Events Archive

Feb
28
2011
"Dynamics of bacterial propulsion: Behavior of the flagellar rotary motor near zero load" by Junhua Yuan
Flagellated bacteria swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. Studies of the physiology of the bacterial flagellar rotary motor have been limited to the regime of relatively high load due to technical limitations. Here, we developed a new technique that allows systematic study of the motor near zero load. Sixty-nanometer-diameter gold spheres were attached to motors lacking flagellar filaments, and a novel laser darkfield setup was used to monitor the sphere rotation. Resurrection experiments were carried out near zero load: paralyzed motors without torque...
Feb
25
2011
"Induction of Cellular Responses by Nanoscopic Environments" by Joachim P. Spatz
Our approach to engineer cellular environments is based on self-organizing spatial positioning of single signaling molecules attached to inorganic or polymeric supports, which offers the highest spatial resolution with respect to the position of single signaling molecules. This approach allows tuning cellular material with respect to its most relevant properties, i.e., viscoelasticity, peptide composition, nanotopography and spatial nanopatterning of signaling molecule. Such materials are defined as “nano-digital materials” since they enable the counting of individual signaling molecules, separated by a biologically inert background. Within these...
Feb
23
2011
"Advances in chaotic dynamics and chaos synchronization" by Hongtao Zhang
Chaotic system, characterized by sensitivity to initial conditions, handles abundant dynamics, sometimes leading to unimagined results in reality. In the past two decades, dynamical analysis and control of chaos attracted a lot of interests of scientists. Specifically, since chaos synchronization was found in 1991, generating and synchronizing chaotic systems has become a hot issue and been intensively studied. In this presentation, I would like to introduce some advances in generating new chaotic attractors and synchronizing chaos. Initially, starting from designing new chaotic systems, I demonstrate three types of systematic approaches to generate multi-scroll attractors and...
Feb
23
2011
"The First Galaxies" by John Wise
The first stars and galaxies had a profound impact on the universe, leading to reionization and the chemical enrichment of the intergalactic medium.  Here I present results from adaptive mesh refinement radiation hydrodynamics simulations that focus on the formation of the first galaxies with a self-consistent transition from massive metal-free stars to metal-enriched stars that populate the first galaxies. These results provide invaluable insight for interpreting the latest and future galaxy observations prior to reionization.
Feb
21
2011
"The Milky Way as a Dark Matter Laboratory" by Michael Kuhlen
One of the biggest outstanding puzzles in physics today is the nature of dark matter. Although there is compelling evidence for its existence over a wide range of scales, from the Cosmic Microwave Background to dwarf galaxies, we still do not fully understand what exactly it is. Our own Milky Way galaxy and its Local Group environment presents an ideal laboratory for the study of dark matter: numerous ground and space-based experiments and observatories are gearing up to probe dark matter on Earth, in the Solar System, at the Galactic Center, in dwarf satellite galaxies, and beyond. In this talk I will describe how recent ultra-high resolution numerical...
Feb
16
2011
From transient to fully turbulent shear flows
The classical picture of the transition to turbulence in fluid flows is that of successive instabilities where starting from a stationary basic state complexity arises via a sequence of bifurcations. In contrast, shear flows undergo a sudden and direct transition from laminar to turbulent motion as the driving velocity increases. In this talk we examine the richness of this transition in pipe flow. We show that turbulence, which is transient at low Reynolds numbers, becomes sustained at a distinct critical point. The turbulent state emerging through this phase transition is a large-scale pattern consisting of localized chaotic clusters that may relaminarize, merge, nucleate new...
Feb
16
2011
"Unraveling the Extragalactic Gamma-ray Sky in the Era of the Fermi Space Telescope and VERITAS" by Luis Reyes
The field of high-energy astrophysics is experiencing a revolution due to recent observations that have revealed a Universe that is surprisingly rich, variable and complex at gamma-ray energies. We employ these new observations to address long-standing science topics including: the inner workings of the Universe's most powerful accelerators; the nature of dark matter; and the total amount of light that has been emitted in the Universe since the first stars were formed.  This revolution has come about due to the launch of the Fermi Gamma-ray Space Telescope and the full-fledged operation of a new generation of ground-based instruments like VERITAS, H.E.S.S. and MAGIC. Among the...
Feb
14
2011
"The First Billion Years of the Universe" by Marcelo Alvarez
Our earliest image of the universe - the cosmic microwave background - shows that hundreds of thousands of years after the big bang, it was a relatively simple place. At that time, there were no planets, stars, or galaxies. Space was permeated by an expanding, nearly homogeneous mixture of dark matter and mostly hydrogen gas, devoid of the heavier elements common in daily life. How then did the complex structure we see today develop? I will focus on a key aspect of this problem, namely the story of the very first stars, galaxies, and black holes - how they formed, and how they influenced the subsequent evolution of cosmic structure. In particular, I will show...
Feb
09
2011
"Stability of vortex and wave flows from bifurcation diagrams exploiting a variational argument" by Paolo Luzzatto-Fegiz
Steady fluid solutions can play a special role in characterizing the dynamics of a flow: stable states might be realized in practice, while unstable ones may act as attractors in the unsteady evolution. Unfortunately, determining stability is often a process substantially more laborious than computing steady flows; this is highlighted by the fact that, for several comparatively simple flows, stability properties have been the subject of protracted disagreement (see e.g. Dritschel et al. 2005, and references therein). In this talk, we build on some ideas of Lord Kelvin, who, over a century ago, proposed an energy-based stability argument for steady flows. In essence, Kelvin’s...
Feb
09
2011
"Gamma-Ray Astrophysics at the Energy Threshold of Cherenkov Telescopes" by Nepomuk Otte
For the last seven years, gamma-ray astronomy from the ground provides us with fantastic results, which address questions in astroparticle physics, cosmology, and fundamental physics.  The workhorses in the field are imaging atmospheric Cherenkov telescopes, which are the most sensitive instruments to explore the gamma-ray sky above 100 GeV in pointed observations. Amongst others I discuss the efforts to lower the energy threshold of Cherenkov telescopes, and the detection of the Crab Pulsar as one of the merits of these efforts. I close by describing ongoing efforts to develop the next generation of Cherenkov telescopes, the Cherenkov Telescope Array (...

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