32,720 research outputs found
Singularity free gravitational collapse in an effective dynamical quantum spacetime
We model the gravitational collapse of heavy massive shells including its
main quantum corrections. Among these corrections, quantum improvements coming
from Quantum Einstein Gravity are taken into account, which provides us with an
effective quantum spacetime. Likewise, we consider dynamical Hawking radiation
by modeling its back-reaction once the horizons have been generated. Our
results point towards a picture of gravitational collapse in which the
collapsing shell reaches a minimum non-zero radius (whose value depends on the
shell initial conditions) with its mass only slightly reduced. Then, there is
always a rebound after which most (or all) of the mass evaporates in the form
of Hawking radiation. Since the mass never concentrates in a single point, no
singularity appears.Comment: 19 pages, 5 figure
Growing interfaces: A brief review on the tilt method
The tilt method applied to models of growing interfaces is a useful tool to
characterize the nonlinearities of their associated equation. Growing
interfaces with average slope , in models and equations belonging to
Kardar-Parisi-Zhang (KPZ) universality class, have average saturation velocity
when .
This property is sufficient to ensure that there is a nonlinearity type square
height-gradient. Usually, the constant is considered equal to the
nonlinear coefficient of the KPZ equation. In this paper, we show
that the mean square height-gradient ,
where for the continuous KPZ equation and otherwise, e.g.
ballistic deposition (BD) and restricted-solid-on-solid (RSOS) models. In order
to find the nonlinear coefficient associated to each system, we
establish the relationship and we test it through the
discrete integration of the KPZ equation. We conclude that height-gradient
fluctuations as function of are constant for continuous KPZ equation and
increasing or decreasing in other systems, such as BD or RSOS models,
respectively.Comment: 11 pages, 4 figure
Evaporation of (quantum) black holes and energy conservation
We consider Hawking radiation as due to a tunneling process in a black hole
were quantum corrections, derived from Quantum Einstein Gravity, are taken into
account. The consequent derivation, satisfying conservation laws, leads to a
deviation from an exact thermal spectrum. The non-thermal radiation is shown to
carry information out of the black hole. Under the appropriate approximation, a
quantum corrected temperature is assigned to the black hole. The evolution of
the quantum black hole as it evaporates is then described by taking into
account the full implications of energy conservation as well as the
back-scattered radiation. It is shown that, as a critical mass of the order of
Planck's mass is reached, the evaporation process decelerates abruptly while
the black hole mass decays towards this critical mass.Comment: 16 pages, 2 figure
The mechanism why colliders could create quasi-stable black holes
It has been postulated that black holes could be created in particle
collisions within the range of the available energies for nowadays colliders
(LHC). In this paper we analyze the evaporation of a type of black holes that
are candidates for this specific behaviour, namely, small black holes on a
brane in a world with large extra-dimensions. We examine their evolution under
the assumption that energy conservation is satisfied during the process and
compare it with the standard evaporation approach. We claim that, rather than
undergoing a quick total evaporation, black holes become quasi-stable. We
comment on the (absence of) implications for safety of this result. We also
discuss how the presence of black holes together with the correctness of the
energy conservation approach might be experimentally verified.Comment: 16 pages, 3 figure
A Hamiltonian functional for the linearized Einstein vacuum field equations
By considering the Einstein vacuum field equations linearized about the
Minkowski metric, the evolution equations for the gauge-invariant quantities
characterizing the gravitational field are written in a Hamiltonian form by
using a conserved functional as Hamiltonian; this Hamiltonian is not the analog
of the energy of the field. A Poisson bracket between functionals of the field,
compatible with the constraints satisfied by the field variables, is obtained.
The generator of spatial translations associated with such bracket is also
obtained.Comment: 5 pages, accepted in J. Phys.: Conf. Serie
Spontaneous violation of the energy conditions
A decade ago, it was shown that a wide class of scalar-tensor theories can
pass very restrictive weak field tests of gravity and yet exhibit
non-perturbative strong field deviations away from General Relativity. This
phenomenon was called `Spontaneous Scalarization' and causes the (Einstein
frame) scalar field inside a neutron star to rapidly become inhomogeneous once
the star's mass increases above some critical value. For a star whose mass is
below the threshold, the field is instead nearly uniform (a state which
minimises the star's energy) and the configuration is similar to the General
Relativity one. Here, we show that the spontaneous scalarization phenomenon is
linked to another strong field effect: a spontaneous violation of the weak
energy condition.Comment: 10 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Local continuity laws on the phase space of Einstein equations with sources
Local continuity equations involving background fields and variantions of the
fields, are obtained for a restricted class of solutions of the
Einstein-Maxwell and Einstein-Weyl theories using a new approach based on the
concept of the adjoint of a differential operator. Such covariant conservation
laws are generated by means of decoupled equations and their adjoints in such a
way that the corresponding covariantly conserved currents possess some
gauge-invariant properties and are expressed in terms of Debye potentials.
These continuity laws lead to both a covariant description of bilinear forms on
the phase space and the existence of conserved quantities. Differences and
similarities with other approaches and extensions of our results are discussed.Comment: LaTeX, 13 page
Conformal mapping of ultrasonic crystals: confining ultrasound and cochlear-like wave guiding
Conformal mapping of a slab of a two-dimensional ultrasonic crystal generate
a closed geometrical arrangement of ultrasonic scatterers with appealing
acoustic properties. This acoustic shell is able to confine ultrasonic modes.
Some of these internal resonances can be induced from an external wave source.
The mapping of a linear defect produces a wave-guide that exhibits a
spatial-frequency selection analogous to that characteristic of a synthetic
"cochlea". Both, experimental and theoretical results are reported here.Comment: 4 pages, 4 figure
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