168 research outputs found
Hawking radiation of nonsingular black holes in two dimensions
In this letter we study the process of Hawking radiation of a black hole
assuming the existence of a limiting physical curvature scale. The particular
model is constructed using the Limiting Curvature Hypothesis (LCH) and in the
context of two-dimensional dilaton gravity. The black hole solution exhibits
properties of the standard Schwarzschild solution at large values of the radial
coordinate. However, near the center, the black hole is nonsingular and the
metric becomes that of de Sitter spacetime. The Hawking temperature is
calculated using the method of complex paths. We find that such black holes
radiate eternally and never completely evaporate. The final state is an
eternally radiating relic, near the fundamental scale, which should make a
viable dark matter candidate. We briefly comment on the black hole information
loss problem and the production of such black holes in collider experiments.Comment: 8 pages, 4 figures; minor revisions; references added; version to
appear in JHE
Effect of the Generalized Uncertainty Principle on Post-Inflation Preheating
We examine effects of the Generalized Uncertainty Principle, predicted by
various theories of quantum gravity to replace the Heisenberg's uncertainty
principle near the Planck scale, on post inflation preheating in cosmology, and
show that it can predict either an increase or a decrease in parametric
resonance and a corresponding change in particle production. Possible
implications are considered.Comment: v1: 9 pages, revtex4, no figures, accepted for publication in JCAP;
v2: one reference added and various cosmetic (but no physics) changes to
match published versio
Calculation of the two-photon decay width of the f_0(980) scalar meson
The applicability of the quasi-static approximation for calculating the
two-photon annihilation rate of the scalar f_0(980) meson envisaged as a K\bar
K molecule is critically re-examined. It is shown that the validity of this
approximation depends on the detailed interplay between the momentum dependence
of the annihilation amplitude and the momentum transform of the bound state
wave function of the annihilating pair. The approximation becomes invalid when
these two scales of variation are similar. An improved method of calculation
based on the inclusion of electromagnetic corrections to the kernel of the
Bethe-Salpeter equation for the interacting K\bar K pair is outlined to cover
this case and applied to re-evaluate the two-photon decay width for f_0(980) in
a one boson exchange model for the interkaon interaction. The corrections are
significant and result in a much better agreement with experiment.Comment: 14 pages, 3 figures. Fig.3 replaced. Additional remarks with
reference
On Black Holes and Cosmological Constant in Noncommutative Gauge Theory of Gravity
Deformed Reissner-Nordstr\"om, as well as Reissner-Nordstr\"om de Sitter,
solutions are obtained in a noncommutative gauge theory of gravitation. The
gauge potentials (tetrad fields) and the components of deformed metric are
calculated to second order in the noncommutativity parameter. The solutions
reduce to the deformed Schwarzschild ones when the electric charge of the
gravitational source and the cosmological constant vanish. Corrections to the
thermodynamical quantities of the corresponding black holes and to the radii of
different horizons have been determined. All the independent invariants, such
as the Ricci scalar and the so-called Kretschmann scalar, have the same
singularity structure as the ones of the usual undeformed case and no smearing
of singularities occurs. The possibility of such a smearing is discussed. In
the noncommutative case we have a local disturbance of the geometry around the
source, although asymptotically at large distances it becomes flat.Comment: Based on a talk given at the International Conference on Fundamental
and Applied Research in Physics "Farphys 2007", 25-28 October 2007, Iasi,
Romani
Static spectroscopy of a dense superfluid
Dense Bose superfluids, as HeII, differ from dilute ones by the existence of
a roton minimum in their excitation spectrum. It is known that this roton
minimum is qualitatively responsible for density oscillations close to any
singularity, such as vortex cores, or close to solid boundaries. We show that
the period of these oscillations, and their exponential decrease with the
distance to the singularity, are fully determined by the position and the width
of the roton minimum. Only an overall amplitude factor and a phase shift are
shown to depend on the details of the interaction potential. Reciprocally, it
allows for determining the characteristics of this roton minimum from static
"observations" of a disturbed ground state, in cases where the dynamics is not
easily accessible. We focus on the vortex example. Our analysis further shows
why the energy of these oscillations is negligible compared to the kinetic
energy, which limits their influence on the vortex dynamics, except for high
curvatures.Comment: 14 pages, 4 figures, extended version, published in J. Low Temp. Phy
High frequency sound in superfluid 3He-B
We present measurements of the absolute phase velocity of transverse and
longitudinal sound in superfluid 3He-B at low temperature, extending from the
imaginary squashing mode to near pair-breaking. Changes in the transverse phase
velocity near pair-breaking have been explained in terms of an order parameter
collective mode that arises from f-wave pairing interactions, the so-called
J=4- mode. Using these measurements, we establish lower bounds on the energy
gap in the B-phase. Measurement of attenuation of longitudinal sound at low
temperature and energies far above the pair-breaking threshold, are in
agreement with the lower bounds set on pair-breaking. Finally, we discuss our
estimations for the strength of the f-wave pairing interactions and the Fermi
liquid parameter, F4s.Comment: 15 pages, 8 figures, accepted to J. Low Temp. Phy
Fine-structure constant variability, equivalence principle and cosmology
It has been widely believed that variability of the fine-structure constant
alpha would imply detectable violations of the weak equivalence principle. This
belief is not justified in general. It is put to rest here in the context of
the general framework for alpha variability [J. D. Bekenstein, Phys. Rev. D 25,
1527 (1982)] in which the exponent of a scalar field plays the role of the
permittivity and inverse permeability of the vacuum. The coupling of particles
to the scalar field is necessarily such that the anomalous force acting on a
charged particle by virtue of its mass's dependence on the scalar field is
cancelled by terms modifying the usual Coulomb force. As a consequence a
particle's acceleration in external fields depends only on its charge to mass
ratio, in accordance with the principle. And the center of mass acceleration of
a composite object can be proved to be independent of the object's internal
constitution, as the weak equivalence principle requires. Likewise the widely
employed assumption that the Coulomb energy of matter is the principal source
of the scalar field proves wrong; Coulomb energy effectively cancels out in the
continuum description of the scalar field's dynamics. This cancellation
resolves a cosmological conundrum: with Coulomb energy as source of the scalar
field, the framework would predict a decrease of alpha with cosmological
expansion, whereas an increase is claimed to be observed. Because of the said
cancellation, magnetic energy of cosmological baryonic matter is the main
source of the scalar field. Consequently the expansion is accompanied by an
increase in alpha; for reasonable values of the framework's sole parameter,
this occurs at a rate consistent with the observers' claims.Comment: RevTeX-4, 22 pages, no figures, added a section on caveats as well as
several new references with discussion of them in body. To appear in Phys.
Rev.
Transverse spin dynamics in a spin-polarized Fermi liquid
The linear equations for transverse spin dynamics in weakly polarised
degenerate Fermi liquid with arbitrary relationship between temperature and
polarization are derived from Landau-Silin phenomenological kinetic equation
with general form of two-particle collision integral. The temperature and
polarization dependence of the spin current relaxation time is established. It
is found in particular that at finite polarization transverse spin wave damping
has a finite value at T=0. The analogy between temperature dependences of spin
waves attenuation and ultrasound absorption in degenerate Fermi liquid at
arbitrary temperature is presented. We also discuss spin-polarized Fermi liquid
in the general context of the Fermi-liquid theory and compare it with "Fermi
liquid" with spontaneous magnetization.Comment: 10 page
Determining the density of states for classical statistical models: A random walk algorithm to produce a flat histogram
We describe an efficient Monte Carlo algorithm using a random walk in energy
space to obtain a very accurate estimate of the density of states for classical
statistical models. The density of states is modified at each step when the
energy level is visited to produce a flat histogram. By carefully controlling
the modification factor, we allow the density of states to converge to the true
value very quickly, even for large systems. This algorithm is especially useful
for complex systems with a rough landscape since all possible energy levels are
visited with the same probability. In this paper, we apply our algorithm to
both 1st and 2nd order phase transitions to demonstrate its efficiency and
accuracy. We obtained direct simulational estimates for the density of states
for two-dimensional ten-state Potts models on lattices up to
and Ising models on lattices up to . Applying this approach to
a 3D spin glass model we estimate the internal energy and entropy at
zero temperature; and, using a two-dimensional random walk in energy and
order-parameter space, we obtain the (rough) canonical distribution and energy
landscape in order-parameter space. Preliminary data suggest that the glass
transition temperature is about 1.2 and that better estimates can be obtained
with more extensive application of the method.Comment: 22 pages (figures included
Wave function of the Universe in the early stage of its evolution
In quantum cosmological models, constructed in the framework of
Friedmann-Robertson-Walker metrics, a nucleation of the Universe with its
further expansion is described as a tunneling transition through an effective
barrier between regions with small and large values of the scale factor at
non-zero (or zero) energy. The approach for describing this tunneling consists
of constructing a wave function satisfying an appropriate boundary condition.
There are various ways for defining the boundary condition that lead to
different estimates of the barrier penetrability and the tunneling time.
In order to describe the escape from the tunneling region as accurately as
possible and to construct the total wave function on the basis of its two
partial solutions unambiguously, we use the tunneling boundary condition that
the total wave function must represent only the outgoing wave at the point of
escape from the barrier, where the following definition for the wave is
introduced: the wave is represented by the wave function whose modulus changes
minimally under a variation of the scale factor . We construct a new method
for a direct non-semiclassical calculation of the total stationary wave
function of the Universe, analyze the behavior of this wave function in the
tunneling region, near the escape point and in the asymptotic region, and
estimate the barrier penetrability. We observe oscillations of modulus of wave
function in the external region starting from the turning point which decrease
with increasing of and which are not shown in semiclassical calculations.
The period of such an oscillation decreases uniformly with increasing and
can be used as a fully quantum dynamical characteristic of the expansion of the
Universe.Comment: 19 pages, 21 files for 10 EPS figures, LaTeX svjour style. The Sec.2
(formalism of Wheeler-De Witt equation) is reduced. In Sec.3.1 definition of
the outgoing wave from barrier is defined more accurately. In Sec.4.1
semiclassical calculations of wavew function and penetrability are performed
and comparison with results in fully quantum approach is adde
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