Eric Sembrat's Test Bonanza

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 through their annihilation or decay into photons and into electrons and positrons. I will present recent results on these searches.

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 computational simulations since finding them in EM searches proved to be a difficult task. I will discuss how current and future developments in theoretical understanding of observational signatures of SBHBs can help in future observational searches.

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 anisotropy at multi-TeV energy can be used to probe the boundary region between the heliosphere and interstellar medium and that it can indirectly link to their origin in our galactic neighborhood, is also discussed.

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 (CTA), which will achieve ten times better sensitivity than existing telescopes.

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 different classes of gamma-ray sources observed by these instruments, some active galactic nuclei (AGN) and Gamma-ray Bursts (GRBs) stand out as the most energetic and variable objects observed at any wavelength.  In my talk I will describe how the complementary capabilities of space and ground-based instruments are leading us to a better understanding of AGN and GRBs as high-energy sources, and as a cosmological tool to probe the background radiation known as extragalactic background light (EBL). Finally, I will discuss the important scientific return that next-generation instruments like the Cherenkov Telescope Array (CTA) and the High-Altitude Water Cherenkov (HAWC) experiment would bring to the field of extragalactic gamma-ray astrophysics.

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 results from numerical simulations that detail how the first stars and black holes illuminated their surroundings with ultraviolet and X-ray radiation, completely reionizing the universe by only a billion years after the big bang.  These large scale, massively parallel computations allow us to obtain unique theoretical insights into an epoch of the distant universe which is only just now coming into view of the most powerful telescopes on earth and in space.

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 simulations of the formation of the dark matter component of a galaxy like our Milky Way have provided theoretical expectations that guide these observational efforts. In particular these simulations predict a staggering abundance of gravitationally self-bound clumps of dark matter orbiting in the Milky Way's potential. I will discuss the implications of this substructure on direct and indirect detection efforts, and then show how hydrodynamical galaxy formation simulations help to explain the discrepancy between the number of observed satellite galaxies and the much greater abundance of dark matter clumps predicted by simulations.

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.

Cells are highly ordered and organized. Much of the cell’s order relies on the active transport of material by molecular motors. Disruption in intracellular transport can be detrimental to cells, and is a common early theme in neurodegeneration. While molecular motors have been studied in isolated, cell-free system, how they act in groups in cells, and how their group functions are regulated or disrupted, are not yet understood. To address these questions in a concrete, experimentally tractable system, we studied the effects of a neurodegenerative mutation (“Legs at Odd Angles”, or Loa) on the major molecular motor, dynein. Combining single molecule, live cell imaging, and nanometer-level particle tracking techniques, we find that the Loa mutation significantly inhibits dynein travel distance, both at the single molecule level, and as ensemble in the cellular environment of neurons. Our theoretical modeling (constrained by the measured single dynein run lengths) successfully predicted the measured travel defect in mutant neurons. These results validate our current model of multiple-motor based transport, and provide the first direct evidence for a link between single motor run length and disease.

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 lie between these extremes, the optimal degree of excitability is not known.

In this colloquium I will begin with a selective history of the role of physics in modern neuroscience.  Then I will present new experimental evidence that brain dynamics undergo a phase transition as neural excitability is tuned from low to high.  Importantly, the critical excitability at which the phase transition occurs also results in optimal information processing.  These results suggest that the optimal excitability is that which places the brain closest to the phase transition.  Moreover, many mental disorders such as epilepsy, Down syndrome, and autism may be caused by deviation from this optimal excitability.