Events at Physics |
Events on Thursday, October 22nd, 2009
- R. G. Herb Condensed Matter Seminar
- Fluctuations, Response, Entropy, and "Temperature" in Granular Packings
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Bulbul Chakraborty, Brandeis University
- Abstract: To understand the mechanical response of granular materials, one needs a theoretical approach that can bridge the gap between microscopic, grain-level quantities and macroscopic, collective properties. Fluctuations are inherently related to the number of microscopic states available under a given set of macroscopic parameters. In equilibrium thermodynamics, the microcanonical entropy, or its derivatives in other ensembles, is the measure used to calculate fluctuations and response. In disordered systems such as spin glasses, the concept of complexity has been used to understand collective properties. In mean-field models, free-energy minima are separated by barriers that diverge in the thermodynamic limit, and one can in principle count the number of states unambiguously. Can we identify a physical variable in granular materials that is conserved under any local dynamics, and therefore leads to a definition of complexity? For mechanically stable packings, there is a topological conservation law that allows us to define the analog of complexity. In this talk, I will describe a framework for calculating stress fluctuations in frictional and frictionless granular packings, based on a model calculation of complexity, and compare the predictions of our theory to experiments and simulations.
- Host: Susan Coppersmith
- Whitford Lecture
- WMAP and Beyond
- Time: 2:00 pm - 3:30 pm
- Place: 3425 Sterling Hall
- Speaker: Dr. David Spergel, Princeton University
- Abstract: CMB experiments have made an accurate full-sky measurement of the microwave background temperature and polarization fluctuations. These measurements probe both the physics of the very early universe and the basic properties of the universe today. These measurements 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).
I will review the results from WMAP and describe recent results from the Atacama Cosmology Telescope (ACT) and look forward to the upcoming results from Planck. ACT small scale measurements not only probe early universe physics but offer a new tool for studying the evolution of clusters and large-scale structure. - Host: Prof Richard Townsend
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- The Search for the Fundamental Nature of Dark Matter
- Time: 4:00 pm
- Place: 4274 Chamberlin
- Speaker: Dan McKinsey, Yale University
- Abstract: Astrophysical evidence on a variety of distance scales clearly shows that we cannot account for a large fraction of the mass of the universe. This matter is "dark", not emitting or absorbing any electromagnetic radiation. A compelling explanation for this missing mass is the existence of Weakly Interacting Massive Particles (WIMPs).
These particles are well motivated by particle physics theories beyond the Standard Model, and the discovery of WIMPs would have enormous impact on both astrophysics and particle physics. WIMPs, if they exist, would occasionally interact with normal matter. With a mass in the range of 1 to 1000 times the mass of the proton, and moving at speeds relative to the Earth on the order of 200 km/s, WIMPs would only deposit a small amount of energy when scattering with nuclei.
Detectors that are low in radioactivity and sensitive to small energy depositions can search for the rare nuclear recoil events predicted by WIMP models. In recent years, several new efforts on direct dark matter detection have begun in which the detection material is a noble liquid. Advantages include: large nuclear recoil signals in both scintillation and ionization channels, good scalability to large target masses, effective discrimination against gamma ray backgrounds, easy purification, and reasonable cost. - Host: Karsten Heeger
- Introductory Graduate Seminar
- Condensed Matter Theory
- Time: 5:30 pm
- Place: 2223 Chamberlin Hall
- Speaker: Bruch, Chubukov, Coppersmith, Joynt, Perkins, Vavilov, University of Wisconsin Department of Physics