717 research outputs found
Extended Quintessence with non-minimally coupled phantom scalar field
We investigate evolutional paths of an extended quintessence with a
non-minimally coupled phantom scalar field to the Ricci curvature. The
dynamical system methods are used to investigate typical regimes of dynamics at
the late time. We demonstrate that there are two generic types of evolutional
scenarios which approach the attractor (a focus or a node type critical point)
in the phase space: the quasi-oscillatory and monotonic trajectories approach
to the attractor which represents the FRW model with the cosmological constant.
We demonstrate that dynamical system admits invariant two-dimensional
submanifold and discussion that which cosmological scenario is realized depends
on behavior of the system on the phase plane . We formulate
simple conditions on the value of coupling constant for which
trajectories tend to the focus in the phase plane and hence damping
oscillations around the mysterious value . We describe this condition in
terms of slow-roll parameters calculated at the critical point. We discover
that the generic trajectories in the focus-attractor scenario come from the
unstable node. It is also investigated the exact form of the parametrization of
the equation of state parameter (directly determined from dynamics)
which assumes a different form for both scenarios.Comment: revtex4, 15 pages, 9 figures; (v2) published versio
Uniqueness and Self-Conjugacy of Dirac Hamiltonians in arbitrary Gravitational Fields
Proofs of two statements are provided in this paper. First, the authors prove
that the formalism of the pseudo-Hermitian quantum mechanics allows describing
the Dirac particles motion in arbitrary stationary gravitational fields.
Second, it is proved that using the Parker weight operator and the subsequent
transition to the \eta -representation gives the transformation of the
Schroedinger equation for nonstationary metric, when the evolution operator
becomes self-conjugate. The scalar products in the \eta -representation are
flat, which makes possible the use of a standard apparatus for the Hermitian
quantum mechanics. Based on the results of this paper the authors draw a
conclusion about solution of the problem of uniqueness and self-conjugacy of
Dirac Hamiltonians in arbitrary gravitational fields including those dependent
on time. The general approach is illustrated by the example of Dirac
Hamiltonians for several stationary metrics, as well as for the cosmologically
flat and the open Friedmann models.Comment: 23 page
Rotation and pseudo-rotation
Eigenvectors of stress-energy tensor (the source in Einstein's equations)
form privileged bases in description of the corresponding space-times. When one
or more of these vector fields are rotating (the property well determined in
differential geometry), one says that the space-time executes this rotation.
Though the rotation in its proper sense is understood as that of a timelike
congruence (vector field), the rotation of a spacelike congruence is not a less
objective property if it corresponds to a canonical proper basis built of the
just mentioned eigenvectors. In this last case, we propose to speak on
pseudo-rotation. Both properties of metric, its material sources, and
space-time symmetries are considered in this paper.Comment: 13 pages, no figures, contains parts of the PhD Thesis of H. Vargas
Rodr\'igue
Thermal and Dynamical Equilibrium in Two-Component Star Clusters
We present the results of Monte Carlo simulations for the dynamical evolution
of star clusters containing two stellar populations with individual masses m1
and m2 > m1, and total masses M1 and M2 < M1. We use both King and Plummer
model initial conditions and we perform simulations for a wide range of
individual and total mass ratios, m2/m1 and M2/M1. We ignore the effects of
binaries, stellar evolution, and the galactic tidal field. The simulations use
N = 10^5 stars and follow the evolution of the clusters until core collapse. We
find that the departure from energy equipartition in the core follows
approximately the theoretical predictions of Spitzer (1969) and Lightman & Fall
(1978), and we suggest a more exact condition that is based on our results. We
find good agreement with previous results obtained by other methods regarding
several important features of the evolution, including the pre-collapse
distribution of heavier stars, the time scale on which equipartition is
approached, and the extent to which core collapse is accelerated by a small
subpopulation of heavier stars. We briefly discuss the possible implications of
our results for the dynamical evolution of primordial black holes and neutron
stars in globular clusters.Comment: 31 pages, including 13 figures, to appear in Ap
Bernstein modes in a weakly relativistic electron-positron plasma
The kinetic theory of weakly relativistic electron-positron plasmas, producing dispersion relations for the electrostatic Bernstein modes was addressed. The treatment presented preserves the full momentum dependence of the cyclotron frequency, albeit with a relaxation on the true relativistic form of the distribution function. The implications of this new treatment were confined largely to astrophysical plasmas, where relativistic electronpositron plasmas occur naturally
Active Mass Under Pressure
After a historical introduction to Poisson's equation for Newtonian gravity,
its analog for static gravitational fields in Einstein's theory is reviewed. It
appears that the pressure contribution to the active mass density in Einstein's
theory might also be noticeable at the Newtonian level. A form of its
surprising appearance, first noticed by Richard Chase Tolman, was discussed
half a century ago in the Hamburg Relativity Seminar and is resolved here.Comment: 28 pages, 4 figure
The changing UK careers landscape : tidal waves, turbulence and transformation
This article explores how the UK careers landscape in each of the four home nations is changing in response to neo-liberal policies. In this context, careers services are increasingly under pressure to demonstrate their added value, impact and returns on investment. As fiscal arrangements tighten and governments state their preferences and priorities for national careers services, differing strategic responses are beginning to emerge. A quasi-market, experimental approach is now the dominant discourse in England, in contrast to differing and complementary arrangements in Northern Ireland, Scotland and Wales. The article suggests that insofar as these developments are transforming national careers services, they are also creating significant challenges which require new forms of policy imagery and imagination for high-impact, all-age careers services
Ohm's Law for a Relativistic Pair Plasma
We derive the fully relativistic Ohm's law for an electron-positron plasma.
The absence of non-resistive terms in Ohm's law and the natural substitution of
the 4-velocity for the velocity flux in the relativistic bulk plasma equations
do not require the field gradient length scale to be much larger than the
lepton inertial lengths, or the existence of a frame in which the distribution
functions are isotropic.Comment: 12 pages, plain TeX, Phys. Rev. Lett. 71 3481 (1993
Neutral perfect fluids of Majumdar-type in general relativity
We consider the extension of the Majumdar-type class of static solutions for
the Einstein-Maxwell equations, proposed by Ida to include charged perfect
fluid sources. We impose the equation of state and discuss
spherically symmetric solutions for the linear potential equation satisfied by
the metric. In this particular case the fluid charge density vanishes and we
locate the arising neutral perfect fluid in the intermediate region defined by
two thin shells with respective charges and . With its innermost flat
and external (Schwarzschild) asymptotically flat spacetime regions, the
resultant condenser-like geometries resemble solutions discussed by Cohen and
Cohen in a different context. We explore this relationship and point out an
exotic gravitational property of our neutral perfect fluid. We mention possible
continuations of this study to embrace non-spherically symmetric situations and
higher dimensional spacetimes.Comment: 9 page
Self-Screening Hawking Atmosphere in the Presence of a Bulk Viscosity
The recent theory of 't Hooft [ Nucl. Phys. Suppl. {\bf 68}, 174 (1998)]
models the black hole as a system endowed with an envelope of matter that obeys
an equation of state in the form , and acts as a source in
Einstein's equations. The present paper generalizes the 't Hooft theory so as
to take into account a bulk viscosity in the fluid. It is shown that
even a slight positive value of will suffice to yield complete
agreement with the Hawking formula for the entropy of the black hole, if the
value of the constant takes a value that is slightly less than 4/3.
The value corresponds to a radiation fluid.Comment: 12 pages, LaTeX, no figures, minor extensions of the discussion. To
appear in PR
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