3,741 research outputs found
Radiative spacetimes approaching the Vaidya metric
We analyze a class of exact type II solutions of the Robinson-Trautman family
which contain pure radiation and (possibly) a cosmological constant. It is
shown that these spacetimes exist for any sufficiently smooth initial data, and
that they approach the spherically symmetric Vaidya-(anti-)de Sitter metric. We
also investigate extensions of the metric, and we demonstrate that their order
of smoothness is in general only finite. Some applications of the results are
outlined.Comment: 12 pages, 3 figure
Investigation of the effects of inlet shapes on fan noise radiation
The effect of inlet shape on forward radiated fan tone noise directivities was investigated under experimentally simplified zero flow conditions. Simulated fan tone noise was radiated to the far field through various shaped zero flow inlets. Baseline data were collected for the simplest baffled and unbaffled straight pipe inlets. These data compared well with prediction. The more general inlet shapes tested were the conical, circular, and exponential surfaces of revolution and an asymmetric inlet achieved by cutting a straight pipe inlet at an acute angle. Approximate theories were developed for these general shapes and some comparisons with data are presented. The conical and exponential shapes produced directivities that differed considerably from the baseline data while the circular shape produced directivities similar to the baseline data. The asymmetric inlet produced asymmetric directivities with significant reductions over the straight pipe data for some angles
Non-symmetric trapped surfaces in the Schwarzschild and Vaidya spacetimes
Marginally trapped surfaces (MTSs) are commonly used in numerical relativity
to locate black holes. For dynamical black holes, it is not known generally if
this procedure is sufficiently reliable. Even for Schwarzschild black holes,
Wald and Iyer constructed foliations which come arbitrarily close to the
singularity but do not contain any MTSs. In this paper, we review the Wald-Iyer
construction, discuss some implications for numerical relativity, and
generalize to the well known Vaidya spacetime describing spherically symmetric
collapse of null dust. In the Vaidya spacetime, we numerically locate
non-spherically symmetric trapped surfaces which extend outside the standard
spherically symmetric trapping horizon. This shows that MTSs are common in this
spacetime and that the event horizon is the most likely candidate for the
boundary of the trapped region.Comment: 4 pages, 3 figures; v2: minor modifications; v3: clarified
conclusion
Connected Autonomous Electric Vehicles as Enablers for Low-Carbon Future
Transportation is the main cause of various harmful gases being released into the atmosphere. Due to dependency on fossil fuels, conventional internal-combustion engine vehicles cause major impacts on air pollution and climate change. Achieving greenhouse gas (GHG) reduction targets requires electrification of transportation at the larger scale. Zero-emission vehicles are developing rapidly with consequences for energy use and GHG emissions, and their penetration is rising throughout the world. Such vehicles are widely considered as a promising solution for GHG reduction and a key to low-carbon mobility future. Recent trend in transportation system is a rapid shift toward connected autonomous vehicles. Connected autonomous electric vehicle (CAEV) will play a vital role in emerging revolution in sustainable low-carbon mobility. They can result in major reductions in GHG emissions and be at the forefront of rapid transformation in transportation. CAEVs have great potential to operate with higher vehicle efficiency, if they are charged using renewable energy sources that will significantly reduce emissions and dependency on fossil fuels. This book chapter is intended not only to provide understanding of potential environmental implications of CAEV technologies by reviewing the existing studies and research works but also to discuss environmental impacts including GHG emissions and improvement of vehicle efficiency
Quantization in black hole backgrounds
Quantum field theory in a semiclassical background can be derived as an
approximation to quantum gravity from a weak-coupling expansion in the inverse
Planck mass. Such an expansion is studied for evolution on "nice-slices" in the
spacetime describing a black hole of mass M. Arguments for a breakdown of this
expansion are presented, due to significant gravitational coupling between
fluctuations, which is consistent with the statement that existing calculations
of information loss in black holes are not reliable. For a given fluctuation,
the coupling to subsequent fluctuations becomes of order unity by a time of
order M^3. Lack of a systematic derivation of the weakly-coupled/semiclassical
approximation would indicate a role for the non-perturbative dynamics of
gravity, and possibly for the proposal that such dynamics has an essentially
non-local quality.Comment: 28 pages, 4 figures, harvmac. v2: added refs, minor clarification
Semiclassical Instability of the Cauchy Horizon in Self-Similar Collapse
Generic spherically symmetric self-similar collapse results in strong
naked-singularity formation. In this paper we are concerned with particle
creation during a naked-singularity formation in spherically symmetric
self-similar collapse without specifying the collapsing matter. In the generic
case, the power of particle emission is found to be proportional to the inverse
square of the remaining time to the Cauchy horizon (CH). The constant of
proportion can be arbitrarily large in the limit to marginally naked
singularity. Therefore, the unbounded power is especially striking in the case
that an event horizon is very close to the CH because the emitted energy can be
arbitrarily large in spite of a cutoff expected from quantum gravity. Above
results suggest the instability of the CH in spherically symmetric self-similar
spacetime from quantum field theory and seem to support the existence of a
semiclassical cosmic censor. The divergence of redshifts and blueshifts of
emitted particles is found to cause the divergence of power to positive or
negative infinity, depending on the coupling manner of scalar fields to
gravity. On the other hand, it is found that there is a special class of
self-similar spacetimes in which the semiclassical instability of the CH is not
efficient. The analyses in this paper are based on the geometric optics
approximation, which is justified in two dimensions but needs justification in
four dimensions.Comment: 14 pages, 4 figures, minor errors corrected and some sentences added
in the introduction, accepted for publication in Physical Review
Gravitating magnetic monopole in Vaidya geometry
A magnetic-monopole solution of a non-Abelian gauge theory as proposed by 't
Hooft and Polyakov is studied in the Vaidya spacetime. We find that the
solutions of Einstein equations generates a geometry of the Bonnor-Vaidya
corresponding to magnetically charged null fluid with Higgs field contributing
a cosmological term. In the absence of the scalar fields the corresponding
Wu-Yang solution of the gauge theory still generates the Bonnor-Vaidya
geometry, but with no cosmological term.Comment: 5 RevTeX pages, no figures, minor changes, to appear in Physical
Review
Statistics and UV-IR Mixing with Twisted Poincare Invariance
We elaborate on the role of quantum statistics in twisted Poincare invariant
theories. It is shown that, in order to have twisted Poincare group as the
symmetry of a quantum theory, statistics must be twisted. It is also confirmed
that the removal of UV-IR mixing (in the absence of gauge fields) in such
theories is a natural consequence.Comment: 13 pages, LaTeX; typos correcte
Collapsing strange quark matter in Vaidya geometry
Exact solutions of the gravitational field equations for a mixture of a null
charged strange quark fluid and radiation are obtained in a Vaidya space-time.
The conditions for the formation of a naked singularity are analyzed by
considering the behavior of radial geodesics originating from the central
singularity.Comment: 6 pages, no figure
- …