5,037 research outputs found
Phantom inflation and the "Big Trip"
Primordial inflation is regarded to be driven by a phantom field which is
here implemented as a scalar field satisfying an equation of state
, with . Being even aggravated by the weird properties
of phantom energy, this will pose a serious problem with the exit from the
inflationary phase. We argue however in favor of the speculation that a smooth
exit from the phantom inflationary phase can still be tentatively recovered by
considering a multiverse scenario where the primordial phantom universe would
travel in time toward a future universe filled with usual radiation, before
reaching the big rip. We call this transition the "big trip" and assume it to
take place with the help of some form of anthropic principle which chooses our
current universe as being the final destination of the time transition.Comment: 23 pages, 5 figures, LaTex, Phys. Lett. B (in press
Dynamics of magnetized relativistic tori oscillating around black holes
We present a numerical study of the dynamics of magnetized, relativistic,
non-self-gravitating, axisymmetric tori orbiting in the background spacetimes
of Schwarzschild and Kerr black holes. The initial models have a constant
specific angular momentum and are built with a non-zero toroidal magnetic field
component, for which equilibrium configurations have recently been obtained. In
this work we extend our previous investigations which dealt with purely
hydrodynamical thick discs, and study the dynamics of magnetized tori subject
to perturbations which, for the values of the magnetic field strength
considered here, trigger quasi-periodic oscillations lasting for tens of
orbital periods. Overall, we have found that the dynamics of the magnetized
tori analyzed is very similar to that found in the corresponding unmagnetized
models. The spectral distribution of the eigenfrequencies of oscillation shows
the presence of a fundamental p mode and of a series of overtones in a harmonic
ratio 2:3:.... These simulations, therefore, extend the validity of the model
of Rezzolla et al.(2003a) for explaining the high-frequency QPOs observed in
the spectra of LMXBs containing a black-hole candidate also to the case of
magnetized discs with purely toroidal magnetic field distribution. If
sufficiently compact and massive, these oscillations can also lead to the
emission of intense gravitational radiation which is potentially detectable for
sources within the Galaxy.Comment: 10 pages,7 figures, submitted to MNRA
Fully covariant and conformal formulation of the Z4 system in a reference-metric approach: comparison with the BSSN formulation in spherical symmetry
We adopt a reference-metric approach to generalize a covariant and conformal
version of the Z4 system of the Einstein equations. We refer to the resulting
system as ``fully covariant and conformal", or fCCZ4 for short, since it is
well suited for curvilinear as well as Cartesian coordinates. We implement this
fCCZ4 formalism in spherical polar coordinates under the assumption of
spherical symmetry using a partially-implicit Runge-Kutta (PIRK) method and
show that our code can evolve both vacuum and non-vacuum spacetimes without
encountering instabilities. Our method does not require regularization of the
equations to handle coordinate singularities, nor does it depend on
constraint-preserving outer boundary conditions, nor does it need any
modifications of the equations for evolutions of black holes. We perform
several tests and compare the performance of the fCCZ4 system, for different
choices of certain free parameters, with that of BSSN. Confirming earlier
results we find that, for an optimal choice of these parameters, and for
neutron-star spacetimes, the violations of the Hamiltonian constraint can be
between 1 and 3 orders of magnitude smaller in the fCCZ4 system than in the
BSSN formulation. For black-hole spacetimes, on the other hand, any advantages
of fCCZ4 over BSSN are less evident.Comment: 13 pages, 10 figure
Business Processes and Flexibility: A Theoretical Perspective
We address the intractable problem of flexibility in business process management: how to deal with variations, unique cases and exceptions? We identify and characterise five conceptualisations of business process. We discuss the main elements and relationships, contracts, and existential and representational properties established by each conceptualisation. For each conceptualisation, we analyse how it impacts flexibility and discuss different strategies for increasing flexibility. Finally, we synthesise our findings in an integrated framework, which helps by relating different conceptualisations of business process with strategies to increase flexibility. This paper makes several conceptual and practical contributions. Considering the former, we disentangle various notions of business process, considering in particular differences between ex ante and ex post, and class and instance. We also highlight that flexibility requires integrating variety and a multi-view over the events defined by a process. Regarding contributions to practice, we propose a framework for organisations to analyse how business processes can be related to flexibility
- …