2,064 research outputs found
A Gaussian Weave for Kinematical Loop Quantum Gravity
Remarkable efforts in the study of the semi-classical regime of kinematical
loop quantum gravity are currently underway. In this note, we construct a
``quasi-coherent'' weave state using Gaussian factors. In a similar fashion to
some other proposals, this state is peaked in both the connection and the spin
network basis. However, the state constructed here has the novel feature that,
in the spin network basis, the main contribution for this state is given by the
fundamental representation, independently of the value of the parameter that
regulates the Gaussian width.Comment: 15 pages, 3 figures, Revtex file. Comments added and references
updated. Final version to appear in IJMP-
Gravitational waves from oscillating accretion tori: Comparison between different approaches
Quasi-periodic oscillations of high density thick accretion disks orbiting a
Schwarzschild black hole have been recently addressed as interesting sources of
gravitational waves. The aim of this paper is to compare the gravitational
waveforms emitted from these sources when computed using (variations of) the
standard quadrupole formula and gauge-invariant metric perturbation theory. To
this goal we evolve representative disk models using an existing general
relativistic hydrodynamics code which has been previously employed in
investigations of such astrophysical systems. Two are the main results of this
work: First, for stable and marginally stable disks, no excitation of the black
hole quasi-normal modes is found. Secondly, we provide a simple, relativistic
modification of the Newtonian quadrupole formula which, in certain regimes,
yields excellent agreement with the perturbative approach. This holds true as
long as back-scattering of GWs is negligible. Otherwise, any functional form of
the quadrupole formula yields systematic errors of the order of 10%.Comment: 6 pages and 3 figures, RevTex, accepted for publication in Phys. Rev.
Group Field Theory: An overview
We give a brief overview of the properties of a higher dimensional
generalization of matrix model which arises naturally in the context of a
background independent approach to quantum gravity, the so called group field
theory. We show that this theory leads to a natural proposal for the physical
scalar product of quantum gravity. We also show in which sense this theory
provides a third quantization point of view on quantum gravity.Comment: 10 page
Gauging kinematical and internal symmetry groups for extended systems: the Galilean one-time and two-times harmonic oscillators
The possible external couplings of an extended non-relativistic classical
system are characterized by gauging its maximal dynamical symmetry group at the
center-of-mass. The Galilean one-time and two-times harmonic oscillators are
exploited as models. The following remarkable results are then obtained: 1) a
peculiar form of interaction of the system as a whole with the external gauge
fields; 2) a modification of the dynamical part of the symmetry
transformations, which is needed to take into account the alteration of the
dynamics itself, induced by the {\it gauge} fields. In particular, the
Yang-Mills fields associated to the internal rotations have the effect of
modifying the time derivative of the internal variables in a scheme of minimal
coupling (introduction of an internal covariant derivative); 3) given their
dynamical effect, the Yang-Mills fields associated to the internal rotations
apparently define a sort of Galilean spin connection, while the Yang-Mills
fields associated to the quadrupole momentum and to the internal energy have
the effect of introducing a sort of dynamically induced internal metric in the
relative space.Comment: 32 pages, LaTex using the IOP preprint macro package (ioplppt.sty
available at: http://www.iop.org/). The file is available at:
http://www.fis.unipr.it/papers/1995.html The file is a uuencoded tar gzip
file with the IOP preprint style include
Convective Excitation of Inertial Modes in Binary Neutron Star Mergers
We present the first very long-term simulations (extending up to ~140 ms
after merger) of binary neutron star mergers with piecewise polytropic
equations of state and in full general relativity. Our simulations reveal that
at a time of 30-50 ms after merger, parts of the star become convectively
unstable, which triggers the excitation of inertial modes. The excited inertial
modes are sustained up to several tens of milliseconds and are potentially
observable by the planned third-generation gravitational-wave detectors at
frequencies of a few kilohertz. Since inertial modes depend on the rotation
rate of the star and they are triggered by a convective instability in the
postmerger remnant, their detection in gravitational waves will provide a
unique opportunity to probe the rotational and thermal state of the merger
remnant. In addition, our findings have implications for the long-term
evolution and stability of binary neutron star remnantsComment: 6 pages, 4 figure
Counting surface states in the loop quantum gravity
We adopt the point of view that (Riemannian) classical and (loop-based)
quantum descriptions of geometry are macro- and micro-descriptions in the usual
statistical mechanical sense. This gives rise to the notion of geometrical
entropy, which is defined as the logarithm of the number of different quantum
states which correspond to one and the same classical geometry configuration
(macro-state). We apply this idea to gravitational degrees of freedom induced
on an arbitrarily chosen in space 2-dimensional surface. Considering an
`ensemble' of particularly simple quantum states, we show that the geometrical
entropy corresponding to a macro-state specified by a total area of
the surface is proportional to the area , with being
approximately equal to . The result holds both for case of open
and closed surfaces. We discuss briefly physical motivations for our choice of
the ensemble of quantum states.Comment: This paper is a substantially modified version of the paper `The
Bekenstein bound and non-perturbative quantum gravity'. Although the main
result (i.e. the result of calculation of the number of quantum states that
correspond to one and the same area of 2-d surface) remains unchanged, it is
presented now from a different point of view. The new version contains a
discussion both of the case of open and closed surfaces, and a discussion of
a possibility to generalize the result obtained considering arbitrary surface
quantum states. LaTeX, 21 pages, 6 figures adde
Photons from quantized electric flux representations
The quantum theory of U(1) connections admits a diffeomorphism invariant
representation in which the electric flux through any surface is quantized.
This representation is the analog of the representation of quantum SU(2) theory
used in loop quantum gravity. We investigate the relation between this
representation, in which the basic excitations are `polymer-like', and the Fock
representation, in which the basic excitations are wave-like photons. We show
that normalizable states in the Fock space are associated with `distributional'
states in the quantized electric flux representation. This work is motivated by
the question of how wave-like gravitons in linearised gravity arise from
polymer-like states in non-perturbative loop quantum gravity.Comment: 22 pages, no figure
Continuum spin foam model for 3d gravity
An example illustrating a continuum spin foam framework is presented. This
covariant framework induces the kinematics of canonical loop quantization, and
its dynamics is generated by a {\em renormalized} sum over colored polyhedra.
Physically the example corresponds to 3d gravity with cosmological constant.
Starting from a kinematical structure that accommodates local degrees of
freedom and does not involve the choice of any background structure (e. g.
triangulation), the dynamics reduces the field theory to have only global
degrees of freedom. The result is {\em projectively} equivalent to the
Turaev-Viro model.Comment: 12 pages, 3 figure
Quantum tetrahedra and simplicial spin networks
A new link between tetrahedra and the group SU(2) is pointed out: by
associating to each face of a tetrahedron an irreducible unitary SU(2)
representation and by imposing that the faces close, the concept of quantum
tetrahedron is seen to emerge. The Hilbert space of the quantum tetrahedron is
introduced and it is shown that, due to an uncertainty relation, the ``geometry
of the tetrahedron'' exists only in the sense of ``mean geometry''. A
kinematical model of quantum gauge theory is also proposed, which shares the
advantages of the Loop Representation approach in handling in a simple way
gauge- and diff-invariances at a quantum level, but is completely
combinatorial. The concept of quantum tetrahedron finds a natural application
in this model, giving a possible intepretation of SU(2) spin networks in terms
of geometrical objects.Comment: 11 pages, LaTeX, 1 figure.ps, typos corrected, references added and
updated, a note added, E-mail and postal addresses change
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