438 research outputs found
Optimal -Control for the Global Cauchy Problem of the Relativistic Vlasov-Poisson System
Recently, M.K.-H. Kiessling and A.S. Tahvildar-Zadeh proved that a unique
global classical solution to the relativistic Vlasov-Poisson system exists
whenever the positive, integrable initial datum is spherically symmetric,
compactly supported in momentum space, vanishes on characteristics with
vanishing angular momentum, and for has
-norm strictly below a positive, critical value
. Everything else being equal, data leading to finite time
blow-up can be found with -norm surpassing
for any , with if and
only if . In their paper, the critical value for is calculated explicitly while the value for all other is
merely characterized as the infimum of a functional over an appropriate
function space. In this work, the existence of minimizers is established, and
the exact expression of is calculated in terms of the
famous Lane-Emden functions. Numerical computations of the
are presented along with some elementary asymptotics near
the critical exponent .Comment: 24 pages, 2 figures Refereed and accepted for publication in
Transport Theory and Statistical Physic
Algorithmic construction of static perfect fluid spheres
Perfect fluid spheres, both Newtonian and relativistic, have attracted
considerable attention as the first step in developing realistic stellar models
(or models for fluid planets). Whereas there have been some early hints on how
one might find general solutions to the perfect fluid constraint in the absence
of a specific equation of state, explicit and fully general solutions of the
perfect fluid constraint have only very recently been developed. In this
article we present a version of Lake's algorithm [Phys. Rev. D 67 (2003)
104015; gr-qc/0209104] wherein: (1) we re-cast the algorithm in terms of
variables with a clear physical meaning -- the average density and the locally
measured acceleration due to gravity, (2) we present explicit and fully general
formulae for the mass profile and pressure profile, and (3) we present an
explicit closed-form expression for the central pressure. Furthermore we can
then use the formalism to easily understand the pattern of inter-relationships
among many of the previously known exact solutions, and generate several new
exact solutions.Comment: Uses revtex4. V2: Minor clarifications, plus an additional section on
how to turn the algorithm into a solution generalization technique. This
version accepted for publication in Physical Review D. Now 7 page
The Physics and Mathematics of the Second Law of Thermodynamics
The essential postulates of classical thermodynamics are formulated, from
which the second law is deduced as the principle of increase of entropy in
irreversible adiabatic processes that take one equilibrium state to another.
The entropy constructed here is defined only for equilibrium states and no
attempt is made to define it otherwise. Statistical mechanics does not enter
these considerations. One of the main concepts that makes everything work is
the comparison principle (which, in essence, states that given any two states
of the same chemical composition at least one is adiabatically accessible from
the other) and we show that it can be derived from some assumptions about the
pressure and thermal equilibrium. Temperature is derived from entropy, but at
the start not even the concept of `hotness' is assumed. Our formulation offers
a certain clarity and rigor that goes beyond most textbook discussions of the
second law.Comment: 93 pages, TeX, 8 eps figures. Updated, published version. A summary
appears in Notices of the Amer. Math. Soc. 45 (1998) 571-581, math-ph/980500
The interior spacetimes of stars in Palatini f(R) gravity
We study the interior spacetimes of stars in the Palatini formalism of f(R)
gravity and derive a generalized Tolman-Oppenheimer-Volkoff and mass equation
for a static, spherically symmetric star. We show that matching the interior
solution with the exterior Schwarzschild-De Sitter solution in general gives a
relation between the gravitational mass and the density profile of a star,
which is different from the one in General Relativity. These modifications
become neglible in models for which is a decreasing function of R however. As a result, both Solar System
constraints and stellar dynamics are perfectly consistent with .Comment: Published version, 6 pages, 1 figur
Symmetric hyperbolic systems for a large class of fields in arbitrary dimension
Symmetric hyperbolic systems of equations are explicitly constructed for a
general class of tensor fields by considering their structure as r-fold forms.
The hyperbolizations depend on 2r-1 arbitrary timelike vectors. The importance
of the so-called "superenergy" tensors, which provide the necessary symmetric
positive matrices, is emphasized and made explicit. Thereby, a unified
treatment of many physical systems is achieved, as well as of the sometimes
called "higher order" systems. The characteristics of these symmetric
hyperbolic systems are always physical, and directly related to the null
directions of the superenergy tensor, which are in particular principal null
directions of the tensor field solutions. Generic energy estimates and
inequalities are presented too.Comment: 24 pages, no figure
G\"{o}del-type universes in f(R) gravity
The gravity theories provide an alternative way to explain the current
cosmic acceleration without a dark energy matter component. If gravity is
governed by a theory a number of issues should be reexamined in this
framework, including the violation of causality problem on nonlocal scale. We
examine the question as to whether the gravity theories permit
space-times in which the causality is violated. We show that the field
equations of these gravity theories do not exclude solutions with
breakdown of causality for a physically well-motivated perfect-fluid matter
content. We demonstrate that every perfect-fluid G\"{o}del-type solution of a
generic gravity satisfying the condition is necessarily
isometric to the G\"odel geometry, and therefore presents violation of
causality. This result extends a theorem on G\"{o}del-type models, which has
been established in the context of general relativity. We also derive an
expression for the critical radius (beyond which the causality is
violated) for an arbitrary theory, making apparent that the violation of
causality depends on both the gravity theory and the matter content. As
an illustration, we concretely take a recent gravity theory that is free
from singularities of the Ricci scalar and is cosmologically viable, and show
that this theory accommodates noncausal as well as causal G\"odel-type
solutions.Comment: 7 pages, V3: Version to appear in Phys. Rev. D (2009), typos
corrected, the generality of our main results is emphasized. The illustrative
character of a particular theory is also made explici
Galactic rotation curves in modified gravity with non-minimal coupling between matter and geometry
We investigate the possibility that the behavior of the rotational velocities
of test particles gravitating around galaxies can be explained in the framework
of modified gravity models with non-minimal matter-geometry coupling.
Generally, the dynamics of test particles around galaxies, as well as the
corresponding mass deficit, is explained by postulating the existence of dark
matter. The extra-terms in the gravitational field equations with
geometry-matter coupling modify the equations of motion of test particles, and
induce a supplementary gravitational interaction. Starting from the variational
principle describing the particle motion in the presence of the non-minimal
coupling, the expression of the tangential velocity of a test particle, moving
in the vacuum on a stable circular orbit in a spherically symmetric geometry,
is derived. The tangential velocity depends on the metric tensor components, as
well as of the coupling function between matter and geometry. The Doppler
velocity shifts are also obtained in terms of the coupling function. If the
tangential velocity profile is known, the coupling term between matter and
geometry can be obtained explicitly in an analytical form. The functional form
of this function is obtained in two cases, for a constant tangential velocity,
and for an empirical velocity profile obtained from astronomical observations,
respectively. Therefore, these results open the possibility of directly testing
the modified gravity models with non-minimal coupling between matter and
geometry by using direct astronomical and astrophysical observations at the
galactic or extra-galactic scale.Comment: 8 pages, accepted for publication in PR
Quadratic superconducting cosmic strings revisited
It has been shown that 5-dimensional general relativity action extended by
appropriate quadratic terms admits a singular superconducting cosmic string
solution. We search for cosmic strings endowed with similar and extended
physical properties by directly integrating the non-linear matrix field
equations thus avoiding the perturbative approach by which we constructed the
above-mentioned \textsl{exact} solution. The most general superconducting
cosmic string, subject to some constraints, will be derived and shown to be
mathematically \textsl{unique} up to linear coordinate transformations mixing
its Killing vectors. The most general solution, however, is not globally
equivalent to the old one due to the existence of Killing vectors with closed
orbits.Comment: 6 page
Some notes on the Kruskal - Szekeres completion
The Kruskal - Szekeres (KS) completion of the Schwarzschild spacetime is open
to Synge's methodological criticism that the KS procedure generates "good"
coordinates from "bad". This is addressed here in two ways: First I generate
the KS coordinates from Israel coordinates, which are also "good", and then I
generate the KS coordinates directly from a streamlined integration of the
Einstein equations.Comment: One typo correcte
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