123 research outputs found
Schwarzschild and Synge once again
We complete the historical overview about the geometry of a Schwarzschild
black hole at its horizon by emphasizing the contribution made by J. L. Synge
in 1950 to its clarification.Comment: 2 pages, LaTeX, submitted for publication; 2 references, one Note,
and an Acknowledgement are adde
Editorial note to "The beginning of the world from the point of view of quantum theory"
This is an editorial note to accompany reprinting as a Golden Oldie in the
Journal of General Relativity and Gravitation of the famous note by Georges
Lemaitre on the quantum birth of the universe, published in Nature in 1931. We
explain why this short (457 words) article can be considered to be the true
"Charter" of the modern Big Bang theory.Comment: This is an editorial comment to accompany reprinting of a classical
paper in the Journal of General Relativity and Gravitation. 16 pages, 2
figure
Reissner-Nordstrom and charged gas spheres
The main point of this paper is a suggestion about the proper treatment of
the photon gas in a theory of stellar structure and other plasmas. This problem
arises in the study of polytropic gas spheres, where we have already introduced
some innovations. The main idea, already advanced in the contextof neutral,
homogeneous, polytropic stellar models, is to base the theory firmly on a
variational principle. Another essential novelty is to let mass distribution
extend to infinity, the boundary between bulk and atmosphere being defined by
an abrupt change in the polytropic index, triggered by the density. The logical
next step in this program is to include the effect of radiation, which is a
very significant complication since a full treatment would have to include an
account of ionization, thus fieldsrepresenting electrons, ions, photons,
gravitons and neutral atoms as well. In way of preparation, we consider models
that are charged but homogeneous, involving only gravity, electromagnetism and
a single scalar field that represents both the mass and the electric charge; in
short, anon-neutral plasma. While this work only represents a stage in the
development of a theory of stars, without direct application to physical
systems, it does shed some light on the meaning of the Reissner-Nordstrom
solution of the modified Einstein-Maxwell equations., with an application to a
simple system.Comment: 19 pages, plain te
A tentative derivation of the main cosmological parameters
Based on the assumption that some apparent properties of the observable
universe are accurate at a reasonable level of approximation, a tentative is
made to independently derive the values of the baryon density parameter, the
Hubble constant, the cosmic microwave background temperature and the helium
mass fraction. The obtained values are in excellent agreement with those given
by the most recent observational data.Comment: 13 pages. Accepted for publication in Astrophysics & Space Scienc
On Dual Formulation of Gravity
In this paper we consider a possibility to construct dual formulation of
gravity where the main dynamical field is the Lorentz connection
\omega_\mu^{ab} and not that of tetrad e_\mu^a or metric g_\mu\nu. Our approach
is based on the usual dualization procedure which uses first order parent
Lagrangians but in (Anti) de Sitter space and not in the flat Minkowski one. It
turns out that in d=3 dimensions such dual formulation is related with the so
called exotic parity-violating interactions for massless spin-2 particles.Comment: 7 pages, plain LaTe
Mass loss by a scalar charge in an expanding universe
We study the phenomenon of mass loss by a scalar charge -- a point particle
that acts a source for a noninteracting scalar field -- in an expanding
universe. The charge is placed on comoving world lines of two cosmological
spacetimes: a de Sitter universe, and a spatially-flat, matter-dominated
universe. In both cases, we find that the particle's rest mass is not a
constant, but that it changes in response to the emission of monopole scalar
radiation by the particle. In de Sitter spacetime, the particle radiates all of
its mass within a finite proper time. In the matter-dominated cosmology, this
happens only if the charge of the particle is sufficiently large; for smaller
charges the particle first loses some of its mass, but then regains it all
eventually.Comment: 11 pages, RevTeX4, Accepted for Phys. Rev.
The restricted two-body problem in constant curvature spaces
We perform the bifurcation analysis of the Kepler problem on and .
An analogue of the Delaunay variables is introduced. We investigate the motion
of a point mass in the field of the Newtonian center moving along a geodesic on
and (the restricted two-body problem). When the curvature is small,
the pericenter shift is computed using the perturbation theory. We also present
the results of the numerical analysis based on the analogy with the motion of
rigid body.Comment: 29 pages, 7 figure
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
Regularizing cosmological singularities by varying physical constants
Varying physical constant cosmologies were claimed to solve standard
cosmological problems such as the horizon, the flatness and the
-problem. In this paper, we suggest yet another possible application
of these theories: solving the singularity problem. By specifying some examples
we show that various cosmological singularities may be regularized provided the
physical constants evolve in time in an appropriate way.Comment: 9 pages, 6 figures, Revtex4-1, an improved version to appear in JCA
Numerical simulations of the kappa-mechanism with convection
A strong coupling between convection and pulsations is known to play a major
role in the disappearance of unstable modes close to the red edge of the
classical Cepheid instability strip. As mean-field models of time-dependent
convection rely on weakly-constrained parameters, we tackle this problem by the
means of 2-D Direct Numerical Simulations (DNS) of kappa-mechanism with
convection.
Using a linear stability analysis, we first determine the physical conditions
favourable to the kappa-mechanism to occur inside a purely-radiative layer.
Both the instability strips and the nonlinear saturation of unstable modes are
then confirmed by the corresponding DNS. We next present the new simulations
with convection, where a convective zone and the driving region overlap. The
coupling between the convective motions and acoustic modes is then addressed by
using projections onto an acoustic subspace.Comment: 5 pages, 6 figures, accepted for publication in Astrophysics and
Space Science, HELAS workshop (Rome june 2009
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