2,312 research outputs found
Spherical symmetry in a dark energy permeated space-time
The properties of a spherically symmetric static space-time permeated of dark
energy are worked out. Dark energy is viewed as the strain energy of an
elastically deformable four dimensional manifold. The metric is worked out in
the vacuum region around a central spherical mass/defect in the linear
approximation. We discuss analogies and differences with the analogue in the de
Sitter space time and how these competing scenarios could be differentiated on
an observational ground. The comparison with the tests at the solar system
scale puts upper limits to the parameters of the theory, consistent with the
values obtained applying the classical cosmological tests.Comment: 14 pages, 1 figure, in press on Classical and Quantum Gravit
Cosmological constraints for the Cosmic Defect theory
The Cosmic Defect theory has been confronted with four observational
constraints: primordial nuclear species abundances emerging from the big bang
nucleosynthesis; large scale structure formation in the universe; cosmic
microwave background acoustic scale; luminosity distances of type Ia
supernovae. The test has been based on a statistical analysis of the a
posteriori probabilities for three parameters of the theory. The result has
been quite satisfactory and such that the performance of the theory is not
distinguishable from the one of the Lambda-CDM theory. The use of the optimal
values of the parameters for the calculation of the Hubble constant and the age
of the universe confirms the compatibility of the Cosmic Defect approach with
observations.Comment: 13 pages, 1 figure, in press on IJMP
From the elasticity theory to cosmology and vice versa
The paper shows how a generalization of the elasticity theory to four
dimensions and to space-time allows for a consistent description of the
homogeneous and isotropic universe, including the accelerated expansion. The
analogy is manifested by the inclusion in the traditional Lagrangian of general
relativity of an additional term accounting for the strain induced in the
manifold (i.e. in space-time) by the curvature, be it induced by the presence
of a texture defect or by a matter/energy distribution. The additional term is
sufficient to account for various observed features of the universe and to give
a simple interpretation for the so called dark energy. Then, we show how the
same approach can be adopted back in three dimensions to obtain the equilibrium
configuration of a given solid subject to strain induced by defects or applied
forces. Finally, it is shown how concepts coming from the familiar elasticity
theory can inspire new approaches to cosmology and in return how methods
appropriated to General Relativity can be applied back to classical problems of
elastic deformations in three dimensions.Comment: 11 pages, 3 figure
Angular momentum effects in weak gravitational fields
It is shown that, contrary to what is normally expected, it is possible to
have angular momentum effects on the geometry of space time at the laboratory
scale, much bigger than the purely Newtonian effects. This is due to the fact
that the ratio between the angular momentum of a body and its mass, expressed
as a length, is easily greater than the mass itself, again expressed as a
length.Comment: LATEX, 8 page
Gravitomagnetism, clocks and geometry
New techniques to evaluate the clock effect using light are described. These
are based on the flatness of the cylindrical surface containing the world lines
of the rays constrained to move on circular trajectories about a spinning mass.
The effect of the angular momentum of the source is manifested in the fact that
inertial observers must be replaced by local non rotating observers. Starting
from this an exact formula for circular trajectories is found. Numerical
estimates for the Earth environment show that light would be a better probe
than actual clocks to evidence the angular momentum influence. The advantages
of light in connection with some principle experiments are shortly reviewed.Comment: TCI Latex, 12 pages, 2 figures. To appear in European Journal of
Physic
Angular momentum effects in Michelson-Morley type experiments
The effect of the angular momentum density of a gravitational source on the
times of flight of light rays in an interferometer is analyzed. The calculation
is made imagining that the interferometer is at the equator of the gravity
source and, as long as possible, the metric, provided it is stationary and
axisymmetric, is not approximated. Finally, in order to evaluate the size of
the effect in the case of the Earth a weak field approximation is introduced.
For laboratory scales and non-geodesic paths the correction turns out to be
comparable with the sensitivity expected in gravitational waves interferometric
detectors, whereas it drops under the threshold of detectability when using
free (geodesic) light rays.Comment: 12 pages, LaTeX; more about the detection technique, references
added; accepted for publication in GR
Effective non-additive pair potential for lock-and-key interacting particles: the role of the limited valence
Theoretical studies of self-assembly processes and condensed phases in
colloidal systems are often based on effective inter-particle potentials. Here
we show that developing an effective potential for particles interacting with a
limited number of ``lock-and-key'' selective bonds (due to the specificity of
bio-molecular interactions) requires -- beside the non-sphericity of the
potential -- a (many body) constraint that prevent multiple bonding on the same
site. We show the importance of retaining both valence and bond-selectivity by
developing, as a case study, a simple effective potential describing the
interaction between colloidal particles coated by four single-strand DNA
chains.Comment: 4 pages, 5 figure
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