12,434 research outputs found
Towards a future singularity?
We discuss whether the future extrapolation of the present cosmological state
may lead to a singularity even in case of "conventional" (negative) pressure of
the dark energy field, namely . The discussion is based on an
often neglected aspect of scalar-tensor models of gravity: the fact that
different test particles may follow the geodesics of different metric frames,
and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on
Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and
selected for "Honorable Mention
Formation of Black Holes from Collapsed Cosmic String Loops
The fraction of cosmic string loops which collapse to form black holes is
estimated using a set of realistic loops generated by loop fragmentation. The
smallest radius sphere into which each cosmic string loop may fit is obtained
by monitoring the loop through one period of oscillation. For a loop with
invariant length which contracts to within a sphere of radius , the
minimum mass-per-unit length necessary for the cosmic string
loop to form a black hole according to the hoop conjecture is . Analyzing loops, we obtain the empirical estimate for the fraction of cosmic string
loops which collapse to form black holes as a function of the mass-per-unit
length in the range . We
use this power law to extrapolate to , obtaining the
fraction of physically interesting cosmic string loops which
collapse to form black holes within one oscillation period of formation.
Comparing this fraction with the observational bounds on a population of
evaporating black holes, we obtain the limit on the cosmic string mass-per-unit-length. This limit is consistent
with all other observational bounds.Comment: uuencoded, compressed postscript; 20 pages including 7 figure
Phantom Accretion by Five Dimensional Charged Black Hole
This paper deals with the dynamical behavior of phantom field near five
dimensional charged black hole. We formulate equations of motion for
steady-state spherically symmetric flow of phantom fluids. It is found that
phantom energy accretes onto black holes for . Further, the location of
critical point of accretion are evaluated that leads to mass to charge ratio
for 5D charged black hole. This ratio implies that accretion cannot transform a
black hole into a naked singularity. We would like to mention here that this
work is an irreducible extension of 4D charged black hole.Comment: 8 pages, accepted for publication in Mod. Phys. Lett.
On big rip singularities
In this comment we discuss big rip singularities occurring in typical phantom
models by violation of the weak energy condition. After that, we compare them
with future late-time singularities arising in models where the scale factor
ends in a constant value and there is no violation of the strong energy
condition. In phantom models the equation of state is well defined along the
whole evolution, even at the big rip. However, both the pressure and the energy
density of the phantom field diverge. In contrast, in the second kind of model
the equation of state is not defined at the big rip because the pressure bursts
at a finite value of the energy density.Comment: 8 page
Fermion masses in noncommutative geometry
Recent indications of neutrino oscillations raise the question of the
possibility of incorporating massive neutrinos in the formulation of the
Standard Model (SM) within noncommutative geometry (NCG). We find that the NCG
requirement of Poincare duality constrains the numbers of massless quarks and
neutrinos to be unequal unless new fermions are introduced. Possible scenarios
in which this constraint is satisfied are discussed.Comment: 4 pages, REVTeX; typos are corrected in (19), "Possible Solutions"
and "Conclusion" are modified; additional calculational details are included;
references are update
Gravitational waveforms from the evaporating ACO cosmic string loop
The linearly polarized gravitational waveforms from a certain type of
rotating, evaporating cosmic string - the Allen-Casper-Ottewill loop - are
constructed and plotted over the lifetime of the loop. The formulas for the
waveforms are simple and exact, and describe waves which attenuate
self-similarly, with the amplitude and period of the waves falling off linearly
with time.Comment: 30 pages, 16 figure
Pair Production of Topological anti de Sitter Black Holes
The pair creation of black holes with event horizons of non-trivial topology
is described. The spacetimes are all limiting cases of the cosmological
metric. They are generalizations of the dimensional black hole and have
asymptotically anti de Sitter behaviour. Domain walls instantons can mediate
their pair creation for a wide range of mass and charge.Comment: 4 pages, uses late
Gear mesh compliance modeling
A computer model has been constructed to simulate the compliance and load sharing in a spur gear mesh. The model adds the effect of rim deflections to previously developed state-of-the-art gear tooth deflection models. The effects of deflections on mesh compliance and load sharing are examined. The model can treat gear meshes composed to two external gears or an external gear driving an internal gear. The model includes deflection contributions from the bending and shear in the teeth, the Hertzian contact deformations, and primary and secondary rotations of the gear rims. The model shows that rimmed gears increase mesh compliance and, in some cases, improve load sharing
- âŠ