Feb
16
Teaser:

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
Teaser:

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
Teaser:

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
Teaser:

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
Teaser:

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 (...

Feb
07
Teaser:

Galactic cosmic rays are found to have a broad and faint energy-dependent anisotropy in arrival direction from a few tens of GeV to hundreds TeV. The observations show large angular features across the sky overlapped with finer sub-structures, some of which manifest as highly significant localized excess regions. Currently there is no explanation for this puzzling observation. Depending on the cosmic ray energy and type, causes could be linked to the influence of the heliosphere, or of the interstellar medium. In this presentation the acceleration and propagation of cosmic rays is discussed along with their detection. The intringuing possibility that the...

Feb
02
Teaser:

Galaxy mergers are expected to be a natural channel for the formation of supermassive black hole binaries (SBHBs). Discovery of a statistically significant sample of these objects has important astrophysical implications for a range of questions that pertain to the formation and cosmological evolution of the supermassive black holes, the rate of their coalescences, and associated electromagnetic (EM) and gravitational wave signatures. All are intricately connected to the properties of the environment in which the SBHBs find themselves during the cosmic time. Most of the information about these systems so far had to be derived from theoretical studies and...

Jan
31
Teaser:

The Fermi Large Area Telescope (LAT) has been successfully launched  from Cape Canaveral on 11 June 2008. It is exploring the gamma ray sky in the energy range from 20 MeV to over 300 GeV with unprecedeted sensitivity. One of the most exciting science questions that Fermi LAT will address is the nature of dark matter. Several theoretical models have been proposed that predict the existence of Weakly Interacting Massive Particles (WIMPs) that are excellent dark matter candidates. Fermi LAT investigates the existence of WIMPs indirectly, primarily...

Jan
28
Teaser:

It has recently become possible to compute precise equilibrium, traveling wave, and periodic orbit solutions to pipe and plane Couette flow at Reynolds numbers above the onset of turbulence. These invariant solutions capture the complex dynamics of unstable coherent structures in wall-bounded flows and provide a framework for understanding turbulent flows as dynamical systems. We present a number of weakly unstable equilibria, traveling waves, and periodic orbits of plane Couette flow and visualizations of their physical and state-space dynamics. What emerges is a picture of low-Reynolds turbulence as a walk among a set of weakly unstable invariant solutions...

Jan
26
Teaser:

Directed Migration of cells is vital to a wide array of biological processes: from the coordinated migration of cells during embryo development to the uncontrollable migration of a metastatic cancer.  We investigate directed cell migration in the model organism Dictyostelium aiming to understand the underlying biophysics of their motion, their direction, and the coordination among cell groups.   The problem of directed cell migration is often broken into three independent modules: a compass,...

Jan
25
Teaser:
The non-equilibrium current fluctuations, or shot noise, in ballistic graphene have received much attention since the seminal 2006 paper by Tworzydlo et al.  In that work, it was shown that shot noise can be generated even in a completely impurity-free sheet of graphene.  This result is surprising, as shot noise is expected to vanish in conductors without electron scattering.  The unexpected noise has been attributed to evanescent, that is, exponentially damped waves that backscatter electrons, even in clean graphene.  The predicted shot noise has been verified experimentally, but...
Jan
24
Teaser:

The cerebral cortex is a highly complex network comprised of billions of excitable nerve cells.  The coordinated dynamic interactions of these cells underlie our thoughts, memories, and sensory perceptions.  A healthy brain carefully regulates its neural excitability to optimize information processing and avoid brain disorders.  If excitability is too low, neural interactions are too weak and signals fail to propagate through the brain network.  On the other hand, high excitability can result in excessively strong interactions and, in some cases, epileptic seizures.  While it is commonly supposed that healthy neural excitability must...

Jan
21
Teaser:

12:15 in the M-Building, room 3201 A, for details see www.softmatter.gatech.edu/baglunch.html

Most industrial and biological materials exist, or are processed, in the form of multicomponent, microstructured fluids, and their ultimate function and use is dependent on the understanding of the optics,...

Dec
16
Teaser:

The discovery of neutrino oscillations has been one of the major advances in our understanding of particle physics in recent times, and we are still trying to fully understand them and the insights they may give to physics at very high energies and perhaps even into the matter-anti-matter asymmetry of the universe.  Long Baseline Neutrino Oscillation Experiments are becoming one of the main tools for the study of neutrinos.  The talk will briefly outline the history and physics of neutrino oscillations and long baseline experiments, and then discuss results from the current round of experiments - OPERA, MINOS, and now T2K.  I will then discuss...

Dec
09
Teaser:

Two dimensional turbulence is an idealization of real 3D systems with anisotropy caused by geometric confinement or body forcing.  I will review the current state of understanding of 2D turbulent flows including specific theoretical predictions, numerical simulation results and experimental realizations of quasi-2D turbulent systems.  Relevance to geophysical systems will be discussed.

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