how information from gravitational waves can be combined and compared against other astrophysical and terrestrial probes of neutron star matter to unveil to the properties of the most dense material objects that we know of.

May 3 Wednesday Z. Lucas Uhm
Korea Astronomy and Space Science Institute (KASI)
host: Bing Zhang
Physics of Relativistic Jets in Gamma-Ray Bursts (GRBs) in the Era of Multi-Messenger Astronomy

After the remarkable cosmic event GW170817 / GRB 170817A in the year 2017, observed in both gravitational waves and electromagnetic waves, and with an upcoming O4 run of gravitational wave detectors, we are now living in an exciting era of multi-messenger astronomy. As the GRBs being the invaluable counterpart to the gravitational wave events, the need for understanding the physical mechanism of GRBs is compelling. The difficulties in nailing down their physical mechanism come from the facts that (1) the radiative processes involved remained so elusive as revealed by the observed shape of gamma-ray spectra, (2) there has been no clear observational clue on the composition of relativistic jets launched from the explosion, and (3) there has been no clear evidence on how far from the central engine the prompt gamma-rays of GRBs are emitted while the competing physical mechanisms predict different characteristic distances from the engine. In this talk, some of the recent important developments regarding these questions will be presented. If time allows, I will also talk about a very recent study on broadband modeling of multi-wavelength observations of GRB 221009A. This burst is the most energetic GRB ever detected in human history and thus provides an important opportunity to understand the physics of GRB afterglow emission mechanism.

May 5 Shigeo Kimura
Tohoku University
host: Ali Kheirandish
Neutrinos from accretion flows onto supermassive black holes

Our Universe is filled with cosmic rays, which are high-energy charged particles, but the origins of cosmic rays are unknown for more than a century. Cosmic rays produce neutrinos via hadronuclear and photohadronic interactions, and thus, we can identify source of cosmic rays by using neutrino signals. IceCube Collaboration reported evidence of neutrino signals from a nearby Seyfert galaxy, NGC 1068. Also, there are hint from neutrinos from tidal disruption events. Both system contains accretion flows onto supermassive black holes. In this talk, I will briefly review the current situation of neutrino astronomy and discuss neutrino emission from accretion flows onto supermassive black. We consider stochastic cosmic-ray acceleration by turbulence inside coronae in Seyfert galaxy and hot accretion flows in low-luminosity active galactic nuclei, and find that our model can explain current IceCube data.

May 12

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