5,636 research outputs found
Multiscale mass-spring models of carbon nanotube foams
This article is concerned with the mechanical properties of dense, vertically aligned CNT foams subject to one-dimensional compressive loading. We develop a discrete model directly inspired by the micromechanical response reported experimentally for CNT foams, where infinitesimal portions of the tubes are represented by collections of uniform bi-stable springs. Under cyclic loading, the given model predicts an initial elastic deformation, a non-homogeneous buckling regime, and a densification response, accompanied by a hysteretic unloading path. We compute the dynamic dissipation of such a model through an analytic approach. The continuum limit of the microscopic spring chain defines a mesoscopic dissipative element (micro-meso transition) which represents a finite portion of the foam thickness. An upper-scale model formed by a chain of non-uniform mesoscopic springs is employed to describe the entire CNT foam. A numerical approximation illustrates the main features of the proposed multiscale approach. Available experimental results on the compressive response of CNT foams are fitted with excellent agreement
Management of change, disaster risk, and uncertainty: An overview
This overview, based on the keywords change, disaster risk, and uncertainties, focuses on issues emerging from the 2001-2004 IIASA-DPRI Forum on Integrated Disaster Risk Management; the uncertain effects of global climate change, implications for disasters due to differential vulnerabilities and rate of change. Other subjects addressed are the characterizing the uncertainties inherent in early warning systems, development of more adequate indicators for describing disasters, the need to investigate changes after disasters in order to develop stronger resilience, and movement towards institutional changes in order to address the global dimensions of disasters in a more and more deeply interconnected world
Black hole perturbation in nondynamical and dynamical Chern-Simons gravity
Chern-Simons gravitational theories are extensions of general relativity in
which the parity is violated due to the Chern-Simons term. We study linear
perturbations on the static and spherically symmetric background spacetime both
for nondynamical and dynamical Chern-Simons theories. We do not make an
assumption that the background Chern-Simons scalar field vanishes, which has
been adopted in the literature. By eliminating nondynamical variables using
their constraint equations, we derive the reduced second order action from
which a set of closed evolution equations containing only dynamical variables
are immediately obtained and therefore the number of propagating degrees of
freedom as well. It is found that ghost is present both for the nondynamical
case and for the dynamical case unless the background Chern-Simons scalar field
vanishes. It is also found that if the background scalar field vanishes, ghost
degrees of freedom are killed and all the modes propagate at the speed of
light.Comment: 18 pages; matches the published version in Phys. Rev.
Constraining Primordial Non-Gaussianity With the Abundance of High Redshift Clusters
We show how observations of the evolution of the galaxy cluster number
abundance can be used to constrain primordial non-Gaussianity in the universe.
We carry out a maximum likelihood analysis incorporating a number of current
datasets and accounting for a wide range of sources of systematic error. Under
the assumption of Gaussianity, the current data prefer a universe with matter
density and are inconsistent with at the
level. If we assume , the predicted degree of cluster
evolution is consistent with the data for non-Gaussian models where the
primordial fluctuations have at least two times as many peaks of height
or more as a Gaussian distribution does. These results are robust to
almost all sources of systematic error considered: in particular, the
Gaussian case can only be reconciled with the data if a number of
systematic effects conspire to modify the analysis in the right direction.
Given an independent measurement of , the techniques described here
represent a powerful tool with which to constrain non-Gaussianity in the
primordial universe, independent of specific details of the non-Gaussian
physics. We discuss the prospects and strategies for improving the constraints
with future observations.Comment: Minor revisions to match published ApJ version, 14 pages emulateap
Mapping the galactic gravitational potential with peculiar acceleration
It has been suggested recently that the change in cosmological redshift (the
Sandage test of expansion) could be observed in the next generation of large
telescopes and ultra-stable spectrographs. In a recent paper we estimated the
change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies
and clusters and shown it to be of the same order of magnitude as the typical
cosmological signal. Mapping the acceleration field allows for a reconstruction
of the galactic gravitational potential without assuming virialization. In this
paper we focus on the peculiar acceleration in our own Galaxy, modeled as a
Kuzmin disc and a dark matter spherical halo. We estimate the peculiar
acceleration for all known Galactic globular clusters and find some cases with
an expected velocity shift in excess of 20 cm/sec for observations fifteen
years apart, well above the typical cosmological acceleration. We then compare
the predicted signal for a MOND (modified Newtonian dynamics) model in which
the spherical dark matter halo is absent. We find that the signal pattern is
qualitatively different, showing that the peculiar acceleration field could be
employed to test competing theories of gravity. However the difference seems
too small to be detectable in the near future.Comment: 11 pages, 10 figures, 3 tables, minor changes, accepted for
publication by MNRA
Dark Matter and Dark Energy
I briefly review our current understanding of dark matter and dark energy.
The first part of this paper focusses on issues pertaining to dark matter
including observational evidence for its existence, current constraints and the
`abundance of substructure' and `cuspy core' issues which arise in CDM. I also
briefly describe MOND. The second part of this review focusses on dark energy.
In this part I discuss the significance of the cosmological constant problem
which leads to a predicted value of the cosmological constant which is almost
times larger than the observed value \la/8\pi G \simeq
10^{-47}GeV. Setting \la to this small value ensures that the
acceleration of the universe is a fairly recent phenomenon giving rise to the
`cosmic coincidence' conundrum according to which we live during a special
epoch when the density in matter and \la are almost equal. Anthropic
arguments are briefly discussed but more emphasis is placed upon dynamical dark
energy models in which the equation of state is time dependent. These include
Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model
independent methods to determine the cosmic equation of state and the
Statefinder diagnostic are also discussed. The Statefinder has the attractive
property \atridot/a H^3 = 1 for LCDM, which is helpful for differentiating
between LCDM and rival dark energy models. The review ends with a brief
discussion of the fate of the universe in dark energy models.Comment: 40 pages, 11 figures, Lectures presented at the Second Aegean Summer
School on the Early Universe, Syros, Greece, September 2003, New References
added Final version to appear in the Proceeding
Dark Energy and Dark Matter
It is a puzzle why the densities of dark matter and dark energy are nearly
equal today when they scale so differently during the expansion of the
universe. This conundrum may be solved if there is a coupling between the two
dark sectors. In this paper we assume that dark matter is made of cold relics
with masses depending exponentially on the scalar field associated to dark
energy. Since the dynamics of the system is dominated by an attractor solution,
the dark matter particle mass is forced to change with time as to ensure that
the ratio between the energy densities of dark matter and dark energy become a
constant at late times and one readily realizes that the present-day dark
matter abundance is not very sensitive to its value when dark matter particles
decouple from the thermal bath. We show that the dependence of the present
abundance of cold dark matter on the parameters of the model differs
drastically from the familiar results where no connection between dark energy
and dark matter is present. In particular, we analyze the case in which the
cold dark matter particle is the lightest supersymmetric particle.Comment: 4 pages latex, 2 figure
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