24,176 research outputs found
Homogeneous cosmologies and the Maupertuis-Jacobi principle
A recent work showing that homogeneous and isotropic cosmologies involving
scalar fields are equivalent to the geodesics of certain effective manifolds is
generalized to the non-minimally coupled and anisotropic cases. As the
Maupertuis-Jacobi principle in classical mechanics, such result permits us to
infer some dynamical properties of cosmological models from the geometry of the
associated effective manifolds, allowing us to go a step further in the study
of cosmological dynamics. By means of some explicit examples, we show how the
geometrical analysis can simplify considerably the dynamical analysis of
cosmological models.Comment: 5 page
Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion
This work wants to show how standard General Relativity (GR) is able to
explain galactic rotation curves without the need for dark matter, this
starting from the idea that when Einstein's equations are applied to the
dynamics of a galaxy embedded in an expanding universe they do not reduce to
Poisson's equation but a generalisation of it taking cosmological expansion
into account. A non-linear scheme to perturb Einstein's field equations around
the Robertson-Walker (R-W) metric is devised in order to find their
non-relativistic limit without losing their characteristic non-linearities. The
resulting equation is used to numerically study the gravitational potential of
a cosmological perturbation and applied to a simple galactic model with an
exponentially decreasing baryonic matter distribution. The non-relativistic
limit of GR in a R-W space-time produces a generalised Poisson equation for the
gravitational potential which is non-linear, parabolic and heat-like. It is
shown how its non-linearities generate an effective "dark matter" distribution
caused by both cosmological expansion and the dynamics of the perturbation's
gravitational potential. It is also shown how this dynamical effect gets
completely lost during a linearisation of Einstein's equations. The equation is
then used to successfully fit real galactic rotation curves numerically using a
matter distribution following the shape of a simple S\'ersic luminosity
profile, common to most galaxies, thus without recourse to dark matter. A
relation for the dark to luminous matter ratio is found, explaining the
domination of dark matter in low-mass galaxies. A few rotation curves with a
faster than Newtonian decrease are also presented and successfully fitted,
opening the way to a new possible interpretation of these phenomena in terms of
an effective "anti-gravitational" dark matter distribution, purely geometrical
in origin.Comment: 8 pages, 18 figures, Research Pape
Loran-C approach guidance project current status
There are four areas of work in the Loran-C flight test project. Current results provide performance data on the effects of Signal Noise Ratio (SNR) on the dynamic performance of the receiver filters for Loran-C data, and data on Loran-C grid deformation at a microscale of 100 meters. The Loran-C receiver provides a line of position (LOP) Master and Slave transmitter at an angle 0 to magnetic north. No transformation to latitude-longitude reference frame is required since this is the major source of Loran-C navigation errors. A local coordinate frame is established centered at touchdown point on the runway with directions along and across the runway. A Loran-C data collection system was set up. The Loran-C data are sent directly to an Apple II computer with a 12 inch monitor. The effect of SNR on Loran-C precision is shown for two receiver filters of different frequency response. A set of ground level static readings of touchdown was taken around Hanscom Field and transferred to an accurate detailed layout drawing; this showed local distortions of the average touchdown values
Entomopathogenic nematodes for biological control of codling moth
Entomopathogenic nematodes are often found naturally infecting codling moth larvae. The
effect of an autumn treatment with S. feltiae on the fruit damage in the following summer
was evaluated by treating 4 different apple orchards in October 2004 and 2005 at
application rates of 3.75; 2 and 1.5 billion nematodes in 4000 l / ha. In three of the treated
orchards, one treated with 3.75x109 nematodes/ha the other two treated with 2e9
nematode/ha, reduction in fruit damage was around 50%. In the most heavily infested
orchard, which was treated with 1.5x109 nematode/ha only 33% reduction in fruit damage
was achieved. Compared to previous studies, this was the first assessing the effect on the
fruit damage in the summer following the treatment rather than assessing the mortality of
sentinel larvae fixed to the treated tree trunks
Field Theory on Newton-Cartan Backgrounds and Symmetries of the Lifshitz Vacuum
Holography for Lifshitz space-times corresponds to dual field theories on a
fixed torsional Newton-Cartan (TNC) background. We examine the coupling of
non-relativistic field theories to TNC backgrounds and uncover a novel
mechanism by which a global U(1) can become local. This involves the TNC vector
which sources a particle number current, and which for flat NC
space-time satisfies with a Schroedinger symmetry
realized on . We discuss various toy model field theories on flat NC
space-time for which the new mechanism leads to extra global space-time
symmetries beyond the generic Lifshitz symmetry, allowing for an enhancement to
Schroedinger symmetry. On the holographic side, the source also appears in
the Lifshitz vacuum with exactly the same properties as for flat NC space-time.
In particular, the bulk diffeomorphisms that preserve the boundary conditions
realize a Schroedinger algebra on , allowing for a conserved particle number
current. Finally, we present a probe action for a complex scalar field on the
Lifshitz vacuum, which exhibits Schroedinger invariance in the same manner as
seen in the field theory models.Comment: 55 pages + 2 appendice
Incorporation of Spacetime Symmetries in Einstein's Field Equations
In the search for exact solutions to Einstein's field equations the main
simplification tool is the introduction of spacetime symmetries. Motivated by
this fact we develop a method to write the field equations for general matter
in a form that fully incorporates the character of the symmetry. The method is
being expressed in a covariant formalism using the framework of a double
congruence. The basic notion on which it is based is that of the geometrisation
of a general symmetry. As a special application of our general method we
consider the case of a spacelike conformal Killing vector field on the
spacetime manifold regarding special types of matter fields. New perspectives
in General Relativity are discussed.Comment: 41 pages, LaTe
- …