Eric Sembrat's Test Bonanza

Cosmic rays have been observed on Earth with energies in excess of 10^20 eV. Because cosmic rays are charged particles and are bent by galactic magnetic fields, the origin of these particles has remained a mystery. Gamma-ray bursts are one of a few astronomical sources containing an environment capable of accelerating charged particles to the energies observed. In addition, gamma-ray bursts are the leading candidate due to the fact that the total aggregate power observed in gamma-ray bursts and ultra high energy cosmic rays are the same order of magnitude. Neutrinos can only be created by hadronic interactions, so an observation of neutrinos in coincidence with a gamma-ray burst would provide compelling evidence that hadrons are accelerated in gamma-ray burst fireballs and hence the origin of cosmic rays. Using the IceCube Neutrino Observatory in its 40 string configuration, a stacked search was performed to look for the simultaneous occurrence of muon neutrinos with 117 gamma-ray bursts. No evidence for neutrino emission was found, placing a 90% upper limit 0.82 of the predicted neutrino fluence.

PLEASE NOTE: This is a WEBINAR

The displacement of a liquid by an air bubble is a generic two-phase flow that underpins applications as diverse as microfluidics, thin-film coating, enhanced oil recovery, and biomechanics of the lungs. I will present two intriguing examples of such flows where, firstly, oscillations in the shape of propagating bubbles are induced by a simple change in tube geometry, and secondly, flexible vessel boundaries that enable streamwise variations of the channel depth suppress viscous fingering instability.

The DNA mismatch repair system is critical for accurate DNA replication.  This system is highly conserved across organisms ranging from bacteria to humans reflecting the importance of minimizing genomic defects during cell division. The mismatch repair protein MutS has been identified the key factor that detects base-base mismatches and insertion-deletion mismatches in double stranded DNA and signals for their repair.  Despite intense study, a temporally resolved understanding of the molecular details of the MutS:DNA interactions during mismatch repair initiation has been difficult to obtain because these transient interactions occur within an overwhelming background of properly matched DNA basepairs.

We used single molecule fluorescence resonance energy transfer (smFRET) to characterize conformational changes in MutS as it scans homoduplex DNA, recognizes mismatches, activates to a sliding clamp, and interacts with downstream proteins in the repair signaling pathway. We found a series of sequential conformational changes that provide a mechanistic picture of: i) dynamic DNA bending by MutS, ii) concomitant conformational changes within MutS itself, iii) motion of MutS scanning along DNA, iv) ATP binding states that commit MutS:mismatch DNA complexes to convert to sliding states used in signaling, and v) the modulation of these MutS behaviors by other regulatory factors.

Studies that involve single vortex dynamics, vortex-vortex interactions, and vortex-impurity interactions are essential in developing a deeper understanding of the nature of superfluidity and in particular, superfluid turbulence.  In highly oblate systems, vortex dynamics have a two-dimensional (2D) nature and the resulting superfluid characteristics may be substantially different from those in three-dimensional (3D) superfluids. However, there have been remarkably few experimental studies of 2D vortex dynamics in superfluids. Therefore, to study 2D vortex dynamics and interactions, it is necessary to first develop experimental methods that can generate vortices and vortex distributions in nominally 2D systems,
such as highly oblate Bose-Einstein condensates (BECs).

Several experimental methods that were developed to generate or manipulate quantized vortices in highly oblate dilute-gas BECs will be presented in this seminar.  Two of these experiments generate multiple singly quantized vortices in a relatively stochastic manner leading to disordered vortex distributions. From these two vortex methods, the physics of high vorticity and highly disordered systems may be observed and studied in a highly oblate system. These methods may prove useful in studies of 2D quantum turbulence. The other two experiments involve newly developed techniques for controlled generation and manipulation of vortices. One of these methods creates multiply quantized pinned vortices with a certain degree of control in the generated vorticity. The other method reliably creates a pair of singly quantized vortices of opposite circulation, whose positions can be easily manipulated after creation, such that they can be placed in any location within the BEC. The two techniques may be scalable to higher number of vortices and may prove useful in superfluid dynamics and vortex interactions that require repeatable vortex distributions. Taken together, these tools and methods may be applicable to many further studies of vortex physics in highly oblate BECs.

Activation of T lymphocytes by antigen-presenting cells involves cell spreading driven by large-scale physical rearrangements of the actin cytoskeleton and the cell membrane, and accompanied by the assembly of signaling molecules into dynamic microclusters. Several recent observations suggest that mechanical forces are important for efficient T cell activation. How forces arise from the dynamics of the cytoskeleton and the membrane during contact formation, and their effect on microcluster assembly and signaling activation is not well understood. We imaged the membrane topography, actin dynamics and the spatiotemporal localization of signaling clusters during the very early stages of spreading. We found that the formation of signaling clusters was closely correlated with the movement and topography of the membrane in contact with the activating surface. Further, we observed membrane waves driven by actin polymerization originating at these signaling clusters. These actin-driven membrane protrusions likely play an important role in force generation at the immune synapse. In order to study the role of cytoskeletal forces during T-cell activation, we studied cell spreading on elastic substrates to measure cellular forces during spreading. We found that substrate stiffness influences cell morphology, actin dynamics and cellular traction forces. Efforts to determine the quantitative relationships between cellular forces and signaling are underway. Our results suggest a role for cytoskeleton driven forces during signaling activation in lymphocytes.

X-ROS signaling is a novel redox signaling pathway that links mechanical stress to changes in [Ca2+]i.  This pathway is activated rapidly and locally within a muscle cell under physiological conditions, but can also contribute to Ca2+-dependent arrhythmia in heart and to the dystrophic phenotype in heart and skeletal muscle 1, 2.  Upon physiologic cellular stretch, microtubules serve as mechanotransducers to activate NADPH oxidase 2 (NOX2) in the transverse tubules and sarcolemmal membranes to produce reactive oxygen species (ROS).  In heart, the ROS acts locally to activate ryanodine receptor Ca2+ release channels in the junctional sarcoplasmic reticulum, increasing the Ca2+ spark rate and "tuning" excitation-contraction coupling.  In skeletal muscle, where Ca2+ sparks are not normally observed, the X-ROS signaling process is muted.  However in muscular dystrophies, such as Duchenne Muscular Dystrophy and dysferlinopathy, X-ROS signaling operates at a high level and contributes to myopathy.  Importantly, in skeletal muscle Ca2+ permeable stretch-activated channels are activated by X-ROS and contribute to the cellular pathology.  In brief, X-ROS provides an exciting new mechanism for the mechanical control of redox and Ca2+ signaling in cardiac and skeletal muscle.

1. Khairallah RJ, Shi G, Sbrana F, Prosser BL, Borroto C, Mazaitis MJ, Hoffman EP, Mahurkar A, Sachs F, Sun Y, Chen YW, Raiteri R, Lederer WJ, Dorsey SG, Ward CW. Microtubules underlie dysfunction in duchenne muscular dystrophy. Sci Signal. 2012;5:ra56

2. Prosser BL, Ward CW, Lederer WJ. X-ros signaling: Rapid mechano-chemo transduction in heart. Science. 2011;333:1440-1445

As experiments probe quantum phenomena ever more deeply, it becomes difficult to believe that there is anything other than pure, unitary, quantum time evolution. Without wave-function collapse, the physicist is left with the many-worlds interpretation as the source of quantum indeterminism. (I say "physicist" because I can't defend against all philosophical contortions.)

However, if one is willing to take a radical view of statistical mechanics, it is possible to have only unitary time evolution, and still have a single "world." I will explain just what that radical departure is and how one achieves quantum determinism. A basic assumption/explanation with be "special" microscopic states. Finally, I will describe an experimental test of these ideas that is well within the realm of feasibility.

Gamma-ray bursts (GRBs) have been shown by the Fermi LAT to be a source of gamma rays with energies as high as ~100 GeV in the rest frame. Detection at higher energies may be possible with next-generation ground-based instruments. I will present results from a new simulation of GRB detections with the upcoming Cherenkov Telescope Array (CTA), using models based on the combined observations of Fermi LAT and lower-energy satellite experiments. This simulation allows the prediction of the overall detection rate, how this rate might vary as a function of telescope performance and uncertain GRB statistical properties, and the likely properties of detected GRBs. I will also show a preliminary calculation of the GRB detection rate with the HAWC (High-Altitude Water Cherenkov) observatory using a similar model. Finally, I will end with a brief mention of how new GRB detections in the GeV band could help improve our understanding of UV-optical radiation fields in the universe.

The statistics of newly discovered z>~6 galaxies---their abundance and clustering---have allowed us to study their ensemble properties, but the difficulties inherent in detecting, resolving, and obtaining spectra of these distant, faint objects has prevented a deeper empirical exploration of their internal physics. However, in advance of observatories like GMT and TMT, multi-wavelength observations can be a useful probe of the high-redshift ISM. I will present a novel analytic model for the internal physics of z>~6 galaxies that, when combined with information from galaxy statistics, probes an enormous range of distance scales---from the tens of megaparsec regions over which cosmic variance operates to the tens of parsec-sized photo-dissociative regions within giant molecular clouds---a range that makes investigations difficult with numerical simulations. I will apply this model to understanding the X-ray emission from faint AGN and to predicting future observations of CO emission lines with ALMA.

I will summarize our current understanding on the relation between the evolution of black holes and galaxies in the local universe. I will show that the growth of supermassive black holes as traced by optically luminous active galactic nuclei is strongly linked to the on-going formation of stars in the bulge component of galaxies. It is likely that this co-evolution is driven by the accretion and radial transport of cold gas, but major mergers of galaxies are not the primary mechanism for doing this. I will show that the fuel source for the black hole growth may be mass loss from intermediate mass stars and that the black hole growth may be limited in part by feedback from supernovae. I will also show that the characteristic mass scales for the populations of growing black holes and bulges are substantially lower now than in the past. The most massive black holes are largely quiescent today with low-level activity driven by the slow accretion of hot gas. The weak radio jets such black holes commonly produce may nonetheless may play a key role in suppressing star formation, keeping their surrounding host galaxy "red and dead".