Eric Sembrat's Test Bonanza

Abstract

Ever wonder what it would be like to live and work at one of the coldest, most remote places on Earth? James Casey and Martin Wolf can tell you all about it. They are adjusting to life in more moderate conditions after thirteen months operating the biggest and strangest telescope in the world, the IceCube Neutrino Observatory at the South Pole.

See incredible pictures of their exciting and challenging adventure, and learn what it takes to capture the almost invisible neutrino, nicknamed the ghost particle.

Speakers' Bios

James Casey

James Casey is from Huntsville, Alabama. Before becoming an IceCube winterover for the 2016-2017 South Pole season, James had completed his PhD in physics at Georgia Tech as a member of the IceCube Collaboration. For his graduate studies, his research focused on neutrinos generated in gamma-ray bursts. Besides physics, he also enjoys amateur radio, general aviation, and scuba diving.

Martin Wolf

Martin Wolf grew up in Germany and was part of the IceCube Collaboration for six years—receiving his PhD in astrophysics—before becoming one of the two IceCube winterovers for the 2016-2017 South Pole season. Photography is one of his personal interests, and you can see his talent from the many wonderful photos he took while at the Pole.

This event is sponsored by IceCube and the School of Physics at Georgia Tech: https://meetings.wipac.wisc.edu/Atlanta2018/Home

Abstract:

Radiation from the first stars and galaxies initiated the dramatic phase transition marking an end to the cosmic dark ages. The emission and absorption signatures from the Lyman-alpha (Lyα) transition of neutral hydrogen have been indispensable in extending the observational frontier for high-redshift galaxies into the epoch of reionization. Lyα radiative transfer provides clues about the processes leading to Lyα escape from individual galaxies and the subsequent transmission through the intergalactic medium. Cosmological simulations incorporating Lyα radiative transfer enhance our understanding of fundamental physics by supplying the inferred spectra and feedback on the gas.

In this talk, I will discuss recent advances in Lyα modeling based on current state-of-the-art simulations and observational insights. As a case study I will present post-processing results of cosmological “zoom-in” simulations of z > 5 galaxies created with the hydrodynamics code GIZMO under the framework of Feedback In Realistic Environments (FIRE). I will also discuss my new resonant discrete diffusion Monte Carlo (rDDMC) method designed to break the efficiency barrier of Monte Carlo Lyα radiative transfer in the high optical depth regime. Such efficient and robust algorithms will enable fully coupled 3D Lyα radiation hydrodynamics in the near future.

Abstract

Soft matter is a broad class of materials with many examples found in everyday life: foods, crude oil, many biological materials, granular materials, liquid crystals, plastics. All of these are unified by the property that they're readily deformable because the elastic energy is of the same order of magnitude as the ambient thermal energy. Moreover, they spontaneously assemble into richly ordered structures that respond to many different kinds of external stimuli. Soft materials are therefore ideal candidates for advanced engineering applications including soft, biomimetic robots, self-building machines, shape-shifters, artificial muscles, new high-performance all-optical switches and chemical delivery packages. In each of these, the material must make a dramatic change in shape with an accompanying re-ordering of the material. To optimize the materials and structures, it is necessary to have a detailed understanding of how the microstructure and macroscopic shape co-evolve.

In this talk, I will therefore discuss the interactions between order and shape, as well as the role of the dynamics in determining the final state, with examples primarily drawn from my group's work on liquid crystals and emulsions. To develop the description, we draw upon differential geometry, topology, optimization theory and computer simulations, revealing beautiful and profound connections between mathematics and superficially mundane things in the world immediately around us.

 

Abstract:

Active galactic nuclei (AGN) are powered by the accretion activity of supermassive black holes residing at the centers of galaxies. While observations propose that growth of AGN and galaxies are globally tied, I investigate whether this connection exists in individual galaxies. I also investigate various AGN selection techniques and star formation rate (SFR) estimates using multi-wavelength data from Chandra, Spitzer and rest-frame optical spectra from the Keck telescope. 

I find that combining multi-wavelength identification techniques provides a complete AGN sample, as each selection method suffers from selection biases. In particular, all selection techniques are biased against identifying AGN in low mass galaxies. Once stellar mass selection biases are taken into account, I find that AGN reside in galaxies with similar physical properties (i.e., SFR) as inactive galaxies. I find that while AGN are prevalent in both star-forming and quiescent galaxies, they are more likely to reside in star-forming galaxies. The probability of fueling an AGN does not strongly depend on SFR for a star-forming galaxy, though it decreases when star formation is shut down in quiescent galaxies. I find no evidence for a strong correlation between SFR of the host galaxy and AGN luminosity. These results indicate that while both AGN and galaxy growth are reliant on the same fuel, enhanced star formation activity does not necessarily go hand-in-hand with increased AGN activity. 

While the star formation activity of galaxies can be traced with various indicators, my investigations indicate that at z~2 a combination of mid-infrared and far-infrared data provide a more reliable SFR estimation than the mid-infrared data alone. I also find that the robustness of UV-based SFRs depends on the extinction correction method used.  I find that about 30% of z ∼ 2 galaxies have SFRs from infrared observations that are elevated relative to the dust corrected UV-based SFR. My investigations show that this infrared excess is not due to any contribution from AGN, and is primarily due to polycyclic aromatic hydrocarbon (PAH) emission.

 

 

Abstract

When organisms locomote and interact in nature, they must navigate through complex habitats that vary on many spatial scales, and they are buffeted by turbulent wind or water currents and waves that also vary on a range of spatial and temporal scales.  We have been using the microscopic larvae of bottom-dwelling marine animals to study how the interaction between the swimming or crawling by an organism and the turbulent water flow around them determines how they move through the environment.  Many bottom-dwelling marine animals produce microscopic larvae that are dispersed to new sites by ambient water currents, and then must land and stay put on surfaces in suitable habitats.

 Field and laboratory measurements enabled us to quantify the fine-scale, rapidly-changing patterns of water velocity vectors and of chemical cue concentrations near coral reefs and along fouling communities (organisms growing on docks and ships). We also measured the swimming and crawling performance of larvae of reef-dwelling and fouling community animals, and their responses to chemical and mechanical cues.  We used these data to design agent-based models of larval behavior.  By putting model larvae into our real-world flow and chemical data, which varied on spatial and temporal scales experienced by microscopic larvae, we could explore how different responses by larvae affected their transport and their recruitment into reefs or fouling communities.  The most effective strategy for recruitment depends on habitat.

BIO

Mimi Koehl , a Professor of Integrative Biology at the University of California, Berkeley, earned her Ph.D. in Zoology at Duke University.  She studies the physics of how organisms interact with their environments, focusing on how microscopic creatures swim and capture food in turbulent water flow, how organisms glide in turbulent wind, how wave-battered marine organisms avoid being washed away, and how olfactory antennae catch odors from water or air moving around them. Professor Koehl’s is a member of the National Academy of Sciences and the American Academy of Arts and Sciences.  Her awards include a MacArthur “genius grant”, a Presidential Young Investigator Award, a Guggenheim Fellowship, the John Martin Award (Association for the Sciences of Limnology and Oceanography, for “for research that created a paradigm shift in an area of aquatic sciences”), the Borelli Award (American Society of Biomechanics, for “outstanding career accomplishment”), the Rachel Carson Award (American Geophysical Union, for "cutting-edge ocean science"), and the Muybridge Award (International Society of Biomechanics “highest honor”).

 

 

Abstract:

I will discuss quantum order-by-disorder effect and will present an evidence that the non-linear terms in the anisotropic kagome-lattice antiferromagnets can yield a rare example of the ground state that is different from the one favored by thermal fluctuations. The corresponding order selection will be shown to be generated by the topologically non-trivial tunneling processes, yielding a new energy scale in the system. I will also discuss the effect of the non-linear terms in the spectra of the kagome-lattice systems and will provide an analysis of the spectral properties of realistic kagome-lattice antiferromagnets such as Fe-jarosite, for which a remarkable wipe-out effect for a significant portion of the spectrum should exist due to a resonant-like decay processes involving two flat modes. Recent result concerning the spectrum of the kagome-lattice ferromagnets will also be presented.

Abstract

In this talk, I will discuss a theoretical framework for understanding materials that are fragile. These are marginal solids or highly viscous liquids that emerge out of thermal equilibrium in response to external stresses. Granular materials and non-Brownian colloidal suspensions are well-known examples, however, reconfigurable pathways of force transmission also play an important role in biological systems. In granular materials, external forces such as gravity create rigid and flowing states. The mechanical integrity of these marginal solids is reliant on a filamentary network of stress-bearing structures. An outstanding question in the field has been how the constraints of vectorial force balance influence the response of granular assemblies to stress, and create localized stress pathways.

I will present results of recent work showing that the localized response is a consequence of the disorder in the underlying contact network, and can be mapped on to a ``localization'' problem.  I will also discuss an interpretive theory, based on statistical ensembles, of two transitions driven by frictional contacts: shear jamming and discontinuous shear thickening.

Abstract:

Advances in cooling and trapping for several distinct species of atoms, ions and molecules have gone hand-in-hand for a long time now. Cold dilute gas ensembles to single particle realization of these have led to investigations of diverse problems in many body physics and exotic physics on one hand to unprecedented spectroscopic precision on the other. In most cases, it is the interaction between trapped particles and the confining fields which is used to realize the system of interest. In these systems, it is typical to know the initial position and motional state with precision. Most experiments though are performed with either atoms or ions or molecules.

Combining cold ions, atoms and molecules for experiments has been gaining traction in recent times. Such combined hybrid traps [1] pose very specific challenges for simultaneous trapping and cooling of the multiple species. In addition, the interactions between different species become paramount and it is imperative to understand how the combined systems evolve, how energy is exchanged, what is the final state mediated by these interactions, etc. In this talk, I shall present our experimental system which can simultaneously cool and trap, within a cavity mode, atoms, ion and molecules in any combination [2]. In this experiment interactions between co-trapped species whose long-range interaction range from 1/r to 1/r⁶ can be probed. I shall then present results of how an ion in a Paul trap is cooled by a MOT of atoms. The details of the cooling hold several surprises such that the ultimate temperature of the trapped ions can be decided by the spatial size of the MOT [3,4], the process of resonant charge exchange is very effective of ion cooling [5], etc. I shall conclude with the prospects of experiments in the future, with such ion-atom systems at ultracold temperatures.

Abstract: 

At this writing, LIGO & Virgo have together released information on observations of five coalescing binary black hole systems, one binary system involving at least neutron star, and one "sub-threshold" event likely also from a black hole binary system. Taken apart, several of these events are of great individual interest; taken together, the collection of events is evidence for discerning the mechanisms governing the formation and evolution of compact object binary systems. 

Connecting theoretical models of compact object binary population synthesis with gravitational wave observations is more fraught with uncertainty than it might - at first glance - seem. Gravitational wave detection is more than a new observational tool capable of providing a fresh perspective on astronomical phenomena that can be studied by other means. In the case of black hole binary coalescence they provide they only observational perspective on black hole binary synthesis and evolution, which is already several degrees removed from existing observational study or theoretical understanding.

In cases like this, theoretical prejudice runs an especially strong risk of clouding the rightful interpretation of the plain observations. Phenomenology - the empirical modeling of observations, consistent with broad physical principles but not tied to any specific theory - thus becomes a crucial intermediary between observation and theory, insuring that it is the former that informs the latter. 

Here we describe work in progress toward a phenomenological model of compact binary coalescence that can act as an interface between complex theoretical models of compact binary population synthesis and the growing body of observations of coalescing stellar-mass neutron star and black hole systems. 

 Abstract

Articular cartilage is a remarkable material in its ability with withstand hundreds of millions of loading cycles throughout its lifetime with minimal capacity to self-repair.  This load-bearing capacity arises from its unique structure, which is comprised of interpenetrating networks of stiff collagen and highly charged proteoglycans.  These constituents are arranged heterogeneously throughout the tissue, with composition varying locally based on anatomic location and tissue depth. This heterogeneous composition give rise to local heterogeneities in tissue properties, which are relevant to tissue function and signaling of the cells embedded within it. 

We have developed techniques in confocal elastography and both Raman and FTIR microscopy that enable measurement of local mechanics and composition on the length scale of 10-20 µm. These techniques enabled us to identify large mechanical gradients in the tissue, where shear modulus varies by more than a factor of 100 over a length scale of 100 µm.  By spatially registering this information with data from local composition, we have measured structure-property relationships that implicate molecular connectivity as playing a key role in dictating the transition between stiff and compliant regions of the tissue. These findings facilitate a new understanding of this complex tissue as well as give fundamental insight into mechanisms of tissue damage and pathology that occur in tissues such as osteoarthritis.

BIO

Dr. Bonassar joined Cornell University in 2003 after five years on the faculty of the Center for Tissue Engineering at the University of Massachusetts Medical School. He completed postdoctoral fellowships in the Orthopaedic Research Laboratory at the Massachusetts General Hospital and in the Center for Biomedical Engineering at the Massachusetts Institute of Technology. He currently serves on the editorial board of the journal Tissue Engineering.