Calendar of department colloquia

NOTE: Colloquia highlighted in red are astronomy/cosmology related.

 

FALL 2014 semester:


All colloquia start at 3 pm and take place in Room 136, Science and Technology Center; refreshments served at 2:30 pm in the adjacent room.


Sep 12: Prof. Seth Fraden (Martin A. Fisher School of Physics, Brandeis University)


TITLE: Testing Turing


ABSTRACT: It’s been 61 years since Alan Turing’s groundbreaking paper, The Chemical Basis of Morphogenesis, in which he showed a general mechanism for how a set of identical cells could differentiate into distinct species. Particularly notable was his counterintuitive discovery that diffusion can interact with chemical kinetics to generate temporally stationary, spatially periodic structures (“Turing patterns”), which spawned a plethora of efforts to model biological patterns (e.g. zebra stripes, leopard spots). What is less well appreciated is that it took four decades for the first experimental demonstration of Turing’s predictions, that clearcut experimental evidence of Turing patterns remains rare, and that Turing proposed several other modes of pattern formation. I will introduce the Turing model and describe an experimental reaction-diffusion system ideally suited for testing all of Turing’s ideas. It consists of a microfluidically produced two-dimensional array of diffusively coupled droplets containing the constituents of the oscillatory chemical reaction. We find a remarkable variety of oscillatory and stationary examples of chemical and physical morphogenesis, some predicted by Turing, others not.



Sep 19: Dr. John Callas (NASA JPL - Pasadena City College)


TITLE: The Mars Exploration Rovers: A Decade of Exploration


ABSTRACT: It was inconceivable that a rover mission designed for 90 days of operation would still be operating after a decade in the harsh environment of the Red Planet's frigid surface.  In spite of our limited imagination, the Mars Exploration Rover Opportunity is still functioning and exploring ten years after landing on Mars.  For a decade now, the rover has been dutifully conducting field geology on the Martian surface day after day.  Opportunity and her twin Spirit have traversed great plains, climbed distant mountains, descended into deep craters and survived rover-killing dust storms and frigid, dark winters.  As the rovers traverse, each day becomes a brand new mission with new vistas, new geology and new opportunities for exploration.  Both rovers have made significant scientific contributions to understanding the Red Planet, finding evidence of past wet, habitable environments that could have supported life.  Although Spirit's mission concluded after an unimaginable six years, exciting exploration remains ahead for the still very capable Opportunity rover, even after ten years.



Sep 26: Prof. Chris Quigg (Fermilab)


TITLE: Particle Physics in a Season of Change


ABSTRACT: How the symmetry that links the weak and electromagnetic interactions is spontaneously broken has been for decades one of the most urgent and challenging questions in particle physics. The minimal hypothesis is an elementary scalar field whose self-interactions select a vacuum state in which the full electroweak symmetry is hidden. Recently, the ATLAS and CMS Collaborations, working at CERN’s Large Hadron Collider, have found a narrow state with properties broadly consistent with those expected for the avatar of electroweak symmetry breaking—the Higgs boson. Their discovery opens a new chapter in our quest to understand the attoworld. A discussion of what we have learned and what remains to be learned about electroweak symmetry breaking and the problem of mass. We will consider some other important themes in particle physics in light of new experimental information from the LHC and elsewhere, and will aim to deconstruct some of the central questions we will face in the near future.



Oct 3: Prof. Ryan Hickox (Dartmouth University)


TITLE: The monsters within: The cosmic evolution of black holes, galaxies, and dark matter halos


ABSTRACT: Supermassive black holes are amazingly exotic and yet ubiquitous objects, residing in the centers of essentially all stellar bulges in galaxies. Recent years have seen remarkable advances in our understanding of how these black holes form and grow over cosmic time, and how energy released by active galactic nuclei connects the growth of black holes to their host galaxies and dark matter halos. I will review a few recent observational and theoretical studies that explore AGN activity over a wide range of scales, using a variety of techniques from observations of individual objects to simulations of whole cosmological volumes. Together, these studies are revealing a detailed yet remarkably simple picture of how black holes grow and influence their surroundings, and show that black holes have an important (and perhaps unexpected) role to play in history of the Universe.



Oct 10: Prof. Cristina Marchetti (Syracuse Biomaterials Institute - Syracuse University)


TITLE: Active Matter


ABSTRACT: Systems ranging from bird flocks to bacterial suspensions to colloids propelled by self-catalytic reactions are examples of active matter – individually driven, dissipative units that self-organize in collectives with coordinated motion at large scales. In this talk I will highlight common properties of these diverse systems and describe recent progress in understanding and classifying their complex behavior using modeling and simulations.



Oct 17: Prof. Lenore L. Dai (Arizona State University)


TITLE: Pickering Emulsions and Beyond


ABSTRACT: Emulsions are ubiquitous in natural and industrial processes. Conventional emulsions use organic surfactants as stabilizers. Although solid particle stabilized emulsions (Pickering emulsions) are often encountered in crude oil recovery, oil separation, cosmetic preparation, and wastewater treatment, the phenomenon is not well-understood. Here we investigate the fundamentals and applications of Pickering emulsions. Using laser scanning confocal microscopy, we have studied the self-assembly of solid particles at oil-water and ionic liquid based interfaces. In addition, we have employed molecular dynamics (MD) simulations to understand the self-assembly and dynamics of nanoparticles at these interfaces. Finally, we employ Pickering emulsions as a new and convenient model system to investigate the dynamics of microparticles at liquid-liquid interfaces and develop one- particle and two-particle interfacial microrheology.



Oct 24: Prof. William Oliver (Tufts University)


TITLE: Particle physics (a narrative)


ABSTRACT: The development and verification of important ideas in particle physics is described with no more mathematics than is necessary.



Nov 7: Dr. Mark Lacy (National ALMA Science Center, NRAO)


TITLE: Do supermassive black holes stunt galaxy growth?


ABSTRACT: One of the current mysteries of galaxy formation is the exponential cutoff in the galaxy mass function at >10^11 solar masses. Explanations for this typically invoke supermassive black holes at the center of galaxies, which, as they accrete matter, eject energetic winds and jets during quasar activity that can disrupt or expel the interstellar medium from which stars in the galaxies form. In this talk, I will discuss how we have made an improved census of accreting black holes by selecting quasars in the mid-infrared, and how this can be used to show that supermassive black holes are ubiquitous in massive galaxies. I will also discuss the influence of less dramatic nuclear activity in galaxies, and its likely importance in keeping gas from falling back after quasar events have run their course.



Nov 21: Dr. Jane Luu (MIT, Lincoln Lab)


TITLE: TBD


ABSTRACT: TBD.





SPRING 2014 semester:


All colloquia start at 3 pm and take place in Robinson Hall, Room 253.


Jan 17: Prof. Stefano Profumo (UC - Santa Cruz)


TITLE: New Physics from the Sky: Cosmic Rays, Gamma Rays, and the Hunt for Dark Matter


ABSTRACT: Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental nature of dark matter. I will discuss some of the recent puzzling findings in cosmic-ray electron-positron data and in gamma-ray observations that might be related to dark matter. I will argue that cosmic-ray data, most notably from the AMS, Pamela and Fermi satellites, indicate that previously unaccounted-for powerful sources in the Galaxy inject high-energy electrons and positrons. Interestingly, this new source class might be related to new fundamental particle physics, and specifically to pair-annihilation or decay of galactic dark matter. This exciting scenario is directly constrained by Fermi gamma-ray observations, which also inform us on astrophysical source counterparts that could be responsible for the high-energy electron-positron excess. Observations of the gamma-ray emission from the central regions of the Galaxy as well as claims about a gamma-ray line at around 130 GeV also recently triggered a wide-spread interest: I will address the question of whether we are really observing signals from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.


Jan 24: Dr. Paul Green (Harvard - CfA)


TITLE: Innocent Bystanders and Smoking Guns: the Dwarf Carbon Stars


ABSTRACT: Fellow carbon-based life forms, as far as we know, most carbon throughout the Universe is created and dispersed by (AGB) stars in their final breaths. So it was at first surprising to find that the carbon stars most prevalent in the Galaxy are in fact dwarfs. We suspect that these dC stars are most likely innocent bystanders in post-mass transfer binaries, and may be predominantly metal-poor. As such, they may retain the chemical and orbital imprints of the first stars. Among 1200 C stars found in the SDSS, we confirm 724 dCs, of which a dozen are DA/dC stars in composite spectrum binaries, quadrupling the total sample of these "smoking guns" for AGB binary mass transfer. Whether you think of them as 'enhanced' or 'polluted' we'll review the ubiquitous anomaly of C-enriched main sequence stars and several new projects to study them further.


Jan 31: Prof. Tomasz Taylor (Northeaster University - Boston)


TITLE: Unity of Amplitudes


ABSTRACT: Theoretical understanding of proton-proton collisions at the Large Hadron Collider is based on the standard model - a quantum field theory of quarks and leptons interacting with gauge fields. Some of the most important quantum field-theoretical observables are the amplitudes describing multi-particle, relativistic processes in which matter, antimatter particles and gauge bosons are scattered, created and/or annihilated. Over the last decade, there has been enormous progress in understanding the structure of scattering amplitudes in the standard model. It is less known that some significant progress has been also accomplished in gravity and in string theory, where the scattering amplitudes of hypothetical gravitons offer a testing ground for some new ideas about the relation of gauge theory to quantum gravity. I will describe some recent developments which point towards a fascinating unity of all gauge, gravity and string amplitudes.


Feb 7: Prof. David Nelson (Harvard University)


TITLE: Gene Surfing and Survival of the Luckiest


ABSTRACT: It is widely appreciated that population waves have played a crucial role in the evolutionary history of many species. In parallel with Fokker-Planck descriptions of stochastic processes in physics, population geneticists have developed methods for understanding mutations, genetic drift and selective advantage in such situations. Provided number fluctuations at the frontier are taken into account, neutral genetic markers can be used to infer information about growth, ancestral population size and colonization pathways. Neutral mutations optimally positioned at the front of a growing population wave can increase their abundance via a "surfing" phenomenon. Experimental and theoretical studies of this effect will be presented, using bacteria and yeast as model systems.


Feb 14: Prof. Joao Guimaraes da Costa (Harvard University)


TITLE: A Closer Look at the Higgs Boson with the Large Hadron Collider


ABSTRACT: Scientists at CERN have been exploring the high energy frontier with the Large Hadron Collider since March 2010. The substantial dataset accumulated thus far, albeit at lower energy than initially foreseen, already yielded a Nobel Prize award for the discovery of the Higgs Boson. The new boson, discovered in 2012 by the ATLAS and CMS collaborations, has been shown to behave very much like the long-sought-after Higgs Boson, and hence it completes the discovery of the Standard Model of Particle Physics. Remarkably, no other deviations from the Standard Model have been found, neither in precision measurements nor in direct searches for new particles. The LHC will resume operations in 2015, after a 2-year shutdown, with increased center of mass energy, and thus, with increased potential for new discoveries. In this talk, I will review recent measurements at the LHC, with a focus on the study of the properties of the newly discovered boson, and I will briefly discuss what we expect to learn from the future LHC data.


Feb 21: Dr. Shannon Curry (Space Sciences Laboratory, UC Berkeley)


TITLE: Atmospheric escape on Mars: the Mars Atmosphere and Volatile Evolution mission (MAVEN)


ABSTRACT: Because Mars and Earth underwent similar processes in their formation, Mars serves as an excellent subject for comparing how the planets have evolved and why these planets are so different. Geomorphological evidence suggests that liquid water also existed on Mars when it had a much warmer, thicker atmosphere that has since evolved into the much colder and thinner atmosphere of the present day. While some of this water may be frozen on or below the surface, a portion may have escaped to deep space as neutral or charged particles. Consequently, studying the current atmospheric production and loss of oxygen and hydrogen addresses the bigger question of how the presence of water (H2O) has evolved on Mars. The Mars Atmosphere and Volatile Evolution mission (MAVEN) launched on November 18, 2013, and is on track to arrive at Mars on September 21, 2014. The goal of NASA’s next Mars scout is to explore the atmosphere, ionosphere, and interactions with the Sun and solar wind. A suite of instruments aboard MAVEN will provide insight into the loss rates of volatile compounds from the Martian atmosphere to space. In particular, non-thermal loss mechanisms such as pick-up ion escape and sputtering will be discussed. Understanding these processes that drive atmospheric loss will give scientists insight into the history of Mars' atmosphere and climate, liquid water, and planetary habitability.


Feb 28: Prof. Steven Pollock (University of Colorado at Boulder)


TITLE: A research-validated approach to transforming upper-division physics courses


ABSTRACT: At most universities, including the University of Colorado, upper-division physics courses are taught using a traditional lecture approach that does not make use of many of the instructional techniques that have been found to improve student learning at the introductory level. We are transforming upper-division courses (E&M, quantum, and Classical Mechanics) using principles of active engagement and learning theory, guided by the results of observations, interviews, and analysis of student work at CU and elsewhere. I will outline these reforms including consensus learning goals, clicker questions, tutorials, modified homeworks, and more, as an example of what a transformed upper-division course can look like, and as a tool to offer insights into student difficulties in advanced undergraduate topics. We have examined the effectiveness of these reforms relative to traditional courses, based on grades, interviews, and attitudinal and conceptual surveys. Our results suggest that it is valuable to further investigate how physics is taught at the upper-division, and how education research may be applied in this context.


Mar 7: Prof. Laura Ferrarese (NRC-Canada; Physics and Astronomy Department, University of Victoria)


TITLE: The Virgo Cluster of Galaxies


ABSTRACT: At a distance of 16.5 Mpc and with a gravitating mass of 4.2×10^14 solar masses, the Virgo Cluster is the dominant mass concentration in the local universe, the centre of the Local Supercluster, and the largest concentration of galaxies within ~35 Mpc. With thousands of member galaxies lying at a nearly common distance and spanning virtually all known morphological types, it has historically played a key role in studies of how galaxies form and evolve in dense environments. It is, without question, the most thoroughly studied cluster of galaxies in the universe, and remains a preferred target for a systematic survey of baryonic substructures in the low-redshift universe. In this talk, I will describe an ambitious optical imaging survey of the Virgo cluster, the Next Generation Virgo Cluster Survey (NGVS), that is being carried out using the MegaPrime instrument at the Canada France Hawaii Telescope (CFHT). The NGVS is designed to address a wide range of fundamental astrophysical questions, including: the faint-end shape of the luminosity function, the characterization of galaxy scaling relations over a factor 10^7 in mass, the cluster/intracluster medium/galaxy connection, and the fossil record of star formation and chemical enrichment in dense environments. I will present a brief overview of the NGVS and discuss preliminary results. Some details about the NGVS can be gathered from the survey webpage:

https://www.astrosci.ca/NGVS/The_Next_Generation_Virgo_Cluster_Survey/Home.html.


Mar 14: Prof. Jeff Urbach (Georgetown University)


TITLE: Nonlinear mechanics of stiff biopolymer networks


ABSTRACT: Unlike homogeneous continuous solids, some disordered granular materials show heterogeneous propagation of externally applied stresses along localized linear chains. Similarly, disordered networks of stiff or semi-flexible filaments display unusual mechanical properties, including dramatic stiffening when sheared, but little is known about the spatial distribution of stresses. This talk will introduce the technique of Boundary Stress Microscopy, which adapts the approach of traction force microscopy to rheological measurements in order to quantify the non-uniform surface stresses in sheared soft materials. Our results on networks of the biopolymer collagen, a major component of the extracellular matrix, show stress variations over length scales much larger than the network mesh size. The strain stiffening behavior over a wide range of mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak stress at both the characteristic strain and at yielding are remarkably insensitive to concentration. These results show the power of Boundary Stress Microscopy to reveal the nature of stress propagation in disordered soft materials, which is critical for understanding many important mechanical properties, including the ultimate strength of a material and the nature of appropriate microscopic constitutive equations.


Mar 28: Prof. Marcin Sawicki (Saint’s Mary University)


TITLE: Life and Death at Cosmic High Noon


ABSTRACT: Galaxies are giant machines that turn gas into stars and the rate at which they were doing this was highest around redshift z=2, when the universe was only 1/4 of its present age. This is "cosmic high noon", and I will discuss both starforming (live) and quiescent (dead) galaxies at z~2. My choice of this live/dead terminology here is not just a fanciful analogy but is central to my talk: I will show how the starforming-to-quiescent transition for galaxies follows rules that are very similar to those that govern human death, leading not only to a well-justified "live/dead" terminology for star forming and quiescent galaxies, but to a simple and direct explanation for the observed distribution of masses of quiescent galaxies in the distant Universe.


Apr 11: Dr. Cristian Staii (Tufts University)


TITLE: Dynamics of neuronal growth on controlled substrates


ABSTRACT: Physical stimuli (stiffness of the growth substrate, gradients of various molecular species, geometry of the surrounding environment, traction forces etc.) play a key role in the wiring up of the nervous system. I will present a systematic experimental and theoretical investigation of neuronal growth on substrates with asymmetric geometries and textures. The experimental results show unidirectional axonal growth on these substrates. We demonstrate that the unidirectional bias is imparted by the surface ratchet geometry and quantify the geometrical guidance cues that control neuronal growth. I will also discuss results obtained in our research group, which combine Atomic Force Microscopy and Fluorescence Microscopy measurements to produce systematic, high-resolution elasticity maps for different types of live neuronal cells cultured on glass or biopolymer-based substrates. We measure how the mechanical properties of neurons change both during axonal outgrowth and upon chemical modification (disruption of the cytoskeleton) of the cell. Our results provide new insight into the fundamental role played by physical cues in neuronal growth, and could lead to new methods for stimulating neuronal regeneration and engineering neuronal networks.


Apr 4: Dr. Danilo Marchesini (Tufts University)


TITLE: New Insights into the Formation and Evolution of Today’s Most Massive Galaxies


ABSTRACT: In the past decade, our understanding of the galaxy population in the last 12 billion years of cosmic history has improved enormously, thanks to the increasing ability to construct representative snapshots (in time) from redshift z=4 (when the universe was ~1.5 billion years old) to the local universe. I will summarize our current knowledge of the evolution of massive galaxies in the last 12 billion years of cosmic history (i.e., since z=4), with an emphasis on the recent results from the UltraVISTA survey. I will then present new findings on the evolution of the progenitors of local ultra-massive galaxies over the past 11.2 billion years (i.e., since z=3), challenging previously proposed pictures for the formation and evolution of elliptical galaxies. I will conclude by presenting new exciting observational programs aiming at furthering our understanding on galaxy formation.


Apr 18: Prof. Ned Wingreen (Princeton University)


TITLE: TBA


ABSTRACT: TBA


Apr 25: Dr. Sing Chandralekha ()


TITLE: TBA


ABSTRACT: TBA





FALL 2013 semester:


All colloquia start at 3 pm and take place in the Science and Technology Center, Room 136, EXCEPT for the special colloquium n Thursday October 3, which starts at 3 pm and will take place in Robinson 253.


Sep 6: Prof. Robert Austin (Princeton University)


TITLE: Game Theory and Cancer


ABSTRACT: Game Theory has interesting implications for the competing strategies of cells fighting for survival in a cancer patient. I'll start with a very short tutorial on Game Theory, follow up with some data/observations that might implicate game theory, and conclude with the possibilities  towards a serious synthesis of Game Theory and cancer at a quantitative, testable level.


Sep 13: Prof. Eric Dufresne (Yale University)


TITLE: Young’s Law is Dead…Long Live Young’s Law: Wetting and Adhesion on Soft Surfaces


ABSTRACT: Liquids and solids tend to stick to each other.  When a liquid droplet sticks to a solid surface we call it wetting.  When a solid particle sticks to a solid surface we call it adhesion.  Our classical coarse-grained descriptions of these two phenomena are quite distinct from each other.  Both descriptions assume that solid objects undergo very little deformation during wetting and adhesion.  In this talk, I will show how this assumption breaks down on when solids are sufficiently soft and how wetting and adhesion really are not that different after all.


Sep 20: Prof. Dawn Meredith (University New Hampshire)


TITLE: Rounding off the cow:  A new approach to teaching physics to life science majors


ABSTRACT: The introductory course for life science majors (IPLS) is being revisited and revised at many colleges and universities, prompted in part by national position papers such as Bio2010 and Scientific Foundations for Future Physicians.  I will discuss the challenges and successes that have been encountered in  designing an interdisciplinary IPLS course at the University of New Hampshire and elsewhere,  and conclude with a discussion of our current work on curriculum development for the difficult but essential topic of fluid dynamics.


Sep 20: Prof. Larry Ford (Tufts University)


TITLE: The Physics of Quantum Fluctuations: A Window into Quantum Gravity?


ABSTRACT: The quantum effects of gravity are expected to be extremely small in the present day universe, apparently precluding direct experimental detection. This talk will explore some possible ways to improve this situation. One is the study of analog models in condensed matter systems, where quantum field fluctuations can mimic the effects of quantum gravity fluctuations. Another possibility is the existence of amplification mechanisms which enhance the magnitude of the fluctuations. Some examples will be discussed. The goal of this line of study is both to cast light on quantum gravity effects, and to deepen our understanding of the basic physics of quantum field fluctuations.


Oct 3: Prof. Jack Steinberger (CERN) - SPECIAL COLLOQUIUM


TITLE: TBA


ABSTRACT: TBA.


Oct 4: Prof. David Spergel (Princeton University)


TITLE: Planck and Beyond


ABSTRACT: The Planck satellite has made an accurate full-sky measurement of the microwave background temperature fluctuations. These measurements probe both the physics of the very early universe and the basic properties of the universe today. The Planck measurements confirm the earlier WMAP and ground-based results, rigorously test our standard cosmological model and provide an accurate determination of basic cosmological parameters (the curvature of the universe, its matter density and composition). When combined with other astronomical measurements, the measurements constrain the properties of the dark energy and the mass of the neutrino. The observations also directly probe the physics of inflation: the current data imply that the primordial fluctuations were primarily adiabatic and nearly scale invariant. Many key cosmological questions remain unanswered: what happened during the first moments of the big bang? what is the dark energy? what were the properties of the first stars? I will discuss the role of on-going and future CMB observations in addressing these key cosmological questions and describe how the combination of large-scale structure, supernova and CMB data can be used to address these questions.


Oct 11: No Colloquium


Oct 18: Prof. Zvonimir Dogic (Brandeis University)


TITLE: TBA


ABSTRACT: TBA.


Oct 25: Prof. Arvind Raman (Purdue University)


TITLE: TBA


ABSTRACT: TBA.


Nov 1: Prof. Min Yun (University of Massachusetts - Amherst)


TITLE: Cold Gas Content and Star Formation in Galaxies


ABSTRACT: Cosmological simulations of large scale structures and mass assembly have shown that the gas accretion rate onto DM halos broadly tracks the observationally established cosmic star formation density evolution, and semi-analytic incorporation of baryon physics has been applied to these simulations to model the build up of ISM and stellar mass in galaxies.  I will examine the current data on cold gas content in present day galaxies and their relation to other physical properties such as stellar mass, color, and star formation rate, in the context of understanding the limitations and utilities of such modeling as a tool for studying the mass assembly history of galaxies.


Nov 8: Prof. Gregory Rudnick (University of Kansas)


TITLE: The Transformation of Galaxies in Dense Environments over Cosmic Time


ABSTRACT: Galaxies fall into three main categories.  They are either actively forming stars, have little or no star formation, or are in transition between those two populations. The stellar mass density of the Universe today is dominated by passive galaxies and understanding their nature and evolution is therefore necessary in creating a unified picture of how galaxies evolve.  Indeed, the total stellar mass in the passive population has doubled over the last 8 billion years, implying that star forming galaxies are being transformed into passive ones.  A major unsolved problem in galaxy evolution is what governs this transformation and to what extent is due to a process that is extrinsic to the galaxies.  Galaxy clusters are a useful laboratory for studying how galaxies are altered by their surroundings as clusters are the largest quasi-virialized objects in the Universe. I will summarize what we have learned about the the transformation of galaxies using a detailed study of clusters that extend over 10 billion years of cosmic time.  Using an extensive multi-wavelength data set we are now forming a picture in which infalling cluster galaxies likely have their gas supplies cut off, their morphologies transformed, and may even experience epochs of very frequent mergers. I will briefly conclude by highlighting our recent observational efforts to observe galaxies in the lower density regions surrounding clusters, where the transformation may actually be occurring.


Nov 15: Dr. Chris Hayward (Heidelberg Institute for Theoretical Studies)


TITLE: Advances in galaxy-formation simulations: calculating mock observables & using a more-accurate numerical technique


ABSTRACT: Galaxy formation has been studied using idealized numerical simulations of isolated disk galaxies and galaxy mergers for decades, but most simulations performed to date have suffered from two potentially significant limitations: First, when comparing simulations with observations, physical quantities - rather than observables - from the simulations are used. Second, the most-commonly used techniques, smoothed-particle hydrodynamics (SPH) and adaptive mesh refinement, suffer from numerical inaccuracies that can potentially jeopardize the results of simulations performed with those techniques.

I will discuss methods for solving both of these limitations. I address the first limitation by performing 3-D dust radiative transfer on hydrodynamical simulations to calculate spatially resolved UV-mm spectral energy distributions of simulated galaxies. I will present an application to sub-millimeter galaxies, for which a realistic comparison with observables yields results that are qualitatively different from those of more naive comparisons. I address the second limitation by using the more-accurate moving-mesh hydrodynamics code Arepo. I will discuss how merger simulations performed with the moving-mesh technique differ from otherwise identical simulations performed using SPH. Finally, based on this comparison and other work, I will outline the types of galaxy-formation simulations for which the traditional formulation of SPH is sufficiently accurate and describe when and why this is not the case.


Nov 22: Prof. Tyce DeYoung (Penn State)


TITLE: TBA


ABSTRACT: TBA.




SPRING 2013 semester:


Jan 18: Prof. Daniel Eisenstein (Harvard - CfA)


TITLE: Dark Energy and Cosmic Sound


ABSTRACT: I will discuss how the acoustic oscillations that propagate in the photon-baryon fluid during the first million years of the Universe provide a robust method for measuring the cosmological distance scale. The distance that the sound can travel can be computed to high precision and creates a signature in the late-time clustering of matter that serves as a standard ruler. Galaxy clustering results from the Sloan Digital Sky Survey reveal this feature, giving a geometric distance to a redshift of 0.35 and 0.57 and an accurate measurement of Omega_matter. I will review our recent work on the theory and practice of the acoustic oscillation method and our latest cosmology results from SDSS-II and SDSS-III on the expansion history of the Universe.



Jan 25: Prof. Andrew West (Boston University)


TITLE: What can Low-mass Stars tell us about the Galaxy, the Habitability of Exoplanets and the Evolution of Stellar Dynamos


ABSTRACT: My primary goal of this seminar is to demonstrate that we can do big science with little stars. M and L dwarfs are the smallest, coolest and least massive stars in the Galaxy. Yet despite their diminutive physical properties, low-mass stars make up ~70% of all of the stars in the Milky Way galaxy and have lifetimes that exceed trillions of years. Their dominance in the Galaxy make low-mass stars excellent tracers of both the structure and evolution of the local Milky Way. In addition, low-mass stars have intense stellar flares and strong magnetic fields that allow us to probe their interiors and may have important consequences for their space weather environments and the habitability of planets that orbit them. I will present results from the largest samples of low-mass stars ever assembled. The advent of large surveys such as the Sloan Digital Sky Survey (SDSS) has yielded photometric and spectroscopic catalogs of more than 100 million and 70,000 stars respectively. Specifically, I will highlight work that has used the unprecedented statistical power of the SDSS to examine the structure and kinematics of low-mass stars in the Milky Way, as well as the nature of their magnetic fields (and subsequent "magnetic activity") and what this may tell us about the ages of stars. In addition, I will share some resent results from follow-up observations at Lowell, Magellan, KPNO and the Fred Whipple observatories to calibrate and confirm findings from our survey data. In particular, I will highlight the confirmation of an age-rotation-activity relation that has come from a collaboration with the MEarth planet hunting team, results from follow-up observations of some of the widest binaries in the Milky Way and demonstrate how a large sample of M dwarfs has helped us map the three-dimensional distribution of dust in the local Galaxy.



Feb 1: Prof. Max Tegmark (MIT)


TITLE: Understanding our Quantum Universe


ABSTRACT: I first describe how redshifted radiation from the hyperfine quantum transition in hydrogen can potentially provide the largest 3D map our universe and shed new light on dark matter, dark energy, neutrino masses and our early universe. I then discuss ways in which cosmology can help us understand quantum mechanics better, generalizing the second law of thermodynamics in a way that helps explain the arrow of time and producing the Born rule for calculating probabilities in quantum mechanics.



Feb 8: Cancelled due to snow storm



Feb 15: Prof. Michael Stoelzner (University of South Carolina)



Feb 22: Prof. Elena D’Onghia (University of Wisconsin-Madison)


TITLE: Self-perpetuating Spiral Arms in Disk Galaxies


ABSTRACT: The causes of spiral structure in galaxies remain uncertain. Leaving aside the grand bi-symmetric spirals with their own well-known complications, here we consider the possibility that multi-armed spiral features originate from density inhomogeneities orbiting within disks. Using high-resolution N-body simulations, we follow the motions of stars under the influence of gravity, and show that mass concentrations with properties similar to those of giant molecular clouds can induce the development of spiral arms through a process termed swing amplification. However, unlike in earlier work, we demonstrate that the eventual response of the disk can be highly non-linear, significantly modifying the formation and longevity of the resulting patterns. Contrary to expectations, ragged spiral structures can thus survive at least in a statistical sense long after the original perturbing influence has been removed.



Mar 1: Prof. Sergio Fantini (Tufts University)


Mar 8: Prof. Jane Kondev (Brandeis University)


Mar 15: no colloquium


Mar 22: SPRING BREAK - no colloquium


Mar 29: Prof. Stamatis Vokos (Seattle Pacific University)


Apr 5: Dr. Jennifer Lotz (Space Telescope Science Institute)


TITLE: Galaxy Mergers through Cosmic Time


ABSTRACT: Galaxies grow with time through both discrete galaxy mergers and smooth gas accretion. When and how this growth occurs, and the role of mergers in defining the properties of today's galaxies, remain outstanding observational questions.  Observational estimates of the galaxy merger rate and its evolution can vary by factors of 10, depending upon the method and assumptions used to count mergers. Using physical-motivated timescales from a large suite of galaxy merger simulations, I am able to reconcile the discrepancies between different measurements of the galaxy merger rate at z<1.  The frequency of gas-rich mergers has increased strongly from z~0 to z~1, while the global galaxy merger rate evolved more modestly. Finally, I will discuss the challenges with identifying galaxy mergers at z~2 and beyond with the Cosmic Assembly Near-infrared Extragalactic Legacy Survey.



Apr 12: Prof. Gerald Guralnik (Brown University)



Apr 19: Prof. Susan Coppersmith (University of Wisconsin - Madison)



Apr 26: Prof. Bruce Partridge (Haverford College)



May 3: Prof. Krzysztof Sliwa (Tufts University)




FALL 2012 semester:


Sep 7: Prof. Noah Finkelstein (Colorado University - Boulder)


Sep 14: Prof. Christoph Paus (MIT)


Sep 21: Dr. Chinedum Osuji (Yale  Univerisity)


Oct 5: Prof. Arjun G. Yodh (UPenn)


Oct 12: Prof. Rachel Somerville (Rutgers University)


TITLE: The mysterious intertwined life-cycle of galaxies and their supermassive black holes


ABSTRACT: It is now widely believed that most massive galaxies harbor supermassive black holes in their nuclei, and that the mass of the black hole is strongly correlated with galaxy properties such as mass or luminosity. In addition, the evolution of the global star formation rate density over cosmic time seems to closely trace that of the global black hole accretion rate, suggesting that galaxies and their black holes grew together. However, in individual objects, star formation and black hole growth often appear to be uncorrelated. Moreover, many questions remain about the origin and evolution of supermassive black holes in galaxies, for example: what are the masses and physical origin of the first seed black holes? How is black hole activity triggered and regulated? How does the energy released by accreting black holes shape their host galaxies? I will address these questions by presenting predictions from theoretical models that attempt to track the intertwined growth of galaxies and their black holes in a cosmological context, and confronting these predictions with recent observations from multi-wavelength surveys.


Oct 19: Prof. Mohammad F. Islam (Carnegie Mellon)


Oct 26: Prof. Sheldon Stone


Oct 26: Prof. Sean Carroll (Caltech)


TITLE: The Origin of the Universe and the Arrow of Time


ABSTRACT: Over a century ago, Boltzmann and others provided a microscopic understanding for the tendency of entropy to increase. But this understanding relies ultimately on an empirical fact about cosmology: the early universe had a very low entropy. Why was it like that? Cosmologists aspire to provide a dynamical explanation for the observed state of the universe, but have had very little to say about the dramatic asymmetry between early times and late times. I will discuss whether the problem of low entropy initial conditions can be alleviated within the context of a multiverse.


Nov 9: Prof. Edo Berger (Harvard - CfA)


TITLE: Shake, Rattle and Explode: Short Gamma-Ray Bursts and the Electromagnetic Counterparts of Gravitational Wave Sources


ABSTRACT: The bi-modality of gamma-ray burst (GRB) durations points to distinct progenitor classes for the long- and short-duration GRBs. While the progenitors of long-duration GRBs are now known to be massive stars, the progenitors of short-duration GRBs remain unidentified. In this talk I will discuss the discovery of short GRB afterglow and their host galaxies, detailed studies of their environments from parsec to galactic scales, and studies of their energetics and beaming. Taken together, these observations point to the coalescence of neutron star and black hole binaries as the most likely progenitors.  With the upcoming Advanced LIGO/VIRGO gravitational wave detectors it is therefore possible that short GRBs will be the first detected sources, and I will discuss various approaches to making this connection between gravitational wave and electromagnetic sources.


Nov 16: Prof. Martha Haynes (Cornell University)


TITLE: The ALFALFA Census of Gas-Bearing Galaxies at z=0


ABSTRACT: Capitalizing on the huge collecting area of the Arecibo telescope and the survey capability of the 7-beam Arecibo L-band Feed Array (ALFA), the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic HI 21cm line survey aims to produce a census of HI-bearing objects found over 7000 square degrees of the high galactic latitude sky out to z < 0.06. The survey observations were completed in Oct 2012 and a catalog is available for about 54% of the final survey area. I will review the nature of the ALFALFA population and discuss some of its more enigmatic objects, including "dark galaxy" candidates and possible very low mass "mini-halos" in the Local Group. Most surprisingly, ALFALFA detects many more high HI mass objects than predicted by previous HIMF results, a result of particular importance since it directly impacts, in a positive sense, estimates of the expected HI detection rate at high z with the Square Kilometer Array (SKA) and its pathfinders. With its completion, ALFALFA will provide the first robust census of gas-bearing halos over a cosmologically significant volume. Our coordinated multiwavelength program will yield important insight into how some massive galaxies maintain huge gas reservoirs without converting their gas into stars and how isolated low mass halos are able to retain some HI gas despite their fragile thermal state and shallow potential wells.


Nov 30: Prof. Andrew Tolley (Case Western Reserve University)


TITLE: New Cosmologies on the Horizon


ABSTRACT: In recent years there has been a considerable effort to look for modifications to Einstein's theory of gravity at cosmological scales, motivated by the vexing puzzle of dark energy. One such approach looks into the possibility of giving the graviton, the fundamental exchange particle for the gravitational force, a mass. In the last few years we have seen the development of the first consistent theory of massive gravity in four dimensions. I will review how these ideas can be important for cosmology, for addressing the cosmological constant problem, and how to go about constructing such consistent theories of gravity.