3,688 research outputs found

### Affect of brane thickness on microscopic tidal-charged black holes

We study the phenomenological implications stemming from the dependence of
the tidal charge on the brane thickness $L$ for the evaporation and decay of
microscopic black holes. In general, the larger $L$, the longer are the black
hole life-times and the greater their maximum mass for those cases in which the
black hole can grow. In particular, we again find that tidal-charged black
holes might live long enough to escape the detectors and even the gravitational
field of the Earth, thus resulting in large amounts of missing energy. However,
under no circumstances could TeV-scale black holes grow enough to enter the
regime of Bondi accretion.Comment: 6 pages, 2 figures, Clarification of tidal charge expression.
Additional justification of constraint

### Generating perfect fluid spheres in general relativity

Ever since Karl Schwarzschild's 1916 discovery of the spacetime geometry
describing the interior of a particular idealized general relativistic star --
a static spherically symmetric blob of fluid with position-independent density
-- the general relativity community has continued to devote considerable time
and energy to understanding the general-relativistic static perfect fluid
sphere. Over the last 90 years a tangle of specific perfect fluid spheres has
been discovered, with most of these specific examples seemingly independent
from each other. To bring some order to this collection, in this article we
develop several new transformation theorems that map perfect fluid spheres into
perfect fluid spheres. These transformation theorems sometimes lead to
unexpected connections between previously known perfect fluid spheres,
sometimes lead to new previously unknown perfect fluid spheres, and in general
can be used to develop a systematic way of classifying the set of all perfect
fluid spheres.Comment: 18 pages, 4 tables, 4 figure

### The Proton Distribution Function in Weakly Magnetized Turbulent Plasmas

We calculate the proton distribution function due to heating by subsonic
(Alfvenic) turbulence in a weakly magnetized collisionless plasma. The
distribution function is nonthermal. For non-relativistic energies, it is an
exponential of the magnitude of the proton velocity. For ultra-relativistic
energies, it can be characterized as a power law with a momentum-dependent
slope.Comment: 7 pages, submitted to Ap

### Buchdahl-like transformations for perfect fluid spheres

In two previous articles [Phys. Rev. D71 (2005) 124307 (gr-qc/0503007), and
gr-qc/0607001] we have discussed several "algorithmic" techniques that permit
one (in a purely mechanical way) to generate large classes of general
relativistic static perfect fluid spheres. Working in Schwarzschild curvature
coordinates, we used these algorithmic ideas to prove several
"solution-generating theorems" of varying levels of complexity. In the present
article we consider the situation in other coordinate systems: In particular,
in general diagonal coordinates we shall generalize our previous theorems, in
isotropic coordinates we shall encounter a variant of the so-called "Buchdahl
transformation", while in other coordinate systems (such as Gaussian polar
coordinates, Synge isothermal coordinates, and Buchdahl coordinates) we shall
find a number of more complex "Buchdahl-like transformations" and
"solution-generating theorems" that may be used to investigate and classify the
general relativistic static perfect fluid sphere. Finally by returning to
general diagonal coordinates and making a suitable ansatz for the functional
form of the metric components we place the Buchdahl transformation in its most
general possible setting.Comment: 23 page

### On a common misunderstanding of the Birkhoff theorem and light deflection calculation: generalized Shapiro delay and its possible laboratory test

In Newtonian gravity (NG) it is known that the gravitational field anywhere
inside a spherically symmetric distribution of mass is determined only by the
enclosed mass. This is also widely believed to be true in general relativity
(GR), and the Birkhoff theorem is often invoked to support this analogy between
NG and GR. Here we show that such an understanding of the Birkhoff theorem is
incorrect and leads to erroneous calculations of light deflection and delay
time through matter. The correct metric, matching continuously to the location
of an external observer, is determined both by the enclosed mass and mass
distribution outside. The effect of the outside mass is to make the interior
clock run slower, i.e., a slower speed of light for external observer. We also
discuss the relations and differences between NG and GR, in light of the
results we obtained in this Lettework. Finally we discuss the Generalized
Shapiro delay, caused by the outside mass, and its possible laboratory test.Comment: 12 pages, 4 figures, invited talk in the 2nd Galileo-Xu Guangqi
Meeing, Italy, 2011, IJMPD in pres

### Region of the anomalous compression under Bondi-Hoyle accretion

We investigate the properties of an axisymmetric non-magnetized gas flow
without angular momentum on a small compact object, in particular, on a
Schwarzschild black hole in the supersonic region near the object; the velocity
of the object itself is assumed to be low compared to the speed of sound at
infinity. First of all, we see that the streamlines intersect (i.e., a caustic
forms) on the symmetry axis at a certain distance $r_x$ from the center on the
front side if the pressure gradient is neglected. The characteristic radial
size of the region, in which the streamlines emerging from the sonic surface at
an angle no larger than $\theta_0$ to the axis intersect, is $\Delta r=
r_x\theta^2_0/3.$ To refine the flow structure in this region, we numerically
compute the system in the adiabatic approximation without ignoring the
pressure. We estimate the parameters of the inferred region with anomalously
high matter temperature and density accompanied by anomalously high energy
release.Comment: 10 pages, 2 figure

### A Solution to the Protostellar Accretion Problem

Accretion rates of order 10^-8 M_\odot/yr are observed in young protostars of
approximately a solar mass with evidence of circumstellar disks. The accretion
rate is significantly lower for protostars of smaller mass, approximately
proportional to the second power of the stellar mass, \dot{M}_accr\propto M^2.
The traditional view is that the observed accretion is the consequence of the
angular momentum transport in isolated protostellar disks, controlled by disk
turbulence or self--gravity. However, these processes are not well understood
and the observed protostellar accretion, a fundamental aspect of star
formation, remains an unsolved problem. In this letter we propose the
protostellar accretion rate is controlled by accretion from the large scale gas
distribution in the parent cloud, not by the isolated disk evolution.
Describing this process as Bondi--Hoyle accretion, we obtain accretion rates
comparable to the observed ones. We also reproduce the observed dependence of
the accretion rate on the protostellar mass. These results are based on
realistic values of the ambient gas density and velocity, as inferred from
numerical simulations of star formation in self--gravitating turbulent clouds.Comment: 4 pages, 2 figures, ApJ Letters, in pres

### Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney

A recent criticism of cosmological methodology and achievements by Disney
(2000) is assessed. Some historical and epistemological fallacies in the said
article have been highlighted. It is shown that---both empirically and
epistemologically---modern cosmology lies on sounder foundations than it is
portrayed. A brief historical account demonstrates that this form of
unsatisfaction with cosmology has had a long tradition, and rather meagre
results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel.
Grav., accepted for publicatio

### 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

### Spherical Accretion

We compare different examples of spherical accretion onto a gravitating mass.
Limiting cases include the accretion of a collisionally dominated fluid and the
accretion of collisionless particles. We derive expressions for the accretion
rate and density profile for semi-collisional accretion which bridges the gap
between these limiting cases. Particle crossing of the Hill sphere during the
formation of the outer planets is likely to have taken place in the
semi-collisional regime.Comment: ApJ Letters, 3 page

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