334 research outputs found
Dark Energy and the mass of galaxy clusters
Up to now, Dark Energy evidences are based on the dynamics of the universe on
very large scales, above 1 Gpc. Assuming it continues to behave like a
cosmological constant on much smaller scales, I discuss its effects
on the motion of non-relativistic test-particles in a weak gravitational field
and I propose a way to detect evidences of at the scale of
about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.Comment: 5 pages, no figures, references adde
Oscillatory Thickness Dependence of Magnetic Moments and interface-induced Changes of the Exchange Coupling in Co/Cu and Co-Ni/Cu Multilayers
We perform first-principles calculations for the three multilayer systems
(100)-Co_1/Cu_n, -NiCo_2Ni/Cu_n and -Co_4/Cu_n, and find from a comparison x of
the results for system 2 and 3 that amplitude and phase of the exchange
coupling are sensitive to the magnetic-slab/nonmagnetic-spacer interface.
Moreover, we observe that for the system 1 and 2 the averaged magnetic moment
of the magnetic slab oscillates with the spacer thickness similarly as the
exchange coupling.Comment: 5 pages (Latex, to be applied 2 times) + 2 figures (.ps-files
Gravitational Collapse with a Cosmological Constant
We consider the effect of a positive cosmological constant on spherical
gravitational collapse to a black hole for a few simple, analytic cases. We
construct the complete Oppenheimer-Snyder-deSitter (OSdS) spacetime, the
generalization of the Oppenheimer-Snyder solution for collapse from rest of a
homogeneous dust ball in an exterior vacuum. In OSdS collapse, the cosmological
constant may affect the onset of collapse and decelerate the implosion
initially, but it plays a diminishing role as the collapse proceeds. We also
construct spacetimes in which a collapsing dust ball can bounce, or hover in
unstable equilibrium, due to the repulsive force of the cosmological constant.
We explore the causal structure of the different spacetimes and identify any
cosmological and black hole event horizons which may be present.Comment: 7 pages, 10 figures; To appear in Phys. Rev.
Static Einstein-Maxwell Solutions in 2+1 dimensions
We obtain the Einstein-Maxwell equations for (2+1)-dimensional static
space-time, which are invariant under the transformation
. It is shown that the
magnetic solution obtained with the help of the procedure used in
Ref.~\cite{Cataldo}, can be obtained from the static BTZ solution using an
appropriate transformation. Superpositions of a perfect fluid and an electric
or a magnetic field are separately studied and their corresponding solutions
found.Comment: 8 pages, LaTeX, no figures, to appear in Physical Review
Approach to the extremal limit of the Schwarzschild-de Sitter black hole
The quasinormal-mode spectrum of the Schwarzschild-de Sitter black hole is
studied in the limit of near-equal black-hole and cosmological radii. It is
found that the mode_frequencies_ agree with the P"oschl-Teller approximation to
one more order than previously realized, even though the effective_potential_
does not. Whether the spectrum approaches the limiting one uniformly in the
mode index is seen to depend on the chosen units (to the order investigated). A
perturbation framework is set up, in which these issues can be studied to
higher order in future.Comment: REVTeX4, 4pp., no figures. N.B. "Alec" is my first, and "Maassen van
den Brink" my family name. v2: added numerical verificatio
Cosmological Constant, Conical Defect and Classical Tests of General Relativity
We investigate the perihelion shift of the planetary motion and the bending
of starlight in the Schwarzschild field modified by the presence of a
-term plus a conical defect. This analysis generalizes an earlier
result obtained by Islam (Phys. Lett. A 97, 239, 1983) to the case of a pure
cosmological constant. By using the experimental data we obtain that the
parameter characterizing the conical defect is less than
and , respectively, on the length scales associated with such
phenomena. In particular, if the defect is generated by a cosmic string, these
values correspond to limits on the linear mass densities of and
, respectively.Comment: 9 pages, no figures, revte
The geodesic structure of the Schwarzschild Anti-de Sitter black hole
In the present work we found the geodesic structure of an AdS black hole. By
means of a detailed analyze of the corresponding effective potentials for
particles and photon, we found all the possible motions which are allowed by
the energy levels. Radial and non radial trajectories were exactly evaluated
for both geodesics. The founded orbits were plotted in order to have a direct
visualization of the allowed motions. We show that the geodesic structure of
this black hole presents new type of motions not allowed by the Schwarzschild
spacetime.Comment: 17 pages, 11 figure
A Cosmological No-Hair Theorem
A generalisation of Price's theorem is given for application to Inflationary
Cosmologies. Namely, we show that on a Schwarzschild--de Sitter background
there are no static solutions to the wave or gravitational perturbation
equations for modes with angular momentum greater than their intrinsic spin.Comment: 9 pages, NCL94 -TP4, (Revtex
All Static Circularly Symmetric Perfect Fluid Solutions of 2+1 Gravity
Via a straightforward integration of the Einstein equations with cosmological
constant, all static circularly symmetric perfect fluid 2+1 solutions are
derived. The structural functions of the metric depend on the energy density,
which remains in general arbitrary. Spacetimes for fluids fulfilling linear and
polytropic state equations are explicitly derived; they describe, among others,
stiff matter, monatomic and diatomic ideal gases, nonrelativistic degenerate
fermions, incoherent and pure radiation. As a by--product, we demonstrate the
uniqueness of the constant energy density perfect fluid within the studied
class of metrics. A full similarity of the perfect fluid solutions with
constant energy density of the 2+1 and 3+1 gravities is established.Comment: revtex4, 8 page
The family of regular interiors for non-rotating black holes with
We find the general solution for the spacetimes describing the interior of
static black holes with an equation of state of the type (
being the stress-energy tensor). This form is the one expected from taking into
account different quantum effects associated with strong gravitational fields.
We recover all the particular examples found in the literature. We remark that
all the solutions found follow the natural scheme of an interior core linked
smoothly with the exterior solution by a transient region. We also discuss
their local energy properties and give the main ideas involved in a possible
generalization of the scheme, in order to include other realistic types of
sources.Comment: 31 pages, 1 figure, version to appear in Physical Review
- …