Current Term Colloquium Videos
March 10, 2017
Ryan Hickox, Dartmouth College
Title: "The Hidden Monsters: New Windows on the Cosmic Evolution of Supermassive Black Holes" (Video)
Abstract: At the heart of essentially every large galaxy in the Universe lies a supermassive black hole. In the past decade, surveys of the extragalactic sky have made great progress in understanding the cosmic growth of these black holes, as they "eat" surrounding material and radiate as active galactic nuclei (AGN). However, our picture of black hole evolution has remained incomplete, due to the challenges of detecting black holes that are highly obscured by gas and dust or hidden beneath the light of their host galaxies. With the advent of new resources including hard X-ray observations from NuSTAR, mid-infrared data from WISE, and new insights from theoretical models, we can now identify millions of these “hidden” growing black holes across much the sky, and characterize the nature of their obscuration and their role in the formation of galaxies. I will describe recent efforts to characterize these "hidden” black holes, particularly highlighting work by postdocs, graduate students, and undergraduates here at Dartmouth, and will present evidence that (at least some) powerful obscured AGN represent an evolutionary phase in the evolution of their host galaxies. Finally, I will point to the exciting potential for future of AGN population studies with the next generation of extragalactic surveys, including with NASA's Lynx concept X-ray mission.
March 7, 2017
Jason TenBarge, University of Maryland
Title: "Kinetic Plasma Turbulence in Space and Astrophysical Plasmas" (Video)
Abstract: Turbulence is a ubiquitous process in space and astrophysical plasmas that serves to mediate the transfer of large-scale motions to small scales at which the turbulence can be dissipated and the plasma heated. The nature of the dissipation and heating ultimately determines the amount and type of radiation we observe from many distant astrophysical objects, such as black hole accretion discs. Closer to home, the dissipation of turbulence may be responsible for the anomalous heating of the solar corona, wherein the upper atmosphere of the sun is orders of magnitude hotter than the solar surface. Most of these systems of interest are weakly collisional, implying that they may be far from thermal equilibrium, which necessitates a kinetic rather than fluid treatment. In this talk, I will discuss recent progress toward understanding dissipation in kinetic plasmas, both within the confines of an asymptotically ordered kinetic system, gyrokinetics, and in full, continuum kinetic models.
March 3, 2017
David Hatch, University of Texas at Austin
Title: "Kinetic Plasma Turbulence: New Insights Into Its Fundamental Nature and Implications for Fusion Energy" (Video)
Abstract: Turbulence is ubiquitous in both space and laboratory plasmas. These plasmas are often hot and/or diffuse, which requires the use of kinetic theory, the description of particle distribution functions in a high-dimensional phase space. Turbulence in this phase space exhibits a rich variety of dynamics, altering standard fluid turbulence paradigms in fascinating ways. I will describe new insights into the fundamental nature of kinetic plasma turbulence (applicable to both natural and laboratory plasmas). I will also discuss breakthroughs in understanding and modeling plasma turbulence in fusion devices, addressing the question: what do plasma turbulence simulations say about the prospects for fusion energy and how can such simulations advance these prospects?
February 28, 2017
Yi-Min Huang, Princeton University
Title: New Perspectives on Magnetic Reconnection — From Plasmoid Instability to Self-Generated Turbulence (Video)
Abstract: Magnetic reconnection is a fundamental physical process that allows magnetic field lines to break the frozen-in constraint of ideal magnetohydrodynamics (MHD), changing magnetic topology while at the same time converting stored magnetic energy into plasma energy. It is generally believed to be the underlying mechanism that powers energetic events over a wide range of scales, including sawtooth crashes in magnetic fusion devices, magnetospheric substorms, solar flares, and coronal mass ejections. In recent years, theoretical analysis and numerical simulations provide strong evidence that large-scale, high-Lundquist-number magnetic reconnection is susceptible to the plasmoid instability and becomes sporadic, therefore the traditional picture of two-dimensional laminar reconnection needs a major revision. In this talk, I will give an overview of recent progress in the roles of plasmoid instability on magnetic reconnection. I will first discuss the linear tearing instability in a reconnecting current sheet that eventually leads to a disruption of the current sheet and triggers onset of fast reconnection. Then I will discuss various scenarios of plasmoid-mediated reconnection in fully nonlinear regimes, with resistive and Hall MHD simulations. Finally, I will discuss results from a recent 3D simulation, where plasmoid instability is shown to facilitate self-generated turbulent reconnection.
February 24, 2017
Zackaria Chacko, University of Maryland College Park
Title: Neutral Naturalness (Video)
Abstract: I explain the hierarchy problem of the standard model of particle physics, and discuss some of the ideas which have been put forward to resolve it. I then show that a specific class of theories, built around a framework known as neutral naturalness, can help address this problem while remaining consistent with all current experimental tests. I explain that while certain theories in this class give rise to striking signals, others are extremely difficult to test, and require a detailed study of the properties of the Higgs boson. I consider the implications of these results for the Large Hadron Collider, and for future experimental programs.
February 10, 2017
Yi-Hsin Liu , NASA Goddard Space Flight
Title: Magnetic Reconnection in Plasmas (Video)
Abstract: Magnetic reconnection is the process whereby a change in topology of magnetic field lines allows for a rapid conversion of magnetic energy into thermal and kinetic energy of the surrounding plasma. This physical process plays a key role in many astrophysical and laboratory contexts, ranging from magnetospheric substorms, solar eruptions, sawtooth crashes in fusion devices and potentially the super-flares in Crab Nebula. The recently launched Magnetospheric Multiscale (MMS) mission provides an unprecedented view of the three-dimensional fine-scale structure of magnetic reconnection. It offers the amazing opportunity to combine key insights from numerical models with high quality in-situ measurements that promise to greatly increase our understanding of this crucial physical process.
A fundamental and long-standing mystery has been why the reconnection energy conversion rate is remarkably efficient and near-universal across a wide spectrum of plasma regimes. In this talk, I demonstrate for the first time how the reconnection geometry determines the energy release, setting an upper limit for the energy conversion rate that is insensitive to the details of the system. This model is in excellent qualitative and quantitative agreement with the universal fast rate observed in disparate systems. I will also present new insights into the complex three-dimensional nature of magnetic reconnection revealed by means of petascale supercomputing, and will interpret my results in the context of MMS. Finally, I describe the next steps we need to take to answer some key open questions in magnetic reconnection.
February 3, 2017
Jack Sankey, McGill University
Title: Toward Optically Defined Micromechanical Systems (Video)
Abstract: Mechanical systems are ubiquitous throughout society, from oscillators in timekeeping devices to accelerometers and electronic filters in automobiles and cell phones. They also represent an indispensable set of tools for fundamental science, providing a means of sensing atomic-scale forces and masses or even the minuscule spacetime distortions from passing gravitational waves. In the field of optomechanics, we exploit the forces exerted by radiation to gain a new level of control over these systems at all size scales.
In this talk I will discuss our group's efforts to create delicate micromechanical systems and then influence their motion with extremely low levels of light. To this end, we have fabricated nanogram-scale "trampolines" having extraordinarily low damping parameters, ringing for six minutes when struck, and record force sensitivities below 20 attonewtons at room temperature. These trampolines also have excellent optical properties and are well-suited for optomechanical applications, notably providing access to a regime in which an average of a single cavity photon could significantly alter the trampoline's damping parameter (and, as a result, its trajectory). I will also discuss plans to apply an optical spring to these and related systems to control frequency, improve mechanical performance, and define the shape and mass of a mechanical mode.
January 27, 2017
Andrew Fitzpatrick, Boston University
Title: Effective Field Theory and the Phenomenology of Dark Matter Direct Detection (Video)
Abstract: Direct detection searches for dark matter have made significant advances in their level of sensitivity and are expected to improve further in the near future. The results of these experiments are often interpreted in a limited context of certain kinds of interactions between dark matter and standard matter, but the full range of possible interactions is much richer. We discuss the theoretical and experimental motivation for considering a broad range of possible structure in the dark matter content of the universe, and a framework for systematically describing this wider range of possibilities. We describe results from on-going work bringing together this framework with nuclear physics results in order to provide a complete `dictionary’ for the experimental predictions in this larger parameter space.
January 13, 2017
Title: Pushing Einstein's Boundaries: Gravitational Approaches to the Challenges of Modern Cosmology (Video)
Abstract: Einstein’s general theory of relativity (GR) is one of the most successful and well-tested physical theories ever developed. Nevertheless, modern cosmology poses a range of questions, from the smallest scales to the largest, that remain currently unresolved by GR coupled to the known energy and matter contents of the universe. This raises the logical possibility that GR may require modification on the relevant scales.
I will discuss the status of some modern approaches to alter GR to address cosmological problems. We shall see that these efforts are extremely theoretically constrained, leaving very few currently viable approaches. Meanwhile, observationally, upcoming missions promise to constrain allowed departures from GR in exciting new ways, complementary to traditional tests within the solar system. I will finish by describing some promising very recent ideas.
January 6, 2017
Eric R. Fossum
Title: Quanta Image Sensor: Every Photon Counts (Video)
Abstract: About 10 years after the invention of the CMOS image sensor at the NASA Jet Propulsion Laboratory in the early 1990’s, I was asked to write a book chapter on the future of digital still cameras. I proposed a binary, photon-counting image sensor now called the Quanta Image Sensor. It was a sort of wild idea at the time, but somewhat surprisingly, it now seems technically feasible. In this talk a brief review of the CMOS image sensor and a few fundamental physical principles behind its operation will be given. The Quanta Image Sensor will then be introduced. Conceptually it consists of perhaps one billion specialized pixels called “jots” that are read out at perhaps 1000 frames per second. Each jot is sensitive enough to count a single photoelectron. Recent progress at Dartmouth in achieving the QIS will be presented. We have shown the feasibility of making small (visible light) arrays that can count single photoelectrons at room temperature without the use of avalanche multiplication with good accuracy. The devices, implemented in a 65nm technology-node, backside-illuminated, CMOS image sensor foundry, also feature dark current less than 0.1e-/s at room temperature. The jot device and high speed readout electronics will be discussed, as well as the possible paradigm shift in image capture for scientific and consumer imaging that can now be envisioned and enabled.