14,341 research outputs found
Fundamental properties and applications of quasi-local black hole horizons
The traditional description of black holes in terms of event horizons is
inadequate for many physical applications, especially when studying black holes
in non-stationary spacetimes. In these cases, it is often more useful to use
the quasi-local notions of trapped and marginally trapped surfaces, which lead
naturally to the framework of trapping, isolated, and dynamical horizons. This
framework allows us to analyze diverse facets of black holes in a unified
manner and to significantly generalize several results in black hole physics.
It also leads to a number of applications in mathematical general relativity,
numerical relativity, astrophysics, and quantum gravity. In this review, I will
discuss the basic ideas and recent developments in this framework, and
summarize some of its applications with an emphasis on numerical relativity.Comment: 14 pages, 2 figures. Based on a talk presented at the 18th
International Conference on General Relativity and Gravitation, 8-13 July
2007, Sydney, Australi
Stellar and Molecular Radii of a Mira Star: First Observations with the Keck Interferometer Grism
Using a new grism at the Keck Interferometer, we obtained spectrally
dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These
data show that the measured radius of the emission varies substantially from
2.0-2.4 microns. Simple models can reproduce these wavelength-dependent
variations using extended molecular layers, which absorb stellar radiation and
re-emit it at longer wavelengths. Because we observe spectral regions with and
without substantial molecular opacity, we determine the stellar photospheric
radius, uncontaminated by molecular emission. We infer that most of the
molecular opacity arises at approximately twice the radius of the stellar
photosphere.Comment: 12 pages, including 3 figures. Accepted by ApJ
Quantifying structural damage from self-irradiation in a plutonium superconductor
The 18.5 K superconductor PuCoGa5 has many unusual properties, including
those due to damage induced by self-irradiation. The superconducting transition
temperature decreases sharply with time, suggesting a radiation-induced Frenkel
defect concentration much larger than predicted by current radiation damage
theories. Extended x-ray absorption fine-structure measurements demonstrate
that while the local crystal structure in fresh material is well ordered, aged
material is disordered much more strongly than expected from simple defects,
consistent with strong disorder throughout the damage cascade region. These
data highlight the potential impact of local lattice distortions relative to
defects on the properties of irradiated materials and underscore the need for
more atomic-resolution structural comparisons between radiation damage
experiments and theory.Comment: 7 pages, 5 figures, to be published in PR
Optimized Coplanar Waveguide Resonators for a Superconductor-Atom Interface
We describe the design and characterization of superconducting coplanar
waveguide cavities tailored to facilitate strong coupling between
superconducting quantum circuits and single trapped Rydberg atoms. For initial
superconductor-atom experiments at 4.2 K, we show that resonator quality
factors above can be readily achieved. Furthermore, we demonstrate that
the incorporation of thick-film copper electrodes at a voltage antinode of the
resonator provides a route to enhance the zero-point electric fields of the
resonator in a trapping region that is 40 m above the chip surface,
thereby minimizing chip heating from scattered trap light. The combination of
high resonator quality factor and strong electric dipole coupling between the
resonator and the atom should make it possible to achieve the strong coupling
limit of cavity quantum electrodynamics with this system.Comment: 4 pages, 4 figure
X-ray Absorption Fine Structure in Embedded Atoms
Oscillatory structure is found in the atomic background absorption in
x-ray-absorption fine structure (XAFS). This atomic-XAFS or AXAFS arises from
scattering within an embedded atom, and is analogous to the Ramsauer-Townsend
effect. Calculations and measurements confirm the existence of AXAFS and show
that it can dominate contributions such as multi-electron excitations. The
structure is sensitive to chemical effects and thus provides a new probe of
bonding and exchange effects on the scattering potential.Comment: 4 pages plus 2 postscript figures, REVTEX 3.
Marginally trapped tubes and dynamical horizons
We investigate the generic behaviour of marginally trapped tubes (roughly
time-evolved apparent horizons) using simple, spherically symmetric examples of
dust and scalar field collapse/accretion onto pre-existing black holes. We find
that given appropriate physical conditions the evolution of the marginally
trapped tube may be either null, timelike, or spacelike and further that the
marginally trapped two-sphere cross-sections may either expand or contract in
area. Spacelike expansions occur when the matter falling into a black hole
satisfies , where is the area of the horizon while
and are respectively the density and pressure of the matter.
Timelike evolutions occur when is greater than this cut-off and so
would be expected to be more common for large black holes. Physically they
correspond to horizon "jumps" as extreme conditions force the formation of new
horizons outside of the old.Comment: 31 pages, many figures. Final Version to appear in CQG: improvements
include more complete references, a discussion of those references,
Penrose-Carter diagrams for several of the spacetimes, and improved numerics
for the scalar field
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