479 research outputs found
Ideal Stars and General Relativity
We study a system of differential equations that governs the distribution of
matter in the theory of General Relativity. The new element in this paper is
the use of a dynamical action principle that includes all the degrees of
freedom, matter as well as metric. The matter lagrangian defines a relativistic
version of non-viscous, isentropic hydrodynamics. The matter fields are a
scalar density and a velocity potential; the conventional, four-vector velocity
field is replaced by the gradient of the potential and its scale is fixed by
one of the eulerian equations of motion, an innovation that significantly
affects the imposition of boundary conditions. If the density is integrable at
infinity, then the metric approaches the Schwarzschild metric at large
distances. There are stars without boundary and with finite total mass; the
metric shows rapid variation in the neighbourhood of the Schwarzschild radius
and there is a very small core where a singularity indicates that the gas laws
break down. For stars with boundary there emerges a new, critical relation
between the radius and the gravitational mass, a consequence of the stronger
boundary conditions. Tentative applications are suggested, to certain Red
Giants, and to neutron stars, but the investigation reported here was limited
to polytropic equations of state. Comparison with the results of Oppenheimer
and Volkoff on neutron cores shows a close agreement of numerical results.
However, in the model the boundary of the star is fixed uniquely by the
required matching of the interior metric to the external Schwarzschild metric,
which is not the case in the traditional approach.Comment: 26 pages, 7 figure
Thermodynamic Properties of Holographic Multiquark and the Multiquark Star
We study thermodynamic properties of the multiquark nuclear matter. The
dependence of the equation of state on the colour charges is explored both
analytically and numerically in the limits where the baryon density is small
and large at fixed temperature between the gluon deconfinement and chiral
symmetry restoration. The gravitational stability of the hypothetical
multiquark stars are discussed using the Tolman-Oppenheimer-Volkoff equation.
Since the equations of state of the multiquarks can be well approximated by
different power laws for small and large density, the content of the multiquark
stars has the core and crust structure. We found that most of the mass of the
star comes from the crust region where the density is relatively small. The
mass limit of the multiquark star is determined as well as its relation to the
star radius. For typical energy density scale of ,
the converging mass and radius of the hypothetical multiquark star in the limit
of large central density are approximately solar mass and 15-27 km.
The adiabatic index and sound speed distributions of the multiquark matter in
the star are also calculated and discussed. The sound speed never exceeds the
speed of light and the multiquark matters are thus compressible even at high
density and pressure.Comment: 27 pages, 17 figures, 1 table, JHEP versio
Compact Stars - How Exotic Can They Be?
Strong interaction physics under extreme conditions of high temperature
and/or density is of central interest in modern nuclear physics for
experimentalists and theorists alike. In order to investigate such systems,
model approaches that include hadrons and quarks in a unified approach, will be
discussed. Special attention will be given to high-density matter as it occurs
in neutron stars. Given the current observational limits for neutron star
masses, the properties of hyperonic and hybrid stars will be determined. In
this context especially the question of the extent, to which exotic particles
like hyperons and quarks affect star masses, will be discussed.Comment: Contributon to conference "Nuclear Physics: Present and Future", held
in Boppard (Germany), May 201
Instability of black hole formation under small pressure perturbations
We investigate here the spectrum of gravitational collapse endstates when
arbitrarily small perfect fluid pressures are introduced in the classic black
hole formation scenario as described by Oppenheimer, Snyder and Datt (OSD) [1].
This extends a previous result on tangential pressures [2] to the more
physically realistic scenario of perfect fluid collapse. The existence of
classes of pressure perturbations is shown explicitly, which has the property
that injecting any smallest pressure changes the final fate of the dynamical
collapse from a black hole to a naked singularity. It is therefore seen that
any smallest neighborhood of the OSD model, in the space of initial data,
contains collapse evolutions that go to a naked singularity outcome. This gives
an intriguing insight on the nature of naked singularity formation in
gravitational collapse.Comment: 7 pages, 1 figure, several modifications to match published version
on GR
Has the Universe always expanded ?
We consider a cosmological setting for which the currently expanding era is
preceded by a contracting phase, that is, we assume the Universe experienced at
least one bounce. We show that scalar hydrodynamic perturbations lead to a
singular behavior of the Bardeen potential and/or its derivatives (i.e. the
curvature) for whatever Universe model for which the last bounce epoch can be
smoothly and causally joined to the radiation dominated era. Such a Universe
would be filled with non-linear perturbations long before nucleosynthesis, and
would thus be incompatible with observations. We therefore conclude that no
observable bounce could possibly have taken place in the early universe if
Einstein gravity together with hydrodynamical fluids is to describe its
evolution, and hence, under these conditions, that the Universe has always
expanded.Comment: 11 pages, LaTeX-ReVTeX, no figures, submitted to PR
Phase structure of black branes in grand canonical ensemble
This is a companion paper of our previous work [1] where we studied the
thermodynamics and phase structure of asymptotically flat black -branes in a
cavity in arbitrary dimensions in a canonical ensemble. In this work we
study the thermodynamics and phase structure of the same in a grand canonical
ensemble. Since the boundary data in two cases are different (for the grand
canonical ensemble boundary potential is fixed instead of the charge as in
canonical ensemble) the stability analysis and the phase structure in the two
cases are quite different. In particular, we find that there exists an analog
of one-variable analysis as in canonical ensemble, which gives the same
stability condition as the rather complicated known (but generalized from black
holes to the present case) two-variable analysis. When certain condition for
the fixed potential is satisfied, the phase structure of charged black
-branes is in some sense similar to that of the zero charge black -branes
in canonical ensemble up to a certain temperature. The new feature in the
present case is that above this temperature, unlike the zero-charge case, the
stable brane phase no longer exists and `hot flat space' is the stable phase
here. In the grand canonical ensemble there is an analog of Hawking-Page
transition, even for the charged black -brane, as opposed to the canonical
ensemble. Our study applies to non-dilatonic as well as dilatonic black
-branes in space-time dimensions.Comment: 32 pages, 2 figures, various points refined, discussion expanded,
references updated, typos corrected, published in JHEP 1105:091,201
Adiabatic and entropy perturbations propagation in a bouncing Universe
By studying some bouncing universe models dominated by a specific class of
hydrodynamical fluids, we show that the primordial cosmological perturbations
may propagate smoothly through a general relativistic bounce. We also find that
the purely adiabatic modes, although almost always fruitfully investigated in
all other contexts in cosmology, are meaningless in the bounce or null energy
condition (NEC) violation cases since the entropy modes can never be neglected
in these situations: the adiabatic modes exhibit a fake divergence that is
compensated in the total Bardeen gravitational potential by inclusion of the
entropy perturbations.Comment: 25 pages, no figure, LaTe
Breakdown of the adiabatic limit in low dimensional gapless systems
It is generally believed that a generic system can be reversibly transformed
from one state into another by sufficiently slow change of parameters. A
standard argument favoring this assertion is based on a possibility to expand
the energy or the entropy of the system into the Taylor series in the ramp
speed. Here we show that this argumentation is only valid in high enough
dimensions and can break down in low-dimensional gapless systems. We identify
three generic regimes of a system response to a slow ramp: (A) mean-field, (B)
non-analytic, and (C) non-adiabatic. In the last regime the limits of the ramp
speed going to zero and the system size going to infinity do not commute and
the adiabatic process does not exist in the thermodynamic limit. We support our
results by numerical simulations. Our findings can be relevant to
condensed-matter, atomic physics, quantum computing, quantum optics, cosmology
and others.Comment: 11 pages, 5 figures, to appear in Nature Physics (originally
submitted version
The postulates of gravitational thermodynamics
The general principles and logical structure of a thermodynamic formalism
that incorporates strongly self-gravitating systems are presented. This
framework generalizes and simplifies the formulation of thermodynamics
developed by Callen. The definition of extensive variables, the homogeneity
properties of intensive parameters, and the fundamental problem of
gravitational thermodynamics are discussed in detail. In particular, extensive
parameters include quasilocal quantities and are naturally incorporated into a
set of basic general postulates for thermodynamics. These include additivity of
entropies (Massieu functions) and the generalized second law. Fundamental
equations are no longer homogeneous first-order functions of their extensive
variables. It is shown that the postulates lead to a formal resolution of the
fundamental problem despite non-additivity of extensive parameters and
thermodynamic potentials. Therefore, all the results of (gravitational)
thermodynamics are an outgrowth of these postulates. The origin and nature of
the differences with ordinary thermodynamics are analyzed. Consequences of the
formalism include the (spatially) inhomogeneous character of thermodynamic
equilibrium states, a reformulation of the Euler equation, and the absence of a
Gibbs-Duhem relation.Comment: 28 pages, Revtex, no figures. An important sentence and several minor
corrections included. To appear in Physical Review
Neutron Stars in Teleparallel Gravity
In this paper we deal with neutron stars, which are described by a perfect
fluid model, in the context of the teleparallel equivalent of general
relativity. We use numerical simulations to find the relationship between the
angular momentum of the field and the angular momentum of the source. Such a
relation was established for each stable star reached by the numerical
simulation once the code is fed with an equation of state, the central energy
density and the ratio between polar and equatorial radii. We also find a regime
where linear relation between gravitational angular momentum and moment of
inertia (as well as angular velocity of the fluid) is valid. We give the
spatial distribution of the gravitational energy and show that it has a linear
dependence with the squared angular velocity of the source.Comment: 19 pages, 14 figures. arXiv admin note: text overlap with
arXiv:1206.331
- …