130 research outputs found
A Lemaitre-Tolman-Bondi cosmological wormhole
We present a new analytical solution of the Einstein field equations
describing a wormhole shell of zero thickness joining two
Lema{\i}tre-Tolman-Bondi universes, with no radial accretion. The material on
the shell satisfies the energy conditions and, at late times, the shell becomes
comoving with the dust-dominated cosmic substratum.Comment: 5 pages, latex, no figures, to appear in Phys. Rev.
Constraining the evolutionary history of Newton's constant with gravitational wave observations
Space-borne gravitational wave detectors, such as the proposed Laser
Interferometer Space Antenna, are expected to observe black hole coalescences
to high redshift and with large signal-to-noise ratios, rendering their
gravitational waves ideal probes of fundamental physics. The promotion of
Newton's constant to a time-function introduces modifications to the binary's
binding energy and the gravitational wave luminosity, leading to corrections in
the chirping frequency. Such corrections propagate into the response function
and, given a gravitational wave observation, they allow for constraints on the
first time-derivative of Newton's constant at the time of merger. We find that
space-borne detectors could indeed place interesting constraints on this
quantity as a function of sky position and redshift, providing a
{\emph{constraint map}} over the entire range of redshifts where binary black
hole mergers are expected to occur. A LISA observation of an equal-mass
inspiral event with total redshifted mass of 10^5 solar masses for three years
should be able to measure at the time of merger to better than
10^(-11)/yr.Comment: 11 pages, 2 figures, replaced with version accepted for publication
in Phys. Rev. D
Intersecting black branes in strong gravitational waves
We consider intersecting black branes with strong gravitational waves
propagating along their worldvolume in the context of supergravity theories.
Both near-horizon and space-filling gravitational wave modes are included in
our ansatz. The equations of motion (originally, partial differential
equations) are shown to reduce to ordinary differential equations, which
include a Toda-like system. For special arrangements of intersecting black
branes, the Toda-like system becomes integrable, permitting a more thorough
analysis of the gravitational equations of motion.Comment: 17 pages; v2: cosmetic improvements, published versio
Exact solutions of coupled Li\'enard-type nonlinear systems using factorization technique
General solutions of nonlinear ordinary differential equations (ODEs) are in
general difficult to find although powerful integrability techniques exist in
the literature for this purpose. It has been shown that in some scalar cases
particular solutions may be found with little effort if it is possible to
factorize the equation in terms of first order differential operators. In our
present study we use this factorization technique to address the problem of
finding solutions of a system of general two-coupled Li\'enard type nonlinear
differential equations. We describe a generic algorithm to identify specific
classes of Li\'enard type systems for which solutions may be found. We
demonstrate this method by identifying a class of two-coupled equations for
which the particular solution can be found by solving a Bernoulli equation.
This class of equations include coupled generalization of the modified Emden
equation. We further deduce the general solution of a class of coupled ordinary
differential equations using the factorization procedure discussed in this
manuscript.Comment: Accepted for publication in J. Math. Phy
Naked Singularity of the Vaidya-deSitter Spacetime and Cosmic Censorship Hypothesis
We investigate the formation of a locally naked singularity in the collapse
of radiation shells in an expanding Vaidya-deSitter background. This is
achieved by considering the behaviour of non-spacelike and radial geodesics
originating at the singularity. A specific condition is determined for the
existence of radially outgoing, null geodesics originating at the singularity
which, when this condition is satisfied, becomes locally naked. This condition
turns out to be the same as that in the collapse of radiation shells in an
asymptotically flat background. Therefore, we have, at least for the case
considered here, established that the asymptotic flatness of the spacetime is
not essential for the development of a locally naked singularity. Our result
then unequivocally supports the view that no special role be given to
asymptotic observers (or, for that matter, any set of observers) in the
formulation of the Cosmic Censorship Hypothesis.Comment: submitted Oct. 1997, Revised Version, to be published Gen. Rel.
Grav., Latex file, 9 page
Explicit Fermi Coordinates and Tidal Dynamics in de Sitter and Goedel Spacetimes
Fermi coordinates are directly constructed in de Sitter and Goedel spacetimes
and the corresponding exact coordinate transformations are given explicitly.
The quasi-inertial Fermi coordinates are then employed to discuss the dynamics
of a free test particle in these spacetimes and the results are compared to the
corresponding generalized Jacobi equations that contain only the lowest-order
tidal terms. The domain of validity of the generalized Jacobi equation is thus
examined in these cases. Furthermore, the difficulty of constructing explicit
Fermi coordinates in black-hole spacetimes is demonstrated.Comment: 23 pages, 3 figures; v2: expanded version (27 pages, 3 figures
Extended black holes in strong gravitational waves
We describe a large class of solutions in pure gravity, dilaton gravity and
supergravity corresponding to extended higher-dimensional black holes with
strong (non-linear) gravitational waves propagating along their worldvolume.
For pure gravity, the extended black holes are higher-dimensional analogs of
the point-like Schwarzschild black hole in four dimensions. For supergravity,
they are non-extremal p-branes. The gravitational waves can be both
space-filling and localized around the worldvolume of the extended black holes.
The solutions we present contain a large number of arbitrary functions of the
light-cone time describing the amplitudes of different non-linear gravitational
wave modes.Comment: 14 pages; v3: commentary and references extended, published versio
The influence of the cosmological expansion on local systems
Following renewed interest, the problem of whether the cosmological expansion
affects the dynamics of local systems is reconsidered. The cosmological
correction to the equations of motion in the locally inertial Fermi normal
frame (the relevant frame for astronomical observations) is computed. The
evolution equations for the cosmological perturbation of the two--body problem
are solved in this frame. The effect on the orbit is insignificant as are the
effects on the galactic and galactic--cluster scales.Comment: To appear in the Astrophysical Journal, Late
Expansion-induced contribution to the precession of binary orbits
We point out the existence of new effects of global spacetime expansion on
local binary systems. In addition to a possible change of orbital size, there
is a contribution to the precession of elliptic orbits, to be added to the
well-known general relativistic effect in static spacetimes, and the
eccentricity can change. Our model calculations are done using geodesics in a
McVittie metric, representing a localized system in an asymptotically
Robertson-Walker spacetime; we give a few numerical estimates for that case,
and indicate ways in which the model should be improved.Comment: revtex, 7 pages, no figures; revised for publication in Classical and
Quantum Gravity, with minor changes in response to referees' comment
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
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