7,788 research outputs found
Bounding the Hubble flow in terms of the w parameter
The last decade has seen increasing efforts to circumscribe and bound the
cosmological Hubble flow in terms of model-independent constraints on the
cosmological fluid - such as, for instance, the classical energy conditions of
general relativity. Quite a bit can certainly be said in this regard, but much
more refined bounds can be obtained by placing more precise constraints (either
theoretical or observational) on the cosmological fluid. In particular, the use
of the w-parameter (w=p/rho) has become increasingly common as a surrogate for
trying to say something about the cosmological equation of state. Herein we
explore the extent to which a constraint on the w-parameter leads to useful and
nontrivial constraints on the Hubble flow, in terms of constraints on density
rho(z), Hubble parameter H(z), density parameter Omega(z), cosmological
distances d(z), and lookback time T(z). In contrast to other partial results in
the literature, we carry out the computations for arbitrary values of the space
curvature k in [-1,0,+1], equivalently for arbitrary Omega_0 <= 1.Comment: 15 page
Myocardial Uptake of Radioactively Labelled Free Fatty Acids
Structural variations in the carbon chain of free fatty acids influence the uptake of free fatty acids in the myocardium. To enable the use of free fatty acids in nuclear cardiology, various methods of introducing gammaemitting isotopes have been evaluated. The uptake of various free fatty acids is described and structure-activity relationships deduce
Effective refractive index tensor for weak field gravity
Gravitational lensing in a weak but otherwise arbitrary gravitational field
can be described in terms of a 3 x 3 tensor, the "effective refractive index".
If the sources generating the gravitational field all have small internal
fluxes, stresses, and pressures, then this tensor is automatically isotropic
and the "effective refractive index" is simply a scalar that can be determined
in terms of a classic result involving the Newtonian gravitational potential.
In contrast if anisotropic stresses are ever important then the gravitational
field acts similarly to an anisotropic crystal. We derive simple formulae for
the refractive index tensor, and indicate some situations in which this will be
important.Comment: V1: 8 pages, no figures, uses iopart.cls. V2: 13 pages, no figures.
Significant additions and clarifications. This version to appear in Classical
and Quantum Gravit
Fundamental limitations on "warp drive" spacetimes
"Warp drive" spacetimes are useful as "gedanken-experiments" that force us to
confront the foundations of general relativity, and among other things, to
precisely formulate the notion of "superluminal" communication. We verify the
non-perturbative violation of the classical energy conditions of the Alcubierre
and Natario warp drive spacetimes and apply linearized gravity to the
weak-field warp drive, testing the energy conditions to first and second order
of the non-relativistic warp-bubble velocity. We are primarily interested in a
secondary feature of the warp drive that has not previously been remarked upon,
if it could be built, the warp drive would be an example of a "reaction-less
drive". For both the Alcubierre and Natario warp drives we find that the
occurrence of significant energy condition violations is not just a high-speed
effect, but that the violations persist even at arbitrarily low speeds.
An interesting feature of this construction is that it is now meaningful to
place a finite mass spaceship at the center of the warp bubble, and compare the
warp field energy with the mass-energy of the spaceship. There is no hope of
doing this in Alcubierre's original version of the warp-field, since by
definition the point in the center of the warp bubble moves on a geodesic and
is "massless". That is, in Alcubierre's original formalism and in the Natario
formalism the spaceship is always treated as a test particle, while in the
linearized theory we can treat the spaceship as a finite mass object. For both
the Alcubierre and Natario warp drives we find that even at low speeds the net
(negative) energy stored in the warp fields must be a significant fraction of
the mass of the spaceship.Comment: 18 pages, Revtex4. V2: one reference added, some clarifying comments
and discussion, no physics changes, accepted for publication in Classical and
Quantum Gravit
Traversable Wormholes in Geometries of Charged Shells
We construct a static axisymmetric wormhole from the gravitational field of
two charged shells which are kept in equilibrium by their electromagnetic
repulsion. For large separations the exterior tends to the Majumdar-Papapetrou
spacetime of two charged particles. The interior of the wormhole is a
Reissner-Nordstr\"om black hole matching to the two shells. The wormhole is
traversable and connects to the same asymptotics without violation of energy
conditions. However, every point in the Majumdar-Papapetrou region lies on a
closed timelike curve.Comment: 9 pages, LaTeX, 1 figur
High-pressure study of the non-Fermi liquid material U_2Pt_2In
The effect of hydrostatic pressure (p<= 1.8 GPa) on the non-Fermi liquid
state of U_2Pt_2In is investigated by electrical resistivity measurements in
the temperature interval 0.3-300 K. The experiments were carried out on
single-crystals with the current along (I||c) and perpendicular (I||a) to the
tetragonal axis. The pressure effect is strongly current-direction dependent.
For I||a we observe a rapid recovery of the Fermi-liquid T^2-term with
pressure. The low-temperature resistivity can be analysed satisfactorily within
the magnetotransport theory of Rosch, which provides strong evidence for the
location of U_2Pt_2In at an antiferromagnetic quantum critical point. For I||c
the resistivity increases under pressure, indicating the enhancement of an
additional scattering mechanism. In addition, we have measured the pressure
dependence of the antiferromagnetic ordering temperature (T_N= 37.6 K) of the
related compound U_2Pd_2In. A simple Doniach-type diagram for U_2Pt_2In and
U_2Pd_2In under pressure is presented.Comment: 21 pages (including 5 figures); pdf forma
Analog black holes in flowing dielectrics
We show that a flowing dielectric medium with a linear response to an
external electric field can be used to generate an analog geometry that has
many of the formal properties of a Schwarzschild black hole for light rays, in
spite of birefringence. We also discuss the possibility of generating these
analog black holes in the laboratory.Comment: Revtex4 file, 7 pages, 4 eps figures, a few changes in presentation,
some references added, conclusions unchange
A Theoretical Construction of Thin Shell Wormhole from Tidal Charged Black hole
Recently, Dadhich et al [ Phys.Lett.B 487, 1 (2000)] have discovered a black
hole solution localized on a three brane in five dimensional gravity in the
Randall-Sundrum scenario. In this article, we develop a new class of thin shell
wormhole by surgically grafting above two black hole spacetimes. Various
aspects of this thin wormhole are also analyzed.Comment: 14 pages, 6 figures, Accepted in Gen.Rel.Gra
The fate of phonons in freely expanding Bose-Einstein condensates
Phonon-like excitations can be imprinted into a trapped Bose-Einstein
condensate of cold atoms using light scattering. If the condensate is suddenly
let to freely expand, the initial phonons lose their collective character by
transferring their energy and momentum to the motion of individual atoms. The
basic mechanisms of this evaporation process are investigated by using the
Gross-Pitaevskii theory and dynamically rescaled Bogoliubov equations.
Different regimes of evaporation are shown to occur depending on the phonon
wavelength. Distinctive signatures of the evaporated phonons are visible in the
density distribution of the expanded gas, thus providing a new type of
spectroscopy of Bogoliubov excitations.Comment: 13 pages, 16 figure
- …