811 research outputs found
Polarization of Instantons and Gravity
Gravity can arise in a conventional non-Abelian gauge theory in which a
specific phenomenon takes place. Suppose there is a condensation of polarized
instantons and antiinstantons in the vacuum state. Then the excitations of the
gauge field in the classical approximation are described through the variables
of Riemann geometry satisfying the Einstein equations of general relativity.
There are no dimensional coupling constants in the theory.Comment: 3 page
Compressible hydromagnetic nonlinearities in the predecoupling plasma
The adiabatic inhomogeneities of the scalar curvature lead to a compressible
flow affecting the dynamics of the hydromagnetic nonlinearities. The influence
of the plasma on the evolution of a putative magnetic field is explored with
the aim of obtaining an effective description valid for sufficiently large
scales. The bulk velocity of the plasma, computed in the framework of the
LambdaCDM scenario, feeds back into the evolution of the magnetic power spectra
leading to a (nonlocal) master equation valid in Fourier space and similar to
the ones discussed in the context of wave turbulence. Conversely, in physical
space, the magnetic power spectra obey a Schroedinger-like equation whose
effective potential depends on the large-scale curvature perturbations.
Explicit solutions are presented both in physical space and in Fourier space.
It is argued that curvature inhomogeneities, compatible with the WMAP 7yr data,
shift to lower wavenumbers the magnetic diffusivity scale.Comment: 29 page
Evidence for topological nonequilibrium in magnetic configurations
We use direct numerical simulations to study the evolution, or relaxation, of
magnetic configurations to an equilibrium state. We use the full single-fluid
equations of motion for a magnetized, non-resistive, but viscous fluid; and a
Lagrangian approach is used to obtain exact solutions for the magnetic field.
As a result, the topology of the magnetic field remains unchanged, which makes
it possible to study the case of topological nonequilibrium. We find two cases
for which such nonequilibrium appears, indicating that these configurations may
develop singular current sheets.Comment: 10 pages, 5 figure
Rapid dissipation of magnetic fields due to Hall current
We propose a mechanism for the fast dissipation of magnetic fields which is
effective in a stratified medium where ion motions can be neglected. In such a
medium, the field is frozen into the electrons and Hall currents prevail.
Although Hall currents conserve magnetic energy, in the presence of density
gradients, they are able to create current sheets which can be the sites for
efficient dissipation of magnetic fields. We recover the frequency,
, for Hall oscillations modified by the presence of density
gradients. We show that these oscillations can lead to the exchange of energy
between different components of the field. We calculate the time evolution and
show that magnetic fields can dissipate on a timescale of order
. This mechanism can play an important role for magnetic
dissipation in systems with very steep density gradients where the ions are
static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.
A Note on the Cosmological Dynamics in Finite-Range Gravity
In this note we consider the homogeneous and isotropic cosmology in the
finite-range gravity theory recently proposed by Babak and Grishchuk. In this
scenario the universe undergoes late time accelerated expansion if both the
massive gravitons present in the model are tachyons. We carry out the phase
space analysis of the system and show that the late-time acceleration is an
attractor of the model.Comment: RevTex, 4 pages, two figures, New references added, To appear in
IJMP
Multiloop calculations in supersymmetric theories with the higher covariant derivative regularization
Most calculations of quantum corrections in supersymmetric theories are made
with the dimensional reduction, which is a modification of the dimensional
regularization. However, it is well known that the dimensional reduction is not
self-consistent. A consistent regularization, which does not break the
supersymmetry, is the higher covariant derivative regularization. However, the
integrals obtained with this regularization can not be usually calculated
analytically. We discuss application of this regularization to the calculations
in supersymmetric theories. In particular, it is demonstrated that integrals
defining the beta-function are possibly integrals of total derivatives. This
feature allows to explain the origin of the exact NSVZ beta-function, relating
the beta-function with the anomalous dimensions of the matter superfields.
However, integrals for the anomalous dimension should be calculated
numerically.Comment: 8 pages, contribution to ACAT 2011 proceeding
U_A(1) Anomaly at high temperature: the scalar-pseudoscalar splitting in QCD
We estimate the splitting between the spatial correlation lengths in the
scalar and pseudoscalar channels in QCD at high temperature. The splitting is
due to the contribution of the instanton/anti-instanton chains in the thermal
ensemble, even though instanton contributions to thermodynamic quantities are
suppressed. The splitting vanishes at asymptotically high temperatures as
, where is the beta function
coefficient.Comment: 5 p
Numerical evidence of chiral magnetic effect in lattice gauge theory
The chiral magnetic effect is the generation of electric current of quarks
along external magnetic field in the background of topologically nontrivial
gluon fields. There is a recent evidence that this effect is observed by the
STAR Collaboration in heavy ion collisions at RHIC. In our paper we study
qualitative signatures of the chiral magnetic effect using quenched lattice
simulations. We find indications that the electric current is indeed enhanced
in the direction of the magnetic field both in equilibrium configurations of
the quantum gluon fields and in a smooth gluon background with nonzero
topological charge. In the confinement phase the magnetic field enhances the
local fluctuations of both the electric charge and chiral charge densities. In
the deconfinement phase the effects of the magnetic field become smaller,
possibly due to thermal screening. Using a simple model of a fireball we obtain
a good agreement between our data and experimental results of the STAR
Collaboration.Comment: 14 pages, 14 figures, uses RevTeX 4.0; revision: references and
comments added, figures corrected, published versio
A Monte Carlo Study of the 6.4 keV Emission at the Galactic Center
Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sgr
B2 giant molecular cloud located in the Galactic Center region. The large
equivalent width of this line and the lack of an apparent illuminating nearby
object indicate that a time-dependent source, currently in a low-activity
state, is causing the fluorescent emission. It has been suggested that this
illuminator is the massive black hole candidate, Sgr A*, whose X-ray luminosity
has declined by an unprecedented six orders of magnitude over the past 300
years. We here report the results of our Monte Carlo simulations for producing
this line under a variety of source configurations and characteristics. These
indicate that the source may in fact be embedded within Sgr B2, although
external sources give a slightly better fit to the data. The weakened
distinction between the internal and external illuminators is due in part to
the instrument response function, which accounts for an enhanced equivalent
width of the line by folding some of the continuum radiation in with the
intrinsic line intensity. We also point out that although the spectrum may be
largely produced by K emission in cold gas, there is some evidence in
the data to suggest the presence of warm (~10^5 K) emitting material near the
cold cloud.Comment: 11 pages, 4 figure
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