6,008 research outputs found
MOND as the weak-field limit of an extended metric theory of gravity
We show that the Modified Newtonian Dynamics (MOND) regime can be fully
recovered as the weak-field limit of a particular theory of gravity formulated
in the metric approach. This is possible when Milgrom's acceleration constant
is taken as a fundamental quantity which couples to the theory in a very
consistent manner. As a consequence, the scale invariance of the gravitational
interaction is naturally broken. In this sense, Newtonian gravity is the
weak-field limit of general relativity and MOND is the weak-field limit of that
particular extended theory of gravity.Comment: To appear in the proceedings of the 2011 Spanish Relativity Meeting
(ERE2011) held in Madrid, Spain, 4 page
Phase II of the ASCE Benchmark Study on SHM
The task group on structural health monitoring of the Dynamic Committee of ASCE was formed in
1999 at the 12
th
Engineering Mechanics Conference. The task group has designed a number of analytical
studies on a benchmark structure and there are plans to follow these with an experimental program. The
first phase of the analytical studies was completed in 2001. The second phase, initiated in the summer of
2001, was formulated in the light of the experience gained on phase I and focuses on increasing realism in
the simulation of the discrepancies between the actual structure and the mathematical model used in the
analysis. This paper describes the rational that lead the SHM task group to the definition of phase II and
presents the details of the cases that are being considered
Non-relativistic Extended Gravity and its applications across different astrophysical scales
Using dimensional analysis techniques we present an extension of Newton's
gravitational theory built under the assumption that Milgrom's acceleration
constant is a fundamental quantity of nature. The gravitational force converges
to Newton's gravity and to a MOND-like description in two different mass and
length regimes. It is shown that a modification on the force sector (and not in
the dynamical one as MOND does) is more convenient and can reproduce and
predict different phenomena usually ascribed to dark matter at the
non-relativistic level.Comment: 4 pages, 2 figures. To appear in the proceedings of the 2011 Spanish
Relativity Meeting (ERE2011) held in Madrid, Spai
Interference pattern in the collision of structures in the BEC dark matter model: comparison with fluids
In order to explore nonlinear effects on the distribution of matter during
collisions within the Bose-Einstein condensate (BEC) dark matter model driven
by the Schr\"odinger-Poisson system of equations, we study the head-on
collision of structures and focus on the interference pattern formation in the
density of matter during the collision process. We explore the possibility that
the collision of two structures of fluid matter modeled with an ideal gas
equation of state also forms interference patterns and found a negative result.
Given that a fluid is the most common flavor of dark matter models, we conclude
that one fingerprint of the BEC dark matter model is the pattern formation in
the density during a collision of structures.Comment: 7 pages, 22 eps figure
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Modelling the inorganic nitrogen behaviour in a small Mediterranean forested catchment, Fuirosos (Catalonia)
The aim of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4-N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil biological processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. Our simulation highlighted the riparian zone as a possible source of nitrate, especially after the summer drought period, but it can also act as an important sink of nitrate due to denitrification, in particular during the wettest period of the year. The riparian zone was a key element to simulate the catchment nitrate behaviour. The lumped LU4-N model (which does not include the riparian zone) could not be validated, while both the semi-distributed LU4-R-N and SD4-R-N model (which include the riparian zone) gave satisfactory results for the calibration process and acceptable results for the temporal validation process
Effect of processing conditions on the thermal and electrical conductivity of poly (butylene terephthalate) nanocomposites prepared via ring-opening polymerization
Successful preparation of polymer nanocomposites, exploiting graphene-related
materials, via melt mixing technology requires precise design, optimization and
control of processing. In the present work, the effect of different processing
parameters during the preparation of poly (butylene terephthalate)
nanocomposites, through ring-opening polymerization of cyclic butylene
terephthalate in presence of graphite nanoplatelets (GNP), was thoroughly
addressed. Processing temperature (240{\deg}C or 260{\deg}C), extrusion time (5
or 10 minutes) and shear rate (50 or 100 rpm) were varied by means of a full
factorial design of experiment approach, leading to the preparation of
polybutylene terephthalate/GNP nanocomposite in 8 different processing
conditions. Morphology and quality of GNP were investigated by means of
electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry and
Raman spectroscopy. Molecular weight of the polymer matrix in nanocomposites
and nanoflake dispersion were experimentally determined as a function of the
different processing conditions. The effect of transformation parameters on
electrical and thermal properties was studied by means of electrical and
thermal conductivity measurement. Heat and charge transport performance
evidenced a clear correlation with the dispersion and fragmentation of the GNP
nanoflakes; in particular, gentle processing conditions (low shear rate, short
mixing time) turned out to be the most favourable condition to obtain high
conductivity values
Linking and causality in globally hyperbolic spacetimes
The linking number is defined if link components are zero homologous.
Our affine linking invariant generalizes to the case of linked
submanifolds with arbitrary homology classes. We apply to the study of
causality in Lorentz manifolds. Let be a spacelike Cauchy surface in a
globally hyperbolic spacetime . The spherical cotangent bundle
is identified with the space of all null geodesics in
Hence the set of null geodesics passing through a point gives an
embedded -sphere in called the sky of Low observed
that if the link is nontrivial, then are causally
related. This motivated the problem (communicated by Penrose) on the Arnold's
1998 problem list to apply link theory to the study of causality. The spheres
are isotopic to fibers of They are nonzero
homologous and is undefined when is closed, while is well defined. Moreover, if is not an
odd-dimensional rational homology sphere. We give a formula for the increment
of \alk under passages through Arnold dangerous tangencies. If is
such that takes values in and is conformal to having all
the timelike sectional curvatures nonnegative, then are causally
related if and only if . We show that in
nonrefocussing are causally unrelated iff can be deformed
to a pair of -fibers of by an isotopy through skies. Low
showed that if (\ss, g) is refocussing, then is compact. We show that the
universal cover of is also compact.Comment: We added: Theorem 11.5 saying that a Cauchy surface in a refocussing
space time has finite pi_1; changed Theorem 7.5 to be in terms of conformal
classes of Lorentz metrics and did a few more changes. 45 pages, 3 figures. A
part of the paper (several results of sections 4,5,6,9,10) is an extension
and development of our work math.GT/0207219 in the context of Lorentzian
geometry. The results of sections 7,8,11,12 and Appendix B are ne
Dimensional analysis and Rutherford Scattering
Dimensional analysis, and in particular the Buckingham theorem is
widely used in fluid mechanics. In this article we obtain an expression for the
impact parameter from Buckingham's theorem and we compare our result with
Rutherford's original discovery found in the early twentieth century
Dynamical Arrest in Attractive Colloids: The Effect of Long-Range Repulsion
We study gelation in suspensions of model colloidal particles with
short-ranged attractive and long-ranged repulsive interactions by means of
three-dimensional fluorescence confocal microscopy. At low packing fractions,
particles form stable equilibrium clusters. Upon increasing the packing
fraction the clusters grow in size and become increasingly anisotropic until
finally associating into a fully connected network at gelation. We find a
surprising order in the gel structure. Analysis of spatial and orientational
correlations reveals that the gel is composed of dense chains of particles
constructed from face-sharing tetrahedral clusters. Our findings imply that
dynamical arrest occurs via cluster growth and association.Comment: Final version: Phys. Rev. Lett. 94, 208301 (2005
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