Colloquia / Seminars
Tea and coffee are served at the speaker’s reception in Phillips 269, 3:30pm.
Colloquia are held Monday afternoons in 265 Phillips Hall at 4:00pm. Some February Colloquia are held Thursday afternoons in 211 Chapman Hall at 4:00pm.
The student Questions and Answers will also be held in Phillips 269 after the colloquium from 5:15-6:00pm.
“The Schwinger-DeWitt Proper Time Algorithm: A History”
Steven Christensen, University of North Carolina at Chapel Hill
Much of my career in physics has been, in one way or another, concerned with the use of the so-called Schwinger-DeWitt proper time algorithm for studying renormalization in quantum field theory in curved spacetimes. As a student of Bryce DeWitt, at UNC-CH and at UT- Austin, I was introduced to Schwinger’s classic 1951 paper, On Gauge Invariance and Vacuum Polarization, Phys. Rev. 82, Vol. 5, pages 664-679. I became aware of DeWitt’s curved space generalizations of the algorithm through detailed chapters in his book Dynamical Theory of Groups and Fields, Gordon and Breach, 1965, based on his 1963 Les Houches lectures. Since then, I, and many others, have used this technique and extensions of it to understand things like black hole radiation and back reaction, trace and other anomalies, index theorems, and more. In this talk I will present a short history of how the method has been the motivation for developments in physics, mathematics, and even computer software.
“Bioengineering & Nanotechnology Solutions for Chemical and Biological Defense”
Robert Botto, Innovation Department of the Defense Threat Reduction Agency
The Joint Science & Technology Office (JSTO) at the Defense Threat Reduction Agency is the research arm of the Chemical & Biological Defense Program within the Department of Defense. Research within JSTO focuses on developing the science and technology solutions to rapidly detect, identify, protect against and mitigate potential chemical and biological threats. The purpose of this presentation is to provide an overview of the basic research program in medical and physical sciences, and to highlight the role of bioengineering and nanotechnology in advancing our mission needs.
“Transients in Time Domain Astronomy: Supernovae, Kilonovae, and Gravitational Lensing”
Curtis McCully, Las Cumbres Observatory
New technology in time-domain astronomy is transforming our understanding of the universe. High-cadence, wide-field time-domain surveys are discovering both rare and rapid transients, but discovery is only the first step: follow-up is key. Las Cumbres Observatory (LCO) operates a network of 21 robotic telescopes around the world which allows us to routinely get rapid, complete coverage of astronomical transients. Using LCO, we were able detect the first electromagnetic counterpart of a gravitational wave event produced by a neutron star merger even though the ensuing explosion, a “kilonova”, rose and faded in less than a week. I will present our observations from LCO and other spectroscopic facilities of the kilonova and discuss their implications for the amount of heavy r-process elements that were produced in this new type of cosmic explosion. We are also using LCO to observe other types of explosive transients. The Global Supernova Project is building a well-sampled spectroscopic and photometric dataset for an unprecedented sample of nearby supernovae. One class in this sample will be type Iax supernovae which are likely produced the “failed” explosion of a white dwarf with an exotic (helium-rich) donor. I will present our observations which led us to this physical picture including our detection in pre-explosion images which marks the first time the progenitor system of a white dwarf supernova has ever been observed. As future surveys like ZTF, LSST, and WFIRST begin, we will begin to be able to build samples of rare, exotic transients like gravitationally lensed supernovae which are poised to become novel, powerful tools for cosmology. Each of these types of transients, kilonovae and (lensed) supernovae, provide complementary constraints that we will use to probe the nature of dark matter and dark energy. I will discuss how we are preparing for future surveys so that we can utilize these cosmological probes to their full potential.
“Illuminating the Growth of Galaxies and Supermassive Black Holes”
Brooke Simmons, University of California, San Diego
Galaxy growth and evolution may be broadly categorized as either driven by galaxy-galaxy collisions/mergers, or merger-free. Merger-free processes are crucial to galaxy evolution, yet many fundamental open questions remain: how does galaxy star formation halt without a disruptive merger? How do central “supermassive” black holes grow in galaxies without mergers? How do these questions impact each other? This talk will review the field of merger-free galaxy and black hole evolution from 0 < z < 3 and discuss the relative importance of both mergers and completely calm, “secular” evolution on galaxy and black hole growth. New evidence from recent cosmological simulations suggests merger-free process contribute significantly to both the overall growth of supermassive black holes and their co-evolution with their host galaxies, and a new Hubble Space Telescope survey is designed to answer these questions about merger-free evolution, in preparation for the next generation of telescopes and surveys. These results are made possible by the contributions of hundreds of thousands of members of the public in Galaxy Zoo; this talk will discuss the future of these citizen science endeavors.
“Measuring Dark Energy with Supernovae and Kilonovae”
Daniel Scolnic, University of Chicago
The next decade will be the golden age of cosmology with transients. In this talk, I will present new analyses of Type Ia Supernovae that mark the most precise measurement of dark energy to date. I will go over how this analysis ties together with the analysis of the local value of the Hubble constant, for which tension persists with the inferred value from the CMB – an exciting hint at possible departures from the standard cosmological model. I will then discuss the first measurements of the Hubble constant with kilonovae and gravitational waves. I will review the large amount of overlap between the issues that must be tackled for future progress using supernovae and kilonovae to measure cosmological parameters. Finally, I will discuss the roles that surveys like LSST and WFIRST will play and how we can harness the millions of transients discovered to make generation-defining cosmological measurements.
“Gravitational Lensing In the Era of Survey Science”
Matt Bayliss, Massachusetts Institute of Technology
Since the advent of large-area, high-quality astronomical surveys strong gravitational lensing has transitioned from a small-N to a large-N discipline. Galaxy cluster scale strong lensing, in particular, holds tremendous untapped potential because it lies at the intersection of cosmology, the most massive structures in the Universe, and the magnified distant universe. As a founding member of the Sloan Giant Arcs Survey (SGAS) team, I will summarize our recent progress toward unlocking the scientific potential of large samples of strong lensing systems to address fundamental problems in astrophysics and cosmology. Focusing on recent results that highlight our sophisticated lensing analysis toolbox, I will present several pioneering measurements that we have made using the SGAS sample. These results lay the groundwork for future work that will use large numbers of highly magnified galaxies to answer outstanding questions about the physics of star formation and the properties of the interstellar medium in the epoch during which the Universe formed most of its stars. In addition to their value as natural telescopes, the massive structures that are responsible for the lensing action are, themselves, rare and powerful tools for testing the Lambda-CDM cosmological paradigm via the growth of structure and the mass distributions of lensing clusters. Thinking outside the bounds of vanilla Lambda-CDM, I will discuss how cluster lenses are also excellent laboratories for probing exotic, non-standard cosmologies. Upcoming surveys from LSST and the South Pole Telescope will reveal sample of cluster lenses out to high redshift. With these new systems we will directly observe the first generation of massive galaxy clusters, and the first generation of strong lensing clusters. The leverage from high redshift galaxy cluster samples—including mass calibrations informed by strong lensing studies—will usher in a new and powerful new tool for constraining the nature of Dark Energy and the Cold Dark Matter theory of structure formation.
“Exploring Peculiar Events in Stellar Evolution Using the Enigmatic Hot Subdwarf Stars”
Brad Barlow, High Point University
The lesser-known hot subdwarf stars represent one of the least-understood stages of stellar evolution. Theory shows these core helium-fusing, extreme horizontal branch stars likely formed from red giants that were stripped of their outer hydrogen envelopes, due to Roche lobe overflow and common envelope interactions with a nearby companion. Observations generally support this idea as nearly all hot subdwarfs appear to be in binaries. These systems have surprisingly important roles to play in our understanding of several astrophysical phenomena. Some of the shortest-period binaries are candidate progenitor systems for underluminous Type Iax supernovae and might serve as verification sources for gravitational wave observatories like LISA. Other short-period systems contain brown dwarf companions, suggesting substellar objects might be able to survive red giant engulfment. On the opposite end of the spectrum are the wide hot subdwarf binaries, which have orbital periods from 1-3 years and F/G/K main sequence companions. Surprisingly, many of these systems show significant eccentricity, contrary to the predictions of Roche lobe overflow models. Here I will give a brief introduction to hot subdwarfs and discuss ways in which we exploit their formation histories to improve our understanding of broader areas within astrophysics.
“Tracing Planetary Evolution from Infancy to Maturity”
Andrew Mann, Columbia University
Planets are not born in their final state; before reaching a more mature and stable phase, young planets are significantly altered and reshaped by their environment. The first few hundred million years are thought to be the most formative, but planets in this age range are also the most difficult to identify and characterize. Instead, research has focused on inferring the history of exoplanets through patterns in the population of older systems. I will discuss how this paradigm is shifting, as novel search techniques and new missions have enabled the discovery of Earth- to Jupiter-sized planets, some as young as 10 Myr. These discoveries have altered our understanding of how planets migrate and lose atmosphere, but raise further questions about the physical drivers of these changes. New missions will broaden the sample of young planets and enable detailed studies of the structures and atmospheres of infant planets, and providing more detailed answers on the underlying physical drivers of exoplanet evolution. In this talk I will review these important recent discoveries, and discuss new directions for research into exoplanet evolution, highlighting the unique role UNC researchers and facilities can play.
“Hyperpolarized NMR Contrast Agents for Molecular Imaging”
Eduard Chekmenev, Vanderbilt University
The significant (usually orders-of-magnitude) increase in nuclear spin polarization above the thermal-equilibrium level is called hyperpolarization. Because the NMR signal is directly proportional to the nuclear spin polarization, the realized polarization enhancement manifests in the corresponding NMR signal and corresponding gains in detection sensitivity, which can be ~4-8 orders of magnitude depending on the detection field of the MRI scanner. Recent developments in NMR hyperpolarization have enabled a wide range of new in vivo molecular imaging modalities—ranging from functional imaging of the lungs to metabolic imaging of cancer. Our research explores selected advances in methods for the preparation and use of hyperpolarized contrast agents, many of which are already at or near the phase of their clinical validation. Biomedical applications of hyperpolarized contrast agents require (i) high polarization level of relatively long-lived hyperpolarized molecules, (ii) preparation of pure hyperpolarized substrates in (iii) biocompatible administration medium. Preparation of such agents in the context of conventional Parahydrogen Induced Polarization (PHIP) using molecular addition of parahydrogen and Signal Amplification By Reversible Exchange (SABRE) using reversible exchange of parahydrogen demands new advances in synthetic chemistry of contrast agents, i.e. sophisticated isotopic enrichment and synthetic schemes, and advances in chemistry and spin-physics of homogeneous and heterogeneous hydrogenation and exchange catalysis.
The significantly enhanced NMR hyperpolarization-endowed signal can enable high-quality clinical 3D images of hyperpolarized contrast agents to be obtained in as little as a few seconds. Moreover, because the hyperpolarized nuclear state is no longer endowed by the static magnetic field of the detecting NMR magnet, high-field MRI scanners are no longer mandatory, and lower-cost, less-confining, low-field 3D MRI can be used instead—with detection sensitivity potentially approaching or even surpassing that of high-field MRI.
This presentation will focus of our recent advances in (i) development of automated instrumentation for preparation of hyperpolarized contrast agents using parahydrogen and Xenon-129, (ii) developing new chemistries for preparation of hyperpolarized contrast agents to probe lung function using hyperpolarized proton MRI, elevated glycolysis in cancer, brain metabolism, and pH and hypoxia imaging, and (iii) improving hyperpolarized MRI detection sensitivity utilizing low-field MRI.
“The Many Facets of Time Crystals”
Alfred Shapere, University of Kentucky
Time crystals are systems that exhibit spontaneous breaking of time translation symmetry, analogously to ordinary crystals, which break space translation symmetry. Unlike space crystals, time crystals are surprisingly hard to make, both theoretically and experimentally. Starting with the original proposal in 2012, I will review the history of time crystals and survey their manifestations in a variety of contexts, from cold atoms to cosmology, including recent experimental realizations.
“Studying Gender in Physics Education Research: Beyond the Binary”
Jennifer Blue, Miami University
Much work in physics education research, including much of my own, has examined differences in preparation, persistence, and performance between male and female students. I now believe that there are issues with the implied theoretical framework behind this work. When we look outside of physics, we see a growing, rich literature about the non-binary nature of gender, about intersectionality, and about identity formation. With other PER colleagues, I have called on our field to expand our framework and perform richer studies (Physical Review Education Research 12, 020114, 2016). We also propose short-term strategies that all physicists can employ to make our classrooms and labs more inclusive (Physics Today, March 2018).
Graciela Gelmini, University of California, Los Angeles
“Investigating student understanding at the physics-mathematics interface”
John Thompson, University of Maine
Because learning physics concepts often requires the ability to construct, interpret, and manipulate mathematical representations and formalism (e.g., equations, graphs, and diagrams), researchers in physics education and in mathematics education have been examining how students navigate this disciplinary interface. Our own research into student conceptual understanding of physics has led us to investigate how students use and reason about mathematics, especially calculus, to solve physics problems in several physics domains, particularly at the upper division. I will discuss findings from work at UMaine and elsewhere that describe student difficulties and successes at the physics-mathematics interface, as well as models and frameworks from both disciplines that help interpret data and that suggest pedagogical strategies to address students’ difficulties. Examples from student construction of differential quantities in vector calculus as used in electromagnetism as well as other concepts in thermal and statistical physics will be given.
“The Jazz of Physics: The Link Between Music and The Structure of the Universe”
Stephon Alexander, Brown University
In this talk Alexander revisits the ancient interconnection between music and the evolution of astrophysics and the laws of motion. He explores new ways music, in particular jazz music, mirrors modern physics, such as quantum mechanics, general relativity, and the physics of the early universe. Finally, he discusses ways that innovations in physics have been and can be inspired from “improvisational logic” exemplified in Jazz performance and practice.
“Turning Photons Into Polarized Nuclei”
Thad Walker, University of Wisconsin-Madison
Spin-exchange optical pumping is a technique that transfers the angular momentum from high-power laser light to noble-gas nuclei, allowing liters of highly polarized, long-lived nuclear spins to be produced. These dense spin gases enable diverse scientific applications ranging from magnetic-resonance imaging to spin-polarized nuclear physics targets to searches for ultra-light dark matter. This talk will discuss the atomic physics alchemy of photon-to-spin conversion, and illustrate a smattering of the applications.
“Setting Stellar Chronometers: The PTF(+) Open Cluster Survey”
Marcel Agüeros, Columbia University
While we have known for 40 years of the existence of a relation between a solar-mass star’s age, rotation, and magnetic activity, observational limitations have hampered the assembly of uniform samples of rotation and activity measurements for stars spanning a wide range of ages and masses. We are still far from being able to describe fully the evolution of either rotation or activity for low-mass stars, or from being able to use rotation or activity measurements to estimate accurately the ages of isolated field stars. I will describe results from our efforts to assemble a complete sample of rotation and activity measurements for low-mass stars in six nearby open clusters ranging in age from ~100 Myr to ~3 Gyr. I will focus on our recent results for the benchmark clusters Praesepe and the Hyades, on new results for NGC 752, and on tests of models of rotational evolution that these data have enabled.
“Columbia’s Bridge to the Ph.D. Program: A Research-Intensive Path to Graduate School for Underrepresented Minorities”
The Bridge to the Ph.D. Program in the Natural Sciences, now in its tenth year, is designed to increase the participation of underrepresented minorities in STEM graduate programs. In this talk, I will outline the Program structure, focusing in particular on admissions and participant performance evaluation, discuss some of the outcomes for its participants, volunteer some lessons learned, and describe our plans for its next half decade.