Theory Seminar
Fall 2010
15.11, Ingve Simonsen:
The Scattering of Electromagnetic Waves from
Two-Dimensional Randomly Rough Metallic Surfaces:
The Full Angular Intensity Distribution
By a computer simulation approach we study the scattering of $p$- or
$s$-polarized light from a two-dimensional, randomly rough,
metalic surface. A set of coupled inhomogeneous
integral equations for the independent tangential components of the
electric and magnetic fields on the surface are converted into matrix
equations by the method of moments, which are then solved by the
biconjugate gradient stabilized method. The solutions are used to
calculate the mean differential reflection coefficient for given angles
of incidence and specified polarizations of the incident and scattered
fields. The full angular distribution of the intensity of the
scattered light is obtained for strongly randomly rough surfaces by
a rigorous computer simulation approach.
18.10, Michael Kachelriess:
Lessons from recent gamma-ray observations
Combining data from the recently launched Fermi satellite with TeV
observations of atmospheric Cherenkov telescopes and low-energy
observations has improved our understanding of the sources of
high-energy radiation as well as of the conditions in the intergalactic
space. After a review of the basic principles of propagation and
detection of high-energy photons, I will discuss three applications:
First, I examine for the example of the nearest radio galaxy, Cen A,
how electromagnetic cascades shape the observed gamma spectrum and how
they inform us about the conditions in the source.
Second, I show that the non-observation of some TeV blazars
in the GeV range by the Fermi satellite leads to the first lower
limit on the intergalactic magnetic field (IGMF). Moreover, the
IGMF has to fill more than ~50% of the Universe, strongly constraining
possible generation mechanisms.
Finally, I discuss how the observation of a diffuse extragalactic
photon background in the GeV-TeV range can be used to constrain the
flux of high energy neutrinos, providing important information
for the choice of future neutrino experiments.
Friday, 24.9. Sergey Odintsov:
Modified gravity as unification of inflation with dark energy: from conventional theory to Horava-Lifshitz gravity
We give a general review of several 4-dimensional models of modified gravity: F(R) theory, Gauss-Bonnet gravity, non-local gravity and Horava-Lifshitz F(R) theory. Qualitatively, the possibility to unify the inflation with dark energy in such an approach, through cosmological reconstruction procedure, is demonstrated. The models may pass the local and cosmological tests and have a very rich cosmological structure.
13.9. Nan Su:
QCD Thermodynamics at Intermediate Coupling
The weak-coupling expansion of the QCD free energy is known to
order g_s^6*log{g_s}, however, the resulting series is poorly convergent at
phenomenologically relevant temperatures. I will discuss how the gauge
invariant hard-thermal-loop perturbation theory (HTLpt) reorganization of
the calculation improves the convergence of the successive approximations to
the QCD free energy. I will present new results of an HTLpt calculation of
QCD thermodynamics to three loops. The results of this calculation are
consistent with lattice data down to 2-3T_c. This is a non-trivial result
since, in this temperature regime, the QCD coupling constant is neither
infinitesimally weak nor infinitely strong with g_s~2, or equivalently
alpha_s~0.3. Therefore, we have a crucial test of the quasiparticle picture
in the intermediate coupling regime. Our results suggest that HTLpt provides
a systematic framework that can be used to calculate static and dynamic
quantities for temperatures relevant at LHC.
23.8. Lars Erlend Leganger:
Quark matter in compact stars
One of the fates that may befall an aging star is going supernova
by gravitational collapse. Expelling most of its matter, it leaves
behind only a dense neutron core,
tiny in volume compared to the Earth, but with more than a million times the
Earth's mass. It is speculated that the innermost layers of such stars may
contain exotic quark matter, the gravitational pressure forcing the neutrons
into a single gigantic hadron core. We will look at the some of the
hypothesised properties of such matter, and outline how it may be described
by an Effective Field Theory in the 2 Particle Irreducible framework,
applying the Hartree approximation