34,726 research outputs found
Convergence of Scalar-Tensor theories toward General Relativity and Primordial Nucleosynthesis
In this paper, we analyze the conditions for convergence toward General
Relativity of scalar-tensor gravity theories defined by an arbitrary coupling
function (in the Einstein frame). We show that, in general, the
evolution of the scalar field is governed by two opposite mechanisms:
an attraction mechanism which tends to drive scalar-tensor models toward
Einstein's theory, and a repulsion mechanism which has the contrary effect. The
attraction mechanism dominates the recent epochs of the universe evolution if,
and only if, the scalar field and its derivative satisfy certain boundary
conditions. Since these conditions for convergence toward general relativity
depend on the particular scalar-tensor theory used to describe the universe
evolution, the nucleosynthesis bounds on the present value of the coupling
function, , strongly differ from some theories to others. For
example, in theories defined by analytical
estimates lead to very stringent nucleosynthesis bounds on
(). By contrast, in scalar-tensor theories defined by
much larger limits on () are
found.Comment: 20 Pages, 3 Figures, accepted for publication in Class. and Quantum
Gravit
Symplectic Structure of 2D Dilaton Gravity
We analyze the symplectic structure of two-dimensional dilaton gravity by
evaluating the symplectic form on the space of classical solutions. The case
when the spatial manifold is compact is studied in detail. When the matter is
absent we find that the reduced phase space is a two-dimensional cotangent
bundle and determine the Hilbert space of the quantum theory. In the
non-compact case the symplectic form is not well defined due to an unresolved
ambiguity in the choice of the boundary terms.Comment: 12 pgs, Imperial TP/92-93/37, La-Tex fil
Uncertainties in the solar photospheric oxygen abundance
The purpose of this work is to better understand the confidence limits of the
photospheric solar oxygen abundance derived from three-dimensional models using
the forbidden [OI] line at 6300 \AA , including correlations with other
parameters involved. We worked with a three-dimensional empirical model and two
solar intensity atlases. We employed Bayesian inference as a tool to determine
the most probable value for the solar oxygen abundance given the model chosen.
We considered a number of error sources, such as uncertainties in the continuum
derivation, in the wavelength calibration and in the abundance/strength of Ni.
Our results shows correlations between the effects of several parameters
employed in the derivation. The Bayesian analysis provides robust confidence
limits taking into account all of these factors in a rigorous manner. We obtain
that, given the empirical three-dimensional model and the atlas observations
employed here, the most probable value for the solar oxygen abundance is
. However, we note that this uncertainty does
not consider possible sources of systematic errors due to the model choice.Comment: Accepted for publication in Astronomy and Astrophysic
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Multilayer and conformal antennas using synthetic dielectric substrates
This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
Tidal Torques and the Orientation of Nearby Disk Galaxies
We use numerical simulations to investigate the orientation of the angular
momentum axis of disk galaxies relative to their surrounding large scale
structure. We find that this is closely related to the spatial configuration at
turnaround of the material destined to form the galaxy, which is often part of
a coherent two-dimensional slab criss-crossed by filaments. The rotation axis
is found to align very well with the intermediate principal axis of the inertia
momentum tensor at this time. This orientation is approximately preserved
during the ensuing collapse, so that the rotation axis of the resulting disk
ends up lying on the plane traced by the protogalactic material at turnaround.
This suggests a tendency for disks to align themselves so that their rotation
axis is perpendicular to the minor axis of the structure defined by surrounding
matter. One example of this trend is provided by our own Galaxy, where the
Galactic plane is almost at right angles with the supergalactic plane (SGP)
drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole
is just 6 degrees. We have searched for a similar signature in catalogs of
nearby disk galaxies, and find a significant excess of edge-on spirals (for
which the orientation of the disk rotation axis may be determined
unambiguously) highly inclined relative to the SGP. This result supports the
view that disk galaxies acquire their angular momentum as a consequence of
early tidal torques acting during the expansion phase of the protogalactic
material.Comment: 5 pages, 2 figures, accepted for publication in ApJ
- …