Past Colloquia: 2016-2017

Spring 2017

Clusters of Galaxies: Laboratories for Probing the Interplay between Baryons and Dark Matter

January 17, 2017

Esra Bulbul (MIT)
Host: John Wardle

Abstract: As the most massive collapsed structures in the Universe, galaxy clusters are unique laboratories for studying the evolution of baryons in concert with dark matter particles, as well as for exploring the nature of dark matter, and dark energy. The thermodynamical state of the intra-cluster gas has been extensively studied through multi-wavelength X-ray and radio (Sunyaev Zel'dovich effect) observations. I will highlight the most recent measurements of a) the evolution of baryons in the deep potential wells of clusters, b) the potential of utilizing clusters in cosmological studies and indirect searches for dark matter with a particular focus on the candidate 3.5 keV emission line.

Discovering a new approach to cosmology with the Dark Energy Survey and Gravitational Waves

January 24, 2017

Marcelle Soares Santos (FermiLab, Illinois)
Host: John Wardle

Abstract: Motivated by the exciting prospect of new wealth of information that will arise from observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) Collaboration has performed a broad range follow-up program for LIGO/Virgo events using its Camera (DECam). In this talk, I present an overview of this effort, including results of searches for signatures of the first two LIGO-triggered binary black hole mergers in the 2015-2016 observing campaign and status of the ongoing 2016-2017 campaign. I will also discuss plans for upcoming seasons and long term prospects for this exciting emerging field: multi-messenger cosmology with gravitational waves and optical data.

The Past, Present, and Future of 21cm Cosmology

January 26, 2017

Adrian Liu (UC Berkley)
Host: John Wardle

Abstract: Despite tremendous recent progress, gaps remain in our knowledge of our cosmic history. For example, we have yet to make direct observations of Cosmic Dawn or the subsequent Epoch of Reionization. Together, these represent the important period when the first stars and galaxies were formed, dramatically altering their surroundings in the process. Radio telescopes targeting the 21cm line will open up these crucial epochs to direct observations in the next few years, filling in a missing chapter in our cosmic story. I will review our recent results from the Precision Array to Probe the Epoch of Reionization (PAPER) experiment. These results have begun to place limits on heating processes during reionization. I will also motivate unconventional ideas in experiment design that have been proposed and implemented to deal with the unique technical challenges of 21cm cosmology. Cognizant of "lessons learned" from the current generation of instruments, I will describe our recently commenced Hydrogen Epoch of Reionization Array (HERA), including its forecasted promise to provide exquisite constraints on reionization astrophysics as well as on fundamental parameters such as the neutrino mass.

The Extraordinary Lives of Supermassive Black Holes at the Centers of Galaxies

January 31, 2017

Francisco Muller-Sanchez (University of Colorado, Boulder)
Host: John Wardle

Abstract: Some of nature's most powerful objects are well-fed supermassive black holes at the centers of galaxies. Weighing up to billions of times the mass of our sun, they usually outshine the stellar emission of their host galaxies. The discovery of a number of black hole – galaxy relations has shown that the growth of supermassive black holes is closely related to the evolution of galaxies. This evidence has opened a new debate in which the fundamental questions concern the interactions between the central black hole and the interstellar medium within the host galaxy, and can be addressed by studying two crucial processes: feeding and feedback. With the help of NASA’s space observatories and the largest optical telescopes in the world, I will explain how supermassive black holes are fed and how they influence their host galaxies. I will focus on my recent results for a large sample of luminous nearby galaxies, which also includes galaxies from the Sloan Digital Sky Survey with dual/offset emission lines to identify outflows and distinguish them from dual/binary black holes. The prospects for JWST/TMT observations of nearby galaxies will also be discussed, including the feeding of the black hole at sub-parsec scales, the starburst-black hole connection and the launching mechanisms of black hole-driven outflows.

Using the “COSMOS” to Understand Black Hole and Galaxy Co-evolution

February 2, 2017

Francesca Civano (Harvard)
Host: John Wardle

Abstract: Observations indicate that supermassive black holes (SMBHs, 10**6-10**9 Msun) dwell at the centers of most local galaxies. Scaling relations between SMBH mass and several large-scale properties of the host galaxies point to a co-ordindated growth of galaxies and their central engines over cosmic time: they "co-evolve". Who is the leading actor on the the cosmic stage: the black hole or the galaxy? Is black hole activity triggering star-formation or suppressing it? Does the galaxy control the black hole growth? To address these questions, in this talk I will present my work testing this co-evolution scenario, focusing on SMBH growth mechanisms, accretion and mergers. I will use the extraordinarily rich multiwavelength dataset of the Cosmic Evolutionary Survey (COSMOS). I will concentrate on the highest energy data available, the X-ray ones, from the surveys I have led using both the Chandra and NuSTAR NASA satellites. These data provide us with a unique and powerful tool to find and study accreting SMBHs in the distant Universe.

The Merger-Free Co-Evolution of Galaxies and their Supermassive Black Holes

February 7, 2017

Brooke Simmons (UC San Diego)
Host: John Wardle

Abstract: Supermassive black holes and galaxies co-evolve over cosmic time, but despite more than a decade of research the engine(s) for this co-evolution are still not fully understood. The typical co-evolution picture invokes major galaxy mergers to both drive material toward the centre of the gravitational potential and trigger star formation, growing both galaxy and black hole together. This is certainly a plausible explanation for many observed systems, but there is growing evidence from both local ground-based and higher-redshift HST observations that supermassive black holes often grow in systems that cannot have had a major merger. This talk will review the field of black hole-galaxy co-evolution from 0 < z < 3 and across many orders of magnitude in the AGN luminosity and black hole mass function, and discuss the relative importance of both mergers and completely calm, "secular" evolution on black hole growth. New evidence from recent years suggests merger-free process may contribute significantly to both the overall growth of supermassive black holes and their co-evolution with their host galaxies.

How to Find Dark Matter

February 28, 2017

Bjoern Penning (University of Bristol)
Host: John Wardle

Abstract: Dark Matter (DM) is a long standing puzzle in fundamental physics and the goal of a diverse research program. I will review the evidence for DM and how to search for it. Underground and astrophysical searches attempt to detect DM particles in the cosmos directly or by searching for their decay products while particle colliders attempt to produce DM in the laboratory. Each of these detection methods probe different parts of the parameter space with complementary sensitivity in mass, interaction type, and uncertainties. I will show the connection between these searches, theoretical developments that connect their search strategies and how an interdisciplinary effort can probe the entire natural phase space in the near term future.

Assessing and Reducing the Risks of Solar Geoengineering

March 7, 2017

David Keith (Harvard)
Host: W. Benjamin Rogers

Abstract: I will discuss new results suggesting it may be possible to implement solar geoengineering using stratospheric aerosols without ozone loss while significantly reducing some other important side effects. Estimates of the risks and efficacy of solar geoengineering are deeply uncertain. Accurate physically-based models along with laboratory and in situ experiments will be needed to improve estimates of the efficacy and risks of proposed solar geoengineering methods. As an example I will discuss our ongoing laboratory experiments and plans for small perturbative outdoor experiments. Governance poses the greatest challenge for solar geoengineering: I will review some recent work on governance of research and deployment of solar geoengineering and argue in favor of an international open-access and interdisciplinary research program.

Hydrodynamic Quantum Analogs

March 21, 2017

John Bush (MIT)
Host: W. Benjamin Rogers

Abstract: A decade ago, Yves Couder and coworkers discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wavefield, and so represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results in turn revealand rationalize the emergence of quantization and quantum-like statistics from this hydrodynamic pilot-wave system in anumber of settings.

Physics Explains How a Virus Motor Stuffs a Long DNA into a Small Shell

March 28, 2017

Steve Harvey (U Penn)
Joint Quantitative Biology/Department of Physics Colloquium
Host: Michael Hagan

Abstract: Many viruses that infect bacteria (bacteriophages) have similar structures: an icosahedral shell (the capsid) surrounding the double-stranded DNA genome (dsDNA), and a tail structure designed to recognize the target bacterium, penetrate the bacterial cell wall, and deliver the DNA into the bacterial cell through a channel in the tail. The assembly of a new virus has three steps: (1) the capsid assembles spontaneously, nucleated by a protein complex (the motor) at one vertex of the icosahedron; (2) the motor captures dsDNA and drives it into the empty capsid, using the energy of ATP hydrolysis to overcome the strong electrostatic forces that resist DNA confinement; and (3) the tail assembly replaces one of the motor's components, forming the mature virus. We are investigating the second step, asking how the motor drives the DNA into the capsid. Many models have been proposed, most based on the belief that DNA is passive substrate, gripped by the motor and propelled forward with lever-like motions. I have proposed that DNA is an active component of force generation. In the original "scrunchworm" model (1), the DNA is driven cyclically between the standard A-DNA and  B-DNA conformations. A-DNA is shorter than B-DNA, and the cycle of DNA shortening and lengthening is coupled to a protein-DNA grip-and-release cycle that rectifies the motion, generating DNA translocation. Our first computer simulations on DNA within the channel of a viral portal complex provided some support for this model (2). We have now completed simulations on DNA-portal interactions in four different complexes. Those simulations have confirmed the role of the DNA shortening and lengthening motions. But they also show that the proposed mechanism, based on transitions between A-DNA and B-DNA, is not correct. Instead, the DNA conformational changes are the consequence of a surprisingly simple physical mechanism (3).

This talk is the first public presentation of these results.
(1) SC Harvey (2015), J Struct Biol 189:1-8.
(2) JT Waters, HD Kim, JC Gumbart, X-J Lu, and SC Harvey (2016), J Phys Chem B 120:6200-6207.
(3) KS Sharp, SC Harvey, et al. (manuscript in preparation)
How ocean eddies support the growth of phytoplankton

April 4, 2017

Amala Mahadevan (Woods Hole Oceanographic Institution)
Host: Albion Lawrence

Abstract: Oceanic turbulence is strongly affected by the earth’s rotation and by density stratification. Flow at large scales is mostly two dimensional and vertical velocities are suppressed. This talk will examine how the eddy field in the ocean supports the growth of phytoplankton by inducing the vertical supply of nutrients in some regions, and enhancing their exposure to light in other regions. The coupling between physical processes and phytoplankton growth will be explored in the context in different oceanic regimes.

Probing the Dark Universe

April 25, 2017

James Battat (Wellesley)
Host: Gabriella Sciolla

Abstract: Dark energy has a commanding influence on the energy budget of our Universe, forcing the Universe to expand at an ever-growing rate. Dark matter, in the form of an exotic new particle, dominates the matter budget. It's fair to say that there is plenty of opportunity for growth in our understanding of both of these constituents. I will describe two different experiments designed to pull back the curtain on this dark sector. On the one hand, I'll explain how lunar laser ranging can measure the Moon's orbit with millimeter precision to test General Relativity and other fundamental theories. On the other, I'll describe a detector designed to sense the wind of dark matter produced as our Solar System speeds through the galaxy. Together, these complementary measurements aim to shed light on the dark universe.

The Restoration of Early Sound Recordings using Optical Metrology and Image Analysis

May 2, 2017

Carl Haber (Lawrence Berkeley National Laboratory)
Host: Gabriella Sciolla

Abstract: Unlike print and latent image scanning, the playback of mechanical sound carriers has been an inherently invasive process. Some of the earliest sound recordings contain material of great historical interest, may be in obsolete formats, and are damaged, decaying, or are now considered too delicate to play. We will discuss the use of optical metrology and numerical methods to acquire and analyze high resolution digital images of the original media. The results will be illustrated with sounds and images.

Fall 2016

Active Materials : Applying the soft materials paradigm to Biology

September 6, 2016

Aparna Baskaran (Brandeis)
Host: Department of Physics

Abstract: In this talk I will introduce and discuss a recently developed class of microscopically driven materials that have been termed active materials. Drawing lessons from both biology and in vitro experimental systems, I will discuss theoretical challenges and different approaches that have proved fruitful so far. In particular, I will discuss the physics of active brownian particles and active nematics.

Cosmic inflation and quantum gravity

September 13, 2016

Albion Lawrence (Brandeis)
Host: Department of Physics

Abstract: A new generation of cosmic microwave background (CMB) experiments are poised to test “high scale” models of cosmic inflation which are highly sensitive to quantum gravitational effects.  In this talk I will review basic aspects of inflation and its imprint on the CMB, and then discuss the difficulties in constructing high-scale models which are not spoiled by quantum gravity.  I will describe a specific class of models which use nontrivial quantum field theory dynamics to evade these difficulties.

Improving student understanding of quantum mechanics

September 20, 2016

Chandralekha Singh (University of Pittsburgh)
Host: Prof. Matthew Headrick

Abstract: Learning quantum mechanics is challenging, in part due to the non-intuitive nature of the subject matter. Our research shows that the patterns of reasoning difficulties in learning quantum mechanics are often universal similar to the universal nature of reasoning difficulties found in introductory physics. Our research also shows that students often have difficulty in monitoring  their learning while learning quantum mechanics. To help improve student understanding of quantum concepts, we are developing quantum interactive learning tutorials (QuILTs) as well as tools for peer-instruction. The goal of QuILTs and peer-instruction tools is to actively engage students in the learning process and to help them build links between the formalism and the conceptual aspects of quantum physics without compromising the technical content. I will discuss the effectiveness of these learning tools based upon assessment data.

The impact of cell volume and molecular crowding on cell mechanics and gene expression

September 27, 2016

Ming Guo (MIT)
Host: W. Benjamin Rogers

Abstract: Cells alter their mechanical properties in response to their local microenvironment; this plays a role in determining cell function and can even influence stem cell fate. In this talk, I will show a robust and unified relationship between cell stiffness and cell volume. As a cell spreads on a substrate, its volume decreases while its stiffness concomitantly increases. The reduction of cell volume is a result of water efflux which leads to a corresponding increase in intracellular molecular crowding. We find that bulk modulus, cortical shear modulus and cytoplasmic shear modulus of cells all scale with cell volume, and possibly reflect the change in molecular crowding. Moreover, we have directly measured the equation of state of living mammalian cells, and find that it can be described by a hard-sphere equation of state. Finally, we find that changes in cell volume and hence stiffness alter stem-cell differentiation, regardless of the method by which these are induced. These observations reveal a surprising, previously unidentified, relationship between cell stiffness and cell volume which strongly influences cell biology, and highlight the impact of molecular crowding.

Fingers, toes and tongues: the anatomy of interfacial instabilities in viscous fluids

October 11, 2016

Irmgard Bischofberger, (MIT)
Host: W. Benjamin Rogers

Abstract: The invasion of one fluid into another of higher viscosity is unstable and produces complex patterns in a quasi-two dimensional geometry. This viscous-fingering instability, a bedrock of our understanding of pattern formation, has been characterized by a most-unstable wavelength that sets the characteristic width of the fingers. We have shown that a second, previously overlooked, parameter governs the length of the fingers and characterizes the dominant global features of the patterns.Because interfacial tension suppresses short-wavelength fluctuations, its elimination would suggest an instability producing highly ramified singular structures. Our experimental investigations using miscible fluids show the opposite behavior – the interface becomes more stable even as the stabilizing effect of interfacial tension is removed. This is accompanied by slender structures, tongues, that form in the narrow thickness of the fluid. Among the rich variety of global patterns that emerge is a regime of blunt structures, “toes”, that exhibit the unusual features characteristic of proportionate growth. This type of pattern formation, while quite common in mammalian biology, was hitherto unknown in physical systems.

Fault-tolerant quantum computation in the 21st century

October 18, 2016

Daniel Gottesman (Perimeter Institute)
Host: Matthew Headrick

Abstract: Experimentalists are getting better and better at building qubits, but no matter how hard they try, their qubits will never be perfect. In order to build a large quantum computer, we will almost certainly need to encode the qubits using quantum error-correcting codes and encode the quantum circuits using fault-tolerant protocols. This will eventually allow reliable quantum computation even when the individual components are imperfect. I will review the current state of the art of quantum fault tolerance and discuss progress towards answering the most important questions that will enable large fault-tolerant quantum computers.

Localization: Moving Beyond Statistical Mechanics

November 1, 2016

Christopher Laumann (Boston University)
Host: Albion Lawrence

Abstract: The central assumption of statistical mechanics is that interactions between particles establish local equilibrium. Isolated quantum systems, however, need not equilibrate; this happens, for example, when sufficient quenched disorder causes localization. The many-body localized (MBL) phase transports neither heat nor charge; may possess orders disallowed in equilibrium; and, may exhibit quantum coherence even when highly excited. We will review the emerging understanding of how quantum localization can lead to new quantum phenomena even in highly excited states. I will give some theoretical intuition about how this might be used to build a better quantum computer and also review some of the latest experiments investigating localization.

Friction and adhesion in colloids: Yielding, thickening, jamming

November 8, 2016

Jeffrey Morris (CUNY)
Joint IGERT/Physics Department Colloquium
Host: Bulbul Chakraborty

Abstract: In recent work, we have shown [1,2] that frictional interactions provide a rational basis for both continuous and discontinuous shear thickening in viscous suspensions. When the repulsive forces (such as those due to electrostatic or steric colloidal stabilization) are overwhelmed by shearing forces, contact is assumed to occur, and the system transitions from a low-viscosity (lubricated) to a high-viscosity (frictional) state. Contacting particles may experience both adhesive forces as well as friction. We will consider the influence of attractive forces at contact, in combination with the stabilizing repulsive forces. This combination of forces would be seen in the case of particles with van der Waals attraction in combination with colloidal stabilization. For sufficient attractive force a yield stress and shear thinning give way to the shear thickening response, a behavior observed in certain flocculated dispersions. At sufficient yield stress, the shear thickening is completely obscured, as the dispersions shear thins after yielding directly onto the high-viscosity (frictional) plateau. The suggestion that a material may exhibit both yielding at low stress and jamming at large stress [3] is explored. 

  1. R. Seto, R. Mari, J. F. Morris & M. M. Denn 2013 "Discontinuous shear thickening of frictional hard-sphere suspensions" Phys. Rev. Lett. 111 218301
  2. R. Mari, R. Seto J. F. Morris & M. M. Denn 2015 "Discontinuous shear thickening in Brownian suspensions by dynamic simulation" Proc. National Acad. Sci.  112. 15326
  3. N. J. Wagner & J. F. Brady 2009 "Shear thickening in colloidal dispersions" Phys. Today62, 27-32

November 15, 2016

Eisenbud Lecture Series in Mathematics and Physics
Nigel Hitchin (University of Oxford)

Abstract: Euler’s equations for a spinning top are well-known to be solvable by elliptic functions. They form the first example of a much wider range of equations, in particular Nahm’s equations, which are solvable using algebraic curves of higher genus. Nahm’s equations appear in various parts of differential geometry and physics, related to hyperk ahler geometry and magnetic monopoles in particular. Loosely speaking, the equations are linearized on the Jacobian of the curve. However, there are many situations where that curve is singular or non-reduced and this viewpoint is no longer valid. The talk will discuss the geometry of what happens in some of these cases.

November 16, 2016

Eisenbud Lecture Series in Mathematics and Physics, Lecture II
Nigel Hitchin (University of Oxford)

Abstract: The theory of Higgs bundles on a compact Riemann surface provided a natural setting for hyperbolic surfaces within the context of an SU(2)-gauge theory with a complex Higgs field. Replacing the group SU(2) by the group of symplectic diffeomorphisms of the two-sphere provides, thanks to work of Biquard, an infinite-dimensional gen eralization of Teichm ̈uller space, but it is as yet unclear what type of geometry, generalizing hyperbolic metrics, on the surface this parametrizes. The lecture will investigate some of the questions and features involved.

November 18, 2016

Eisenbud Lecture Series in Mathematics and Physics, Lecture III
Nigel Hitchin (University of Oxford)

Abstract: The moduli space of Higgs bundles on a curve, together with its fibration structure as an integrable system, forms a natural example to examine the predictions of mirror symmetry in the approach of Strominger, Yau and Zaslow. The mirror for gauge group G is regarded as being the moduli space for the Langlands dual group LG. Of particular interest is the how this manifests itself in the duality of “branes” on each side. We consider in the talk cases arising from noncompact real forms of complex groups, and also Lagrangians arising from the existence of holomorphic spinor fields.

The Physics of the Deep Ocean Circulation

November 29, 2016

Raffaele Ferrari (MIT)
Host: Albion Lawrence

Abstract: The deep ocean circulation is fed by waters that become dense enough to sink into the ocean abyss at high latitudes and return to the surface through convoluted three dimensional pathways. While the physics behind the sinking of dense waters is well understood, the physics that allows waters to rise back to the surface remains elusive. It is generally believed that small-scale turbulent mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters. However the observational evidence that the turbulent fluxes generated by small-scale turbulent mixing in the stratified ocean interior are more vigorous close to the ocean bottom than above implies that small-scale turbulent mixing converts light waters into denser ones, thus driving a net sinking of abyssal water. Using a combination of numerical models and observations, it will be shown that abyssal waters return to the surface along weakly stratified boundary layers, where the small-scale turbulent mixing of density decays to zero. The net ocean meridional overturning circulation is thus the small residual of a large sinking of waters, driven by small-scale turbulent mixing in the stratified interior, and a comparably large upwelling, driven by the reduced small-scale turbulent mixing along the ocean boundaries.

Shape From Mechanics: Designing and Understanding Self-Folding Origami

December 6, 2016

Chris Santangelo (UMass Amherst)
Host: W. Benjamin Rogers

Abstract: Origami, the ancient art of paper folding, has probably been around as long as paper. Yet, recent advances in materials have enabled the fabrication of self-folding origami structures, sometimes called “4d printing.” These self-folding structures promise to revolutionize the manufacture of complex structures on a variety of scales, yet realizing this has proven challenging. He will discuss recent work with collaborators on self-folding origami structures, focusing on the theory behind the mechanics of such structures, our limited understanding of how to design shape, and prospects for self-folding and responsive structures.