106 research outputs found
Magnetic Resonance Force Microscopy Measurement of Entangled Spin States
We simulate magnetic resonance force microscopy measurements of an entangled
spin state. One of the entangled spins drives the resonant cantilever
vibrations, while the other remote spin does not interact directly with the
quasiclassical cantilever. The Schr\"odinger cat state of the cantilever
reveals two possible outcomes of the measurement for both entangled spins.Comment: 3 pages RevTe
Resonant Hawking radiation in Bose-Einstein condensates
We study double-barrier interfaces separating regions of asymptotically
subsonic and supersonic flow of Bose condensed atoms. These setups contain at
least one black hole sonic horizon from which the analog of Hawking radiation
should be generated and emitted against the flow in the subsonic region.
Multiple coherent scattering by the double-barrier structure strongly modulates
the transmission probability of phonons, rendering it very sensitive to their
frequency. As a result, resonant tunneling occurs with high probability within
a few narrow frequency intervals. This gives rise to highly non-thermal spectra
with sharp peaks. We find that these peaks are mostly associated to decaying
resonances and only occasionally to dynamical instabilities. Even at achievable
nonzero temperatures, the radiation peaks can be dominated by the spontaneous
emission, i.e. enhanced zero-point fluctuations, and not, as often in analog
models, by stimulated emission.Comment: 20 pages, 9 figures, revised versio
Tricritical Phenomena at the Cerium Transition
The isostructural transition in the
CeLaTh system is measured as a function of La alloying
using specific heat, magnetic susceptibility, resistivity, thermal
expansivity/striction measurements. A line of discontinuous transitions, as
indicated by the change in volume, decreases exponentially from 118 K to close
to zero with increasing La doping and the transition changes from being
first-order to continuous at a critical concentration . At the tricritical point, the coefficient of the linear term in the
specific heat and the magnetic susceptibility start to increase
rapidly near = 0.14 and gradually approaches large values at =0.35
signifying that a heavy Fermi-liquid state evolves at large doping. Near ,
the Wilson ratio, , has a value of 3.0, signifying the presence of
magnetic fluctuations. Also, the low-temperature resistivity shows that the
character of the low-temperature Fermi-liquid is changing
Phase Transitions in Rotating Neutron Stars
As rotating neutron stars slow down, the pressure and the density in the core
region increase due to the decreasing centrifugal forces and phase transitions
may occur in the center. We extract the analytic behavior near the critical
angular velocity , where the phase transitions occur in the center of
a neutron star, and calculate the moment of inertia, angular velocity, rate of
slow down, braking index, etc. For a first order phase transition these
quantities have a characteristic behavior, e.g., the braking index diverges as
. Observational consequences for first, second
and other phase transitions are discussed.Comment: 5 pages, one figure included, revtex latex styl
Kaluza-Klein Higher Derivative Induced Gravity
The existence and stability analysis of an inflationary solution in a
-dimensional anisotropic induced gravity is presented in this paper.
Nontrivial conditions in the field equations are shown to be compatible with a
cosmological model in which the 4-dimension external space evolves
inflationary, while, the D-dimension internal one is static. In particular,
only two additional constraints on the coupling constants are derived from the
abundant field equations and perturbation equations. In addition, a compact
formula for the non-redundant 4+D dimensional Friedmann equation is also
derived for convenience. Possible implications are also discussed in this
paper.Comment: 13 pages, typos/errors corrected, three additional appendices adde
How to tell a gravastar from a black hole
Gravastars have been recently proposed as potential alternatives to explain
the astrophysical phenomenology traditionally associated to black holes,
raising the question of whether the two objects can be distinguished at all.
Leaving aside the debate about the processes that would lead to the formation
of a gravastar and the astronomical evidence in their support, we here address
two basic questions: Is a gravastar stable against generic perturbations? If
stable, can an observer distinguish it from a black hole of the same mass? To
answer these questions we construct a general class of gravastars and determine
the conditions they must satisfy in order to exist as equilibrium solutions of
the Einstein equations. For such models we perform a systematic stability
analysis against axial-perturbations, computing the real and imaginary parts of
the eigenfrequencies. Overall, we find that gravastars are stable to axial
perturbations, but also that their quasi-normal modes differ from those of a
black hole of the same mass and thus can be used to discern, beyond dispute, a
gravastar from a black hole.Comment: 16 pages, 13 figures, minor improvemen
Exact Solutions of Regge-Wheeler Equation and Quasi-Normal Modes of Compact Objects
The well-known Regge-Wheeler equation describes the axial perturbations of
Schwarzschild metric in the linear approximation. From a mathematical point of
view it presents a particular case of the confluent Heun equation and can be
solved exactly, due to recent mathematical developments. We present the basic
properties of its general solution. A novel analytical approach and numerical
techniques for study the boundary problems which correspond to quasi-normal
modes of black holes and other simple models of compact objects are developed.Comment: latex file, 25 pages, 4 figures, new references, new results and new
Appendix added, some comments and corrections in the text made. Accepted for
publication in Classical and Quantum Gravity, 2006, simplification of
notations, changes in the norm in some formulas, corrections in reference
Traversable wormholes coupled to nonlinear electrodynamics
In this work we explore the possible existence of static, spherically
symmetric and stationary, axisymmetric traversable wormholes coupled to
nonlinear electrodynamics. Considering static and spherically symmetric (2+1)
and (3+1)-dimensional wormhole spacetimes, we verify the presence of an event
horizon and the non-violation of the null energy condition at the throat. For
the former spacetime, the principle of finiteness is imposed, in order to
obtain regular physical fields at the throat. Next, we analyze the
(2+1)-dimensional stationary and axisymmetric wormhole, and also verify the
presence of an event horizon, rendering the geometry non-traversable.
Relatively to the (3+1)-dimensional stationary and axisymmetric wormhole
geometry, we find that the field equations impose specific conditions that are
incompatible with the properties of wormholes. Thus, we prove the non-existence
of the general class of traversable wormhole solutions, outlined above, within
the context of nonlinear electrodynamics.Comment: 9 pages, Revtex4. V2: major change in title; considerable additions
in the Introduction and in the rotating solution, no physics changes;
correction of a reference, one reference added; now 10 pages. This version to
appear in Classical and Quantum Gravit
Gravastar Solutions with Continuous Pressures and Equation of State
We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial and
tangential pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated
references along with some clarifying remarks in the stability analysi
Can accretion disk properties distinguish gravastars from black holes?
Gravastars, hypothetic astrophysical objects, consisting of a dark energy
condensate surrounded by a strongly correlated thin shell of anisotropic
matter, have been proposed as an alternative to the standard black hole picture
of general relativity. Observationally distinguishing between astrophysical
black holes and gravastars is a major challenge for this latter theoretical
model. In the context of stationary and axially symmetrical geometries, a
possibility of distinguishing gravastars from black holes is through the
comparative study of thin accretion disks around rotating gravastars and
Kerr-type black holes, respectively. In the present paper, we consider
accretion disks around slowly rotating gravastars, with all the metric tensor
components estimated up to the second order in the angular velocity. Due to the
differences in the exterior geometry, the thermodynamic and electromagnetic
properties of the disks (energy flux, temperature distribution and equilibrium
radiation spectrum) are different for these two classes of compact objects,
consequently giving clear observational signatures. In addition to this, it is
also shown that the conversion efficiency of the accreting mass into radiation
is always smaller than the conversion efficiency for black holes, i.e.,
gravastars provide a less efficient mechanism for converting mass to radiation
than black holes. Thus, these observational signatures provide the possibility
of clearly distinguishing rotating gravastars from Kerr-type black holes.Comment: 12 pages, 12 figures. V2: 14 pages, significant discussion and
references added, to appear in Class.Quant.Gra
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