2,671 research outputs found
Rotating and counterrotating relativistic thin disks as sources of stationary electrovacuum spacetimes
A detailed study is presented of the counterrotating model (CRM) for
electrovacuum stationary axially symmetric relativistic thin disks of infinite
extension without radial stress, in the case when the eigenvalues of the
energy-momentum tensor of the disk are real quantities, so that there is not
heat flow. We find a general constraint over the counterrotating tangential
velocities needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counterrotating charged dust fluids. We then show that, in
some cases, this constraint can be satisfied if we take the two counterrotating
tangential velocities as equal and opposite or by taking the two
counterrotating streams as circulating along electro-geodesics. However, we
show that, in general, it is not possible to take the two counterrotating
fluids as circulating along electro-geodesics nor take the two counterrotating
tangential velocities as equal and opposite. A simple family of models of
counterrotating charged disks based on the Kerr-Newman solution are considered
where we obtain some disks with a CRM well behaved. We also show that the disks
constructed from the Kerr-Newman solution can be interpreted, for all the
values of parameters, as a matter distribution with currents and purely
azimuthal pressure without heat flow. The models are constructed using the
well-known "displace, cut and reflect" method extended to solutions of vacuum
Einstein-Maxwell equations. We obtain, in all the cases, counterrotating
Kerr-Newman disks that are in agreement with all the energy conditions.Comment: 22 pages, 7 figures, Late
The Martin-Benito-Mena Marugan-Olmedo prescription for the Dapor-Liegener model of Loop Quantum Cosmology
Recently, an alternative Hamiltonian constraint for Loop Quantum Cosmology
has been put forward by Dapor and Liegener, inspired by previous work on
regularization due to Thiemann. Here, we quantize this Hamiltonian following a
prescription for cosmology proposed by Mart\'{\i}n-Benito, Mena Marug\'an, and
Olmedo. To this effect, we first regularize the Euclidean and Lorentzian parts
of the Hamiltonian constraint separately in the case of a Bianchi I cosmology.
This allows us to identify a natural symmetrization of the Hamiltonian which is
apparent in anisotropic scenarios. Preserving this symmetrization in isotropic
regimes, we then determine the Hamiltonian constraint corresponding to a
Friedmann-Lema\^itre-Robertson-Walker cosmology, which we proceed to quantize.
We compute the action of this Hamiltonian operator in the volume eigenbasis and
show that it takes the form of a fourth-order difference equation, unlike in
standard Loop Quantum Cosmology, where it is known to be of second order. We
investigate the superselection sectors of our constraint operator, proving that
they are semilattices supported only on either the positive or the negative
semiaxis, depending on the triad orientation. Remarkably, the decoupling
between semiaxes allows us to write a closed expression for the generalized
eigenfunctions of the geometric part of the constraint. This expression is
totally determined by the values at the two points of the semilattice that are
closest to the origin, namely the two contributions with smallest eigenvolume.
This is in clear contrast with the situation found for the standard Hamiltonian
of Loop Quantum Cosmology, where only the smallest value is free. This result
indicates that the degeneracy of the new geometric Hamiltonian operator is
equal to two, doubling the possible number of solutions with respect to the
conventional quantization considered until now.Comment: 15 pages, published in Physical Review
High-order gauge-invariant perturbations of a spherical spacetime
We complete the formulation of a general framework for the analysis of
high-order nonspherical perturbations of a four-dimensional spherical spacetime
by including a gauge-invariant description of the perturbations. We present a
general algorithm to construct these invariants and provide explicit formulas
for the case of second-order metric perturbations. We show that the well-known
problem of lack of invariance for the first-order perturbations with l=0,1
propagates to increasing values of l for perturbations of higher order, owing
to mode coupling. We also discuss in which circumstances it is possible to
construct the invariants
Evaluation of tree-based routing Ethernet
Tree-based Routing (TRE) revisits Tree-based Routing Architecture for Irregular Networks (TRAIN)—a forwarding scheme based on a spanning tree that was extended to use some shortcut links.We propose its adaptation to Ethernet, using a new type of hierarchical Ethernet addresses and a procedure to assign them to bridges. We show that compared to RSTP, TRE offers improved throughput. The impact of transient loops in TRE is lower compared to the application of the classical shortest path routing protocols to Ethernet. Finally, TRE is self-configuring and its forwarding process is simpler and more efficient than in standard Ethernet and shortest path routing proposals.Publicad
Hierarchical Up/Down Routing Architecture for Ethernet backbones and campus networks
We describe a new layer two distributed and scalable routing architecture. It uses an automatic hierarchical node identifier assignment mechanism associated to the rapid spanning tree protocol. Enhanced up/down mechanisms are used to prohibit some turns at nodes to break cycles, instead of blocking links like the spannning tree protocol does. The protocol performance is similar or better than other turn prohibition algorithms recently proposed with lower complexity O(Nd) and better scalability. Simulations show that the fraction of prohibited turns over random networks is less than 0.2. The effect of root bridge election on the performance of the protocol is limited both in the random and regular networks studied. The use of hierarchical, tree-descriptive addresses simplifies the routing, and avoids the need of all nodes having a global knowleddge of the network topology. Routing frames through the hierarchical tree at very high speed is possible by progressive decoding of frame destination address, without routing tables or port address learning. Coexistence with standard bridges is achieved using combined devices: bridges that forward the frames having global destination MAC addresses as standard bridges and frames with local MAC frames with the proposed protocol.Publicad
Second and higher-order perturbations of a spherical spacetime
The Gerlach and Sengupta (GS) formalism of coordinate-invariant, first-order,
spherical and nonspherical perturbations around an arbitrary spherical
spacetime is generalized to higher orders, focusing on second-order
perturbation theory. The GS harmonics are generalized to an arbitrary number of
indices on the unit sphere and a formula is given for their products. The
formalism is optimized for its implementation in a computer algebra system,
something that becomes essential in practice given the size and complexity of
the equations. All evolution equations for the second-order perturbations, as
well as the conservation equations for the energy-momentum tensor at this
perturbation order, are given in covariant form, in Regge-Wheeler gauge.Comment: Accepted for publication in Physical Review
Primordial perturbations in the Dapor-Liegener model of hybrid loop quantum cosmology
In this work, we extend the formalism of hybrid loop quantum cosmology for
primordial perturbations around a flat, homogeneous, and isotropic universe to
the new treatment of Friedmann-Lema\^itre-Robertson-Walker geometries proposed
recently by Dapor and Liegener, based on an alternative regularization of the
Hamiltonian constraint. In fact, our discussion is applicable also to other
possible regularization schemes for loop quantum cosmology, although we
specialize our analysis to the Dapor-Liegener proposal and construct explicitly
all involved quantum operators for that case.Comment: 17 page
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