109 research outputs found
Matrix model formulation of four dimensional gravity
The attempt of extending to higher dimensions the matrix model formulation of
two-dimensional quantum gravity leads to the consideration of higher rank
tensor models. We discuss how these models relate to four dimensional quantum
gravity and the precise conditions allowing to associate a four-dimensional
simplicial manifold to Feynman diagrams of a rank-four tensor model.Comment: Lattice 2000 (Gravity), 4 pages,4 figures, uses espcrc2.st
Eigenvalues of the Weyl operator as observables of general relativity
We consider the eigenvalues of the three-dimensional Weyl operator defined in
terms of the (Euclidean) Ashtekar variables, and we study their dependence on
the gravitational field. We notice that these eigenvalues can be used as
gravitational variables, and derive explicit formulas for their Poisson
brackets and their time evolution.Comment: 15 pages, LaTex style, preprint UPRF-94-39
Neutron Star instabilities in full General Relativity using a ideal fluid
We present results about the effect of the use of a stiffer equation of
state, namely the ideal-fluid ones, on the dynamical bar-mode
instability in rapidly rotating polytropic models of neutron stars in full
General Relativity. We determine the change on the critical value of the
instability parameter for the emergence of the instability when the
adiabatic index is changed from 2 to 2.75 in order to mimic the
behavior of a realistic equation of state. In particular, we show that the
threshold for the onset of the bar-mode instability is reduced by this change
in the stiffness and give a precise quantification of the change in value of
the critical parameter . We also extend the analysis to lower values
of and show that low-beta shear instabilities are present also in the
case of matter described by a simple polytropic equation of state.Comment: 16 pages, 16 figure
Dynamical bar-mode instability in rotating and magnetized relativistic stars
We present three-dimensional simulations of the dynamical bar-mode
instability in magnetized and differentially rotating stars in full general
relativity. Our focus is on the effects that magnetic fields have on the
dynamics and the onset of the instability. In particular, we perform
ideal-magnetohydrodynamics simulations of neutron stars that are known to be
either stable or unstable against the purely hydrodynamical instability, but to
which a poloidal magnetic field in the range of -- G is
superimposed initially. As expected, the differential rotation is responsible
for the shearing of the poloidal field and the consequent linear growth in time
of the toroidal magnetic field. The latter rapidly exceeds in strength the
original poloidal one, leading to a magnetic-field amplification in the the
stars. Weak initial magnetic fields, i.e. G, have
negligible effects on the development of the dynamical bar-mode instability,
simply braking the stellar configuration via magnetic-field shearing, and over
a timescale for which we derived a simple algebraic expression. On the other
hand, strong magnetic fields, i.e. G, can suppress the
instability completely, with the precise threshold being dependent also on the
amount of rotation. As a result, it is unlikely that very highly magnetized
neutron stars can be considered as sources of gravitational waves via the
dynamical bar-mode instability.Comment: 18 pages, 13 figure
Modeling Mergers of Known Galactic Systems of Binary Neutron Stars
We present a study of the merger of six different known galactic systems of
binary neutron stars (BNS) of unequal mass with a mass ratio between and
. Specifically, these systems are J1756-2251, J0737-3039A, J1906+0746,
B1534+12, J0453+1559 and B1913+16. We follow the dynamics of the merger from
the late stage of the inspiral process up to 20 ms after the system has
merged, either to form a hyper-massive neutron star (NS) or a rotating black
hole (BH), using a semi-realistic equation of state (EOS), namely the
seven-segment piece-wise polytropic SLy with a thermal component. For the most
extreme of these systems (, J0453+1559), we also investigate the
effects of different EOSs: APR4, H4, and MS1. Our numerical simulations are
performed using only publicly available open source code such as, the Einstein
Toolkit code deployed for the dynamical evolution and the LORENE code for the
generation of the initial models. We show results on the gravitational wave
signals, spectrogram and frequencies of the BNS after the merger and the BH
properties in the two cases in which the system collapse within the simulated
time.Comment: 13 pages, 10 figure
Hamiltonian linearization of the rest-frame instant form of tetrad gravity in a completely fixed 3-orthogonal gauge: a radiation gauge for background-independent gravitational waves in a post-Minkowskian Einstein spacetime
In the framework of the rest-frame instant form of tetrad gravity, where the
Hamiltonian is the weak ADM energy , we define a special
completely fixed 3-orthogonal Hamiltonian gauge, corresponding to a choice of
{\it non-harmonic} 4-coordinates, in which the independent degrees of freedom
of the gravitational field are described by two pairs of canonically conjugate
Dirac observables (DO) , , . We define a Hamiltonian linearization of the
theory, i.e. gravitational waves, {\it without introducing any background
4-metric}, by retaining only the linear terms in the DO's in the
super-hamiltonian constraint (the Lichnerowicz equation for the conformal
factor of the 3-metric) and the quadratic terms in the DO's in . {\it We solve all the constraints} of the linearized theory: this
amounts to work in a well defined post-Minkowskian Christodoulou-Klainermann
space-time. The Hamilton equations imply the wave equation for the DO's
, which replace the two polarizations of the TT
harmonic gauge, and that {\it linearized Einstein's equations are satisfied} .
Finally we study the geodesic equation, both for time-like and null geodesics,
and the geodesic deviation equation.Comment: LaTeX (RevTeX3), 94 pages, 4 figure
Bar-mode instability suppression in magnetized relativistic stars
We show that magnetic fields stronger than about G are able to
suppress the development of the hydrodynamical bar-mode instability in
relativistic stars. The suppression is due to a change in the rest-mass density
and angular velocity profiles due to the formation and to the linear growth of
a toroidal component that rapidly overcomes the original poloidal one, leading
to an amplification of the total magnetic energy. The study is carried out
performing three-dimensional ideal-magnetohydrodynamics simulations in full
general relativity, superimposing to the initial (matter) equilibrium
configurations a purely poloidal magnetic field in the range
G. When the seed field is a few parts in G or above, all the evolved
models show the formation of a low-density envelope surrounding the star. For
much weaker fields, no effect on the matter evolution is observed, while
magnetic fields which are just below the suppression threshold are observed to
slow down the growth-rate of the instability.Comment: 6 pages, 4 figures, to appear on the proceedings of the 4th YRM
(Trieste 2013
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