16,705 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
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
interactions include
StochSoCs: High performance biocomputing simulations for large scale Systems Biology
The stochastic simulation of large-scale biochemical reaction networks is of
great importance for systems biology since it enables the study of inherently
stochastic biological mechanisms at the whole cell scale. Stochastic Simulation
Algorithms (SSA) allow us to simulate the dynamic behavior of complex kinetic
models, but their high computational cost makes them very slow for many
realistic size problems. We present a pilot service, named WebStoch, developed
in the context of our StochSoCs research project, allowing life scientists with
no high-performance computing expertise to perform over the internet stochastic
simulations of large-scale biological network models described in the SBML
standard format. Biomodels submitted to the service are parsed automatically
and then placed for parallel execution on distributed worker nodes. The workers
are implemented using multi-core and many-core processors, or FPGA accelerators
that can handle the simulation of thousands of stochastic repetitions of
complex biomodels, with possibly thousands of reactions and interacting
species. Using benchmark LCSE biomodels, whose workload can be scaled on
demand, we demonstrate linear speedup and more than two orders of magnitude
higher throughput than existing serial simulators.Comment: The 2017 International Conference on High Performance Computing &
Simulation (HPCS 2017), 8 page
- …