20,101 research outputs found
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
Localization of actin in Dictyostelium amebas by immunofluorescence
Antibody prepared against avian smooth muscle actin has been used to localize actin in the slime mold, Dictyostelium discoideum. The distribution of actin in migrating cells is different from that in feeding cells. Migrating amebas display fluorescence primarily in advancing regions whereas feeding amebas show uniform fluorescence throughout. The reaction is specific for actin since the fluorescence observed is blocked when the antibody is absorbed by actin purified from avian skeletal muscle, human platelets, and Dictyostelium. These results, in addition to describing the distribution of actin in D. discoideum, demonstrate that actins from these diverse sources share at least one common antigenic determinant
Reconstructing the Forest of Lineage Trees of Diverse Bacterial Communities Using Bio-inspired Image Analysis
Cell segmentation and tracking allow us to extract a plethora of cell
attributes from bacterial time-lapse cell movies, thus promoting computational
modeling and simulation of biological processes down to the single-cell level.
However, to analyze successfully complex cell movies, imaging multiple
interacting bacterial clones as they grow and merge to generate overcrowded
bacterial communities with thousands of cells in the field of view,
segmentation results should be near perfect to warrant good tracking results.
We introduce here a fully automated closed-loop bio-inspired computational
strategy that exploits prior knowledge about the expected structure of a
colony's lineage tree to locate and correct segmentation errors in analyzed
movie frames. We show that this correction strategy is effective, resulting in
improved cell tracking and consequently trustworthy deep colony lineage trees.
Our image analysis approach has the unique capability to keep tracking cells
even after clonal subpopulations merge in the movie. This enables the
reconstruction of the complete Forest of Lineage Trees (FLT) representation of
evolving multi-clonal bacterial communities. Moreover, the percentage of valid
cell trajectories extracted from the image analysis almost doubles after
segmentation correction. This plethora of trustworthy data extracted from a
complex cell movie analysis enables single-cell analytics as a tool for
addressing compelling questions for human health, such as understanding the
role of single-cell stochasticity in antibiotics resistance without losing site
of the inter-cellular interactions and microenvironment effects that may shape
it
Resource requirements and speed versus geometry of unconditionally secure physical key exchanges
The imperative need for unconditional secure key exchange is expounded by the
increasing connectivity of networks and by the increasing number and level of
sophistication of cyberattacks. Two concepts that are information theoretically
secure are quantum key distribution (QKD) and Kirchoff-law-Johnson-noise
(KLJN). However, these concepts require a dedicated connection between hosts in
peer-to-peer (P2P) networks which can be impractical and or cost prohibitive. A
practical and cost effective method is to have each host share their respective
cable(s) with other hosts such that two remote hosts can realize a secure key
exchange without the need of an additional cable or key exchanger. In this
article we analyze the cost complexities of cable, key exchangers, and time
required in the star network. We mentioned the reliability of the star network
and compare it with other network geometries. We also conceived a protocol and
equation for the number of secure bit exchange periods needed in a star
network. We then outline other network geometries and trade-off possibilities
that seem interesting to explore.Comment: 13 pages, 7 figures, MDPI Entrop
Ageing of Natural Rubber under Stress
We report a dynamical-mechanical study of stress relaxation at small
deformation in a natural (polyisoprene) rubber well above its glass transition
temperature Tg. We find that an almost complete relaxation of stress takes
place over very long periods of time, even though the elastic network integrity
is fully retained. The relaxation rate and the long-time equilibrium modulus
are sensitive functions of temperature which do not follow time-temperature
superposition. Many characteristic features of non-ergodic ageing response are
apparent at both short and very long times. We interpret the observed behaviour
in terms of the properties of rubber crosslinks, capable of isomerisation under
stress, and relate the results to recent models of soft glassy rheology.Comment: Latex 2e (EPJ style), 5 EPS figure
Dual descriptions of spin two massive particles in via master actions
In the first part of this work we show the decoupling (up to contact terms)
of redundant degrees of freedom which appear in the covariant description of
spin two massive particles in . We make use of a master action which
interpolates, without solving any constraints, between a first, second and
third order (in derivatives) self-dual model. An explicit dual map between
those models is derived. In our approach the absence of ghosts in the third
order self-dual model, which corresponds to a quadratic truncation of
topologically massive gravity, is due to the triviality (no particle content)
of the Einstein-Hilbert action in . In the second part of the work, also
in , we prove the quantum equivalence of the gauge invariant sector of a
couple of self-dual models of opposite helicities (+2 and -2) and masses
and to a generalized self-dual model which contains a quadratic
Einstein-Hilbert action, a Chern-Simons term of first order and a Fierz-Pauli
mass term. The use of a first order Chern-Simons term instead of a third order
one avoids conflicts with the sign of the Einstein-Hilbert action.Comment: title and abstract slightly modified, 3 references added, comments on
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