1,522 research outputs found
The neutron star in Cassiopeia A: equation of state, superfluidity, and Joule heating
The thermomagnetic evolution of the young neutron star in Cassiopea A is
studied by considering fast neutrino emission processes. In particular, we
consider neutron star models obtained from the equation of state computed in
the framework of the Brueckner-Bethe-Goldstone many-body theory and variational
methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of
state. It is shown that it is possible to explain a fast cooling regime as the
one observed in the neutron star in Cassiopea A if the Joule heating produced
by dissipation of the small-scale magnetic field in the crust is taken into
account. We thus argue that it is difficult to put severe constraints on the
superfluid gap if the Joule heating is considered.Comment: 4 pages, 2 figures, to appear on A&A Letter
Cosmological Perturbations in Renormalization Group Derived Cosmologies
A linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid Universe with dynamically evolving Newton constant
and cosmological constant is presented. A gauge-invariant formalism
is developed by means of the covariant approach, and the acoustic propagation
equations governing the evolution of the comoving fractional spatial gradients
of the matter density, , and are thus obtained. Explicit solutions
are discussed in cosmologies where both and vary according to
renormalization group equations in the vicinity of a fixed point.Comment: 22 pages, revtex, subeqn.sty, to appear on IJMP
Networks of equities in financial markets
We review the recent approach of correlation based networks of financial
equities. We investigate portfolio of stocks at different time horizons,
financial indices and volatility time series and we show that meaningful
economic information can be extracted from noise dressed correlation matrices.
We show that the method can be used to falsify widespread market models by
directly comparing the topological properties of networks of real and
artificial markets.Comment: 9 pages, 8 figures. Accepted for publication in EPJ
The MEV project: design and testing of a new high-resolution telescope for Muography of Etna Volcano
The MEV project aims at developing a muon telescope expressly designed for
the muography of Etna Volcano. In particular, one of the active craters in the
summit area of the volcano would be a suitable target for this experiment. A
muon tracking telescope with high imaging resolution was built and tested
during 2017. The telescope is a tracker based on extruded scintillating bars
with WLS fibres and featuring an innovative read-out architecture. It is
composed of three XY planes with a sensitive area of \SI{1}{m^2}; the angular
resolution does not exceeds \SI{0.4}{\milli\steradian} and the total angular
aperture is about \SI{45}{\degree}. A special effort concerned the design
of mechanics and electronics in order to meet the requirements of a detector
capable to work in a hostile environment such as the top of a tall volcano, at
a far distance from any facility. The test phase started in January 2017 and
ended successfully at the end of July 2017. An extinct volcanic crater (the
Monti Rossi, in the village of Nicolosi, about 15km from Catania) is the target
of the measurement. The detector acquired data for about 120 days and the
preliminary results are reported in this work
Estimation of Dynamical Noise Power in Unknown Systems
Noise can be modeled as a sequence of random variables defined on a probability space that may be added to a given dynamical system , which is a map on a phase space. In the non-trivial case of dynamical noise , where follows a Gaussian distribution and the system output is , without any specific knowledge or assumption about , the quantitative estimation of the noise power is a challenge. Here, we introduce a formal method based on the nonlinear entropy profile to estimate the dynamical noise power without requiring knowledge of the specific function. We tested the correctness of the proposed method using time series generated from Logistic maps and Pomeau-Manneville systems under different conditions. Our results demonstrate that the proposed estimation algorithm can properly discern different noise levels without any a priori information
Comment on "Feynman Effective Classical Potential in the Schrodinger Formulation"
We comment on the paper "Feynman Effective Classical Potential in the
Schrodinger Formulation"[Phys. Rev. Lett. 81, 3303 (1998)]. We show that the
results in this paper about the time evolution of a wave packet in a double
well potential can be properly explained by resorting to a variational
principle for the effective action. A way to improve on these results is also
discussed.Comment: 1 page, 2eps figures, Revte
Global simulations of Tayler instability in stellar interiors: a long-time multi-stage evolution of the magnetic field
Magnetic fields have been observed in massive Ap/Bp stars and presumably are
also present in the radiative zone of solar-like stars. Yet, to date there is
no clear understanding of the dynamics of the magnetic field in stably
stratified layers. A purely toroidal magnetic field configuration is known to
be unstable, developing mainly non-axisymmetric modes. Rotation and a small
poloidal field component may lead to a stable configuration. Here we perform
global MHD simulations with the EULAG-MHD code to explore the evolution of a
toroidal magnetic field located in a layer whose stratification resembles the
solar tachocline. Our numerical experiments allow us to explore the initial
unstable phase as well as the long-term evolution of the magnetic field. During
the first Alfven cycles, we observe the development of the Tayler instability
with the prominent longitudinal wavenumber, . Rotation decreases the
growth rate of the instability, and eventually suppresses it. However, after a
stable phase, sudden energy surges lead to the development of higher order
modes even for fast rotation. These modes extract energy from the initial
toroidal field. Nevertheless, our results show that sufficiently fast rotation
leads to a lower saturation energy of the unstable modes, resulting in a
magnetic topology with only a small fraction of poloidal field which remains
steady for several hundreds of Alfven travel times. At this stage, the system
becomes turbulent and the field is prone to turbulent diffusion. The final
toroidal-poloidal configuration of the magnetic field may represent an
important aspect of the field generation and evolution in stably-stratified
layers.Comment: 15 pages, 16 figures, submitted to MNRA
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
We discuss various basic conceptual issues related to coarse graining flows
in quantum gravity. In particular the requirement of background independence is
shown to lead to renormalization group (RG) flows which are significantly
different from their analogs on a rigid background spacetime. The importance of
these findings for the asymptotic safety approach to Quantum Einstein Gravity
(QEG) is demonstrated in a simplified setting where only the conformal factor
is quantized. We identify background independence as a (the ?) key prerequisite
for the existence of a non-Gaussian RG fixed point and the renormalizability of
QEG.Comment: 2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum
Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to
appear in General Relativity and Gravitatio
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