1,025 research outputs found
Thermodynamics of four-dimensional black objects in the warped compactification
We reinvestigate the thermodynamics of black objects (holes and strings) in
four-dimensional braneworld models that are originally constructed by Emparan,
Horowitz and Myers based on the anti-de Sitter (AdS) C-metric. After proving
the uniqueness of slicing the AdS C-metric, we derive thermodynamic quantities
of the black objects by means of the Euclidean formulation and find that we
have no necessity of requiring any regularization to calculate their classical
action. We show that there exist the Bekenstein-Hawking law and the
thermodynamic first law. The thermodynamic mass of the localized black hole on
a flat brane is negative, and it differs from the one previously derived. We
discuss the thermodynamic stabilities and show that the BTZ black string is
more stable than the localized black holes in a canonical ensemble, except for
an extreme case. We also find a braneworld analogue of the Hawking-Page
transition between the BTZ black string and thermal AdS branes. The localized
black holes on a de Sitter brane is discussed by considering Nariai instanton,
comparing the study of "black cigar" in the five-dimensional braneworld model.Comment: 15 pages, 4 figures, RevTex4, typos fixed, minor correction
Gravitational Wave Background from Neutrino-Driven Gamma-Ray Bursts
We discuss the gravitational wave background (GWB) from a cosmological
population of gamma-ray bursts (GRBs). Among various emission mechanisms for
the gravitational waves (GWs), we pay a particular attention to the vast
anisotropic neutrino emissions from the accretion disk around the black hole
formed after the so-called failed supernova explosions. The produced GWs by
such mechanism are known as burst with memory, which could dominate over the
low-frequency regime below \sim 10Hz. To estimate their amplitudes, we derive
general analytic formulae for gravitational waveform from the axisymmetric
jets. Based on the formulae, we first quantify the spectrum of GWs from a
single GRB. Then, summing up its cosmological population, we find that the
resultant value of the density parameter becomes roughly \Omega_{GW} \approx
10^{-20} over the wide-band of the low-frequency region, f\sim 10^{-4}-10^1Hz.
The amplitude of GWB is sufficiently smaller than the primordial GWBs
originated from an inflationary epoch and far below the detection limit.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Second order perturbations in the radius stabilized Randall-Sundrum two branes model
The nonlinear gravitational interaction is investigated in the Randall-Sundrum two branes model with the radius stabilization mechanism. As the stabilization model, we assume a single scalar field which has the potential in the bulk and the potential on each brane. We develop the formulation of the second order gravitational perturbations under the assumption of the static and axial-symmetric 5-dimensional metric that is spherically symmetric in the 4-dimensional sense. After deriving the formal solutions for the perturbations, we discuss the gravity on each brane induced by the matter on its own side, taking the limit of the large coupling of the scalar field interaction term on the branes. We show using the Goldberger-Wise stabilization model that the 4-dimensional Einstein gravity is approximately recovered in the second order perturbations
Six-dimensional localized black holes: numerical solutions
To test the strong-gravity regime in Randall-Sundrum braneworlds, we consider
black holes bound to a brane. In a previous paper, we studied numerical
solutions of localized black holes whose horizon radii are smaller than the AdS
curvature radius. In this paper, we improve the numerical method and discuss
properties of the six dimensional (6D) localized black holes whose horizon
radii are larger than the AdS curvature radius. At a horizon temperature
, the thermodynamics of the localized black
hole undergo a transition with its character changing from a 6D Schwarzschild
black hole type to a 6D black string type. The specific heat of the localized
black holes is negative, and the entropy is greater than or nearly equal to
that of the 6D black strings with the same thermodynamic mass. The large
localized black holes show flattened horizon geometries, and the intrinsic
curvature of the horizon four-geometry becomes negative near the brane. Our
results indicate that the recovery mechanism of lower-dimensional Einstein
gravity on the brane works even in the presence of the black holes.Comment: 17 pages, 9 figures, RevTeX4, typos correcte
3-D General Relativistic MHD Simulations of Generating Jets
We have performed a first fully 3-D GRMHD simulation with Schwarzschild black
hole with a free falling corona. The initial simulation results show that a jet
is created as in previous axisymmetric simulations. However, the time to
generate the jet is longer than in the 2-D simulations. We expect that due to
the additional azimuthal dimension the dynamics of jet formation can be
modified.Comment: 4 pages Proc. Oxford Radio Galaxy Workshop ed. R. Laing & K. Blundell
(San Francisco: PASP) in press (revised
Thermal Equilibrium of String Gas in Hagedorn Universe
The thermal equilibrium of string gas is necessary to activate the
Brandenberger-Vafa mechanism, which makes our observed 4-dimensional universe
enlarge. Nevertheless, the thermal equilibrium is not realized in the original
setup, a problem that remains as a critical defect. We study thermal
equilibrium in the Hagedorn universe, and explore possibilities for avoiding
the issue aforementioned flaw. We employ a minimal modification of the original
setup, introducing a dilaton potential. Two types of potential are
investigated: exponential and double-well potentials. For the first type, the
basic evolutions of universe and dilaton are such that both the radius of the
universe and the dilaton asymptotically grow in over a short time, or that the
radius converges to a constant value while the dilaton rolls down toward the
weak coupling limit. For the second type, in addition to the above solutions,
there is another solution in which the dilaton is stabilized at a minimum of
potential and the radius grows in proportion to . Thermal equilibrium is
realized for both cases during the initial phase. These simple setups provide
possible resolutions of the difficulty.Comment: 23 pages,19 figure
Probing anisotropies of gravitational-wave backgroundswith a space-based interferometer II: Perturbative reconstruction of a low-frequency skymap
We present a perturbative reconstruction method to make a skymap of
gravitational-wave backgrounds (GWBs) observed via space-based interferometer.
In the presence of anisotropies in GWBs, the cross-correlated signals of
observed GWBs are inherently time-dependent due to the non-stationarity of the
gravitational-wave detector. Since the cross-correlated signal is obtained
through an all-sky integral of primary signals convolving with the antenna
pattern function of gravitational-wave detectors, the non-stationarity of
cross-correlated signals, together with full knowledge of antenna pattern
functions, can be used to reconstruct an intensity map of the GWBs. Here, we
give two simple methods to reconstruct a skymap of GWBs based on the
perturbative expansion in low-frequency regime. The first one is based on
harmonic-Fourier representation of data streams and the second is based on
"direct" time-series data. The latter method enables us to create a skymap in a
direct manner. The reconstruction technique is demonstrated in the case of the
Galactic gravitational wave background observed via planned space
interferometer, LISA. Although the angular resolution of low-frequency skymap
is rather restricted, the methodology presented here would be helpful in
discriminating the GWBs of galactic origins by those of the extragalactic
and/or cosmological origins.Comment: 23 pages, 12 figures, Phys.Rev.D (2005) in pres
Simplified solution to determination of a binary orbit
We present a simplified solution to orbit determination of a binary system
from astrometric observations. An exact solution was found by Asada, Akasaka
and Kasai by assuming no observational errors. We extend the solution
considering observational data. The generalized solution is expressed in terms
of elementary functions, and therefore requires neither iterative nor numerical
methods.Comment: 15 pages; text improved, Accepted for publication in the Astronomical
Journa
Towers of Gravitational Theories
In this essay we introduce a theoretical framework designed to describe black
hole dynamics. The difficulties in understanding such dynamics stems from the
proliferation of scales involved when one attempts to simultaneously describe
all of the relevant dynamical degrees of freedom. These range from the modes
that describe the black hole horizon, which are responsible for dissipative
effects, to the long wavelength gravitational radiation that drains mechanical
energy from macroscopic black hole bound states. We approach the problem from a
Wilsonian point of view, by building a tower of theories of gravity each of
which is valid at different scales. The methodology leads to multiple new
results in diverse topics including phase transitions of Kaluza-Klein black
holes and the interactions of spinning black hole in non-relativistic orbits.
Moreover, our methods tie together speculative ideas regarding dualities for
black hole horizons to real physical measurements in gravitational wave
detectors.Comment: Awarded second prize for 2006 Gravity Research Foundation essay
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