Aug 31

"High energy photons from Fermi GRBs: Who would have thought that they were produced in the external shock?"
Rodolfo Barniol Duran, University of Texas at Austin

"The Fermi satellite - capable of detecting photons from 8keV to >300GeV - has discovered a number of very interesting and sometimes puzzling observations regarding GRBs. It detected photons with energy up to 13GeV from GRB 080916C. This burst shows two distinct components: the lower-energy one (photon energy less than ~ 1 MeV), which lasts for about 50s and resembles the short-lived prompt gamma-ray emission of the Swift bursts, and the high-energy one (>=100MeV), which starts ~4s after the Fermi/GBM trigger time and shows a steep rise and later a steady decay for ~ 1400s. We show that the high-energy component spectral and temporal decay indices are consistent with the closure relations for the external shock. Moreover, the predicted flux magnitude for forward shock emission at 100 MeV is consistent with the observed level. Before the launch of the Fermi Satellite many exotic and creative mechanisms for the generation of high energy photons were proposed. It turns out that at least for GRB 080916C Nature has adopted the simplest mechanism possible, i.e. synchrotron in the external forward shock. The external shock model provides an explanation for the delay of ~4s for the high-energy photons: it is the deceleration time of the GRB jet.

The late time (>1 day) x-ray and optical emissions from GRBs have long been attributed to the external shock. It turns out that the forward shock parameters that explain the late time afterglow data for GRB 080916C also provide a good fit for the early time high energy observations; the exercise carried out in the other direction - going from early, >100 MeV, data to late x-ray and optical data - also works very well. Finally, we show that the magnetic field required for the external shock synchrotron emission is consistent with a circum-stellar seed magnetic field compressed by the shock."

Sep 21

"A New Perspective on Galaxy Clustering as a Cosmological Probe: General Relativistic Effects"
Jaiyul Yoo, Harvard-Smithsonian Center for Astrophysics

"We present a general relativistic description of galaxy clustering in a FLRW universe. The observed redshift and position of galaxies are affected by the matter fluctuations and the gravity waves between the source galaxies and the observer, and the volume element constructed by using the observables differs from the physical volume occupied by the observed galaxies. Therefore, the observed galaxy fluctuation field contains additional contributions arising from the distortion in observable quantities and these include tensor contributions as well as numerous scalar contributions. We generalize the linear bias approximation to relate the observed galaxy fluctuation field to the underlying matter distribution in a gauge-invariant way. Our full formalism is essential for the consistency of theoretical predictions. As our first application, we compute the angular auto correlation of large-scale structure and its cross correlation with CMB temperature anisotropies. We comment on the possibility of detecting primordial gravity waves using galaxy clustering and discuss further applications of our formalism."

Sep 28

"Cosmology with high (z>1) redshift galaxy surveys"
Donghui Jeong, University of Texas at Austin

"Galaxy redshift surveys are powerful probes of cosmology. Yet, in order to fully exploit the information contained in galaxy surveys, such as the distance scales, the primordial power spectrum, and neutrino masses, we need to improve upon our understanding of the structure formation in the Universe. As galaxies are formed/observed in the late time when the density field is no longer linear, understanding non-linearities, which alter the galaxy power spectrum different from the linear theory prediction, is essential. In this talk, we show that, at high redshifts, we can accurately model the galaxy power spectrum by using the standard cosmological perturbation theory. Once we model the full shape of the galaxy power spectrum in redshift space, we can extract much more cosmological information than simply using small features such as the Baryon Acoustic Oscillations. Going beyond the power spectrum, we can use the three-point function, or the bispectrum, to gain important information on the early universe as well as on the galaxy formation via measurements of primordial non-Gaussianity and galaxy bias. We show that the galaxy bispectrum is more sensitive to primordial non-Gaussianities than previously recognized, making high-redshift galaxy surveys a particularly potent probe of the physics of inflation."

Oct 26

"Is Cold Dark Matter a Bose-Einstein Condensate?"
Paul Shapiro, University of Texas at Austin

"Despite the success of Cold Dark Matter cosmology in explaining many properties of the observed universe, the identity of CDM on the microscopic level remains unknown. Among the leading candidates are neutralinos predicted by supersymmetry and axions. Axions are light, neutral, spin zero bosons which occur in a model to explain the absence of CP violation in the strong interaction, so far invisible in the lab. An important distinction between axionic and neutralino dark matter (DM) is that axions form a Bose-Einstein Condensate (BEC). On large-scales, gravity is believed to form the same structures in BEC/CDM (e.g. axions) as in the standard, "noninteracting" CDM (e.g. neutralinos), but on small scales they must differ, because the BEC is a superfluid. Astronomical measurements may, therefore, distinguish them. We will discuss this possibility , not only for the weakly-interacting case of axions, but also for other forms of BEC dark matter which have been proposed, in the strong-coupling regime. BEC/CDM may explain the flat-density cores of dwarf and LSB galaxies, but the merging halos of the Bullet Cluster may pose a fatal constraint."

Oct 26

"Vortices (and the Angular Momentum Problem) in Bose-Einstein-Condensed Cold Dark Matter Halos"
Tanja Rindler-Daller, University of Cologne (Koln)

"Several suggestions have appeared in the literature that cold dark matter (CDM) may be in the form of a Bose-Einstein condensate (BEC), including axionic and other forms of CDM. This has important implications for the physics of structure formation, notably at small scales where one expects significant deviations from the more standard CDM, due to the superfluidity exhibited by BECs. However, even on the scales of individual galactic halos, the issue of acquiring angular momentum during galaxy formation is affected. Laboratory BECs are known to develop vortices when rotated with sufficient angular velocity. In cosmology, simulations of structure formation in the CDM model show that halos acquire angular momentum as they form, consistent with that expected from gravitational tidal torquing by the surrounding large-scale structure. Vortices could, in principle, then result if the CDM is a BEC. We address this point by calculating the critical angular velocity for vortex creation in some simple models of BEC/CDM galactic halos and comparing the results with the angular velocity expected from cosmological N-body simulations of CDM. We start from the Gross-Pitaevskii equation of motion for the BEC wave function, coupled self-consistently to the Poisson equation, to describe self-gravitating BEC halos of ellipsoidal shape with varying degrees of rotational support. The implications of these results for cosmological models of CDM involving BECs will be discussed."

Nov 02

"Nonlinear Tides in Close Binary Systems"
Eliot Quataert, University of California, Berkeley

"The excitation and dissipation of tides strongly influences the evolution of most close binary systems, from planetary systems to compact object binaries. Despite the fact that the theoretical framework for understanding tides in binary systems is well over a century old, many of the observed orbital properties of close binaries are still not well understood. In this talk I will describe ongoing work that attempts to go beyond the standard linear theory treatment of tides in stars, focusing on solar-type stars with either stellar or planetary (hot Jupiter) companions. I will describe how nonlinear interactions dramatically modify both the excitation and damping of resonantly excited internal gravity waves in stars: the linear approximation used in previous investigations does not hold. I will conclude by discussing the implications of these effects for the orbital properties of solar-type binaries and for the orbital decay of Jupiter-mass planets around sun-like stars."

Nov 16

"Cosmological hydrogen recombination: the effect of high-n states and forbidden transitions"
Daniel Grin, California Institute of Technology

"Thanks to the ongoing Planck mission, a new window will be opened on the properties of the primordial density field, the cosmological parameters, and the physics of reionization. Much of Planck's new leverage on these quantities will come from temperature measurements at small angular scales and from polarization measurements. These both depend on the details of cosmological hydrogen recombination; use of the CMB as a probe of energies greater than 1016 GeV compels us to get the ~eV scale atomic physics right.

One question that remains is how high in hydrogen principle quantum number we have to go to make sufficiently accurate predictions for Planck. Using sparse matrix methods to beat computational difficulties, I have modeled the influence of very high (up to and including n=200) excitation states of atomic hydrogen on the recombination history of the primordial plasma, resolving all angular momentum sub-states separately and including, for the first time, the effect of hydrogen quadrupole transitions. I will review the basic physics, explain the resulting plasma properties, discuss recombination histories, and close by discussing the effects on CMB observables."