148 research outputs found
Revised Conditions for MRI due to Isorotation Theorem
We re-analyze the physical conditions for Magneto-rotational Instability
(MRI) within a steady axisymmetric stratified disk of plasma, in order to
account for the so-called isorotation theory (the spatial profile of
differential angular velocity depends on the magnetic flux surface). We develop
the study of linear stability around an astrophysical background configuration,
following the original derivation in \cite{Ba:1995}, but implementing the
isorotation condition as the orthogonality between the background magnetic
field and the angular velocity gradient. We demonstrate that a dependence on
the background magnetic field direction is restored in the dispersion relation
and, hence, the emergence of MRI is also influenced by field orientation.Comment: 9 pages, 2 figures, accepted for publication in Physics Letters
The Immirzi parameter from an external scalar field
We promote the Immirzi parameter to be a minimally coupled scalar field and
we analyzed the Hamiltonian constraints in the framework of Loop Quantum
Gravity without the time gauge. Proper SU(2) connections can be defined and a
term containing derivatives of the field enters into their definition.
Furthermore, boost degrees of freedom are non-dynamical, while the
super-momentum constraints coincide with the scalar field case. Hence, the
kinematical Hilbert space can be defined as for gravity in presence of a
minimally coupled scalar field. Then, we analyzed the dynamical implications of
this scenario and we outlined how a the dynamical relaxation to a non-vanishing
vacuum expectation value is predicted.Comment: 5 page
Quasi-linear model for the beam-plasma instability: analysis of the self-consistent evolution
We re-analyze the quasi-linear self consistent dynamics for the beam-plasma
instability, by comparing the theory predictions to numerical simulations of
the corresponding Hamiltonian system. While the diffusive features of the
asymptotic dynamics are reliably predicted, the early temporal mesoscale
transport appears less efficient in reproducing the convective feature of the
self-consistent scenario. As a result, we identify the origin of the observed
discrepancy in the underlying quasi-linear model assumption that the
distribution function is quasi-stationary. Furthermore, we provide a correction
to the instantaneous quasi-linear growth rate based on a linear expansion of
the distribution function time dependence, and we successfully test this
revised formulation for the spectral evolution during the temporal mesoscale.Comment: 12 pages, 5 figure
Spin connection as Lorentz gauge field: propagating torsion
We propose a modified gravitational action containing besides the
Einstein-Cartan term some quadratic contributions resembling the Yang-Mills
lagrangian for the Lorentz spin connections. We outline how a propagating
torsion arises and we solve explicitly the linearised equations of motion on a
Minkowski background. We identify among torsion components six degrees of
freedom: one is carried by a pseudo-scalar particle, five by a tachyon field.
By adding spinor fields and neglecting backreaction on the geometry, we point
out how only the pseudo-scalar particle couples directly with fermions, but the
resulting coupling constant is suppressed by the ratio between fermion and
Planck masses. Including backreaction, we demonstrate how the tachyon field
provides causality violation in the matter sector, via an interaction mediated
by gravitational waves.Comment: 7 pages, no figures, new section adde
- …