304 research outputs found
Exapting exaptation
The term exaptation was introduced to encourage biologists to consider alternatives to adaptation to explain the origins of traits. Here, we discuss why exaptation has proved more successful in technological than biological contexts, and propose a revised definition of exaptation applicable to both genetic and cultural evolution
Co-production of synfuels and electricity from coal + biomass with zero net carbon emissions: An Illinois case study
Abstract Energy, carbon, and economic performance are estimated for facilities co-producing Fischer–Tropsch Liquid (FTL) fuels and electricity from a co-feed of biomass and coal in Illinois, with capture and storage of by-product CO 2 . The estimates include detailed models of supply systems for corn stover or mixed prairie grasses and of feedstock conversion facilities. The Illinois results are extrapolated to estimate the potential FTL production in 23 states
A solution for galactic disks with Yukawian gravitational potential
We present a new solution for the rotation curves of galactic disks with
gravitational potential of the Yukawa type. We follow the technique employed by
Toomre in 1963 in the study of galactic disks in the Newtonian theory. This new
solution allows an easy comparison between the Newtonian solution and the
Yukawian one. Therefore, constraints on the parameters of theories of
gravitation can be imposed, which in the weak field limit reduce to Yukawian
potentials. We then apply our formulae to the study of rotation curves for a
zero-thickness exponential disk and compare it with the Newtonian case studied
by Freeman in 1970. As an application of the mathematical tool developed here,
we show that in any theory of gravity with a massive graviton (this means a
gravitational potential of the Yukawa type), a strong limit can be imposed on
the mass (m_g) of this particle. For example, in order to obtain a galactic
disk with a scale length of b ~ 10 kpc, we should have a massive graviton of
m_g << 10^{-59} g. This result is much more restrictive than those inferred
from solar system observations.Comment: 7 pages; 1 eps figure; to appear in General Relativity and
Gravitatio
New limits on the ordered moments in alpha-Pu and Ga-stabilized delta-Pu
We present the first muon spin relaxation measurements ever performed on
elemental Pu, and set the most stringent upper limits to date on the magnitude
of the ordered moment in alpha-Pu and delta-stabilized Pu (alloyed with 4.3 at.
% Ga). Assuming a nominal hyperfine coupling field of 1 kOe per Bohr magneton
we set an upper limit of 0.001 Bohr magnetons for both materials at T = 4 K.Comment: 4 pages, 3 figures. Submitted to 10th International Conference on
Muon Spin Rotation, Relaxation and Resonance, Oxford, UK, August 200
Classical dynamics of a two-species Bose-Einstein condensate in the presence of nonlinear maser processes
The stability analysis of a generalized Dicke model, in the semi-classical
limit, describing the interaction of a two-species Bose-Einstein condensate
driven by a quantized field in the presence of Kerr and spontaneous parametric
processes is presented. The transitions from Rabi to Josephson dynamics are
identified depending on the relative value of the involved parameters.
Symmetry-breaking dynamics are shown for both types of coherent oscillations
due to the quantized field and nonlinear optical processes.Comment: 12 pages, 5 figures. Accepted for publication as chapter in
"Spontaneous Symmetry Breaking, Self-Trapping, and Josephson Oscillations in
Nonlinear Systems
Analysis of Ductile Bursting in Pressure Vessels of Texture-Hardening and Filament-Wrapped Materials
Analyses are presented for predicting the strength governed by the plastic tensile instability (PTI) phenomenon in thin-walled cylindrical and spherical pressure vessels constructed of texture- hardening alloys and with or without over-wrapped filaments. These analyses are important in predicting ductile bursting of pressure vessels used in such high-performance applications as high-pressure storage bottles, liquid-propellant tankage, and solid rocket casings. The analyses cover cylindrical pressure vessels subject to any ratio of biaxial stresses. Also means of estimating the effect of finite length is presented. Spherical vessels of texture- hardening material and cylindrical vessels with filaments over wrapped on a texture-hardening metallic substrate are treated as special cases. The analytical results are compared with available experimental results with good success.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Testing the Void against Cosmological data: fitting CMB, BAO, SN and H0
In this paper, instead of invoking Dark Energy, we try and fit various
cosmological observations with a large Gpc scale under-dense region (Void)
which is modeled by a Lemaitre-Tolman-Bondi metric that at large distances
becomes a homogeneous FLRW metric. We improve on previous analyses by allowing
for nonzero overall curvature, accurately computing the distance to the
last-scattering surface and the observed scale of the Baryon Acoustic peaks,
and investigating important effects that could arise from having nontrivial
Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE),
Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon
Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a
nonzero overall curvature drastically improves the goodness of fit of the Void
model, bringing it very close to that of a homogeneous universe containing Dark
Energy, while by varying the profile one can increase the value of the local
Hubble parameter which has been a challenge for these models. We also try to
gauge how well our model can fit the large-scale-structure data, but a
comprehensive analysis will require the knowledge of perturbations on LTB
metrics. The model is consistent with the CMB dipole if the observer is about
15 Mpc off the centre of the Void. Remarkably, such an off-center position may
be able to account for the recent anomalous measurements of a large bulk flow
from kSZ data. Finally we provide several analytical approximations in
different regimes for the LTB metric, and a numerical module for CosmoMC, thus
allowing for a MCMC exploration of the full parameter space.Comment: 70 pages, 12 figures, matches version accepted for publication in
JCAP. References added, numerical values in tables changed due to minor bug,
conclusions unaltered. Numerical module available at
http://web.physik.rwth-aachen.de/download/valkenburg
Inflationary signatures of single-field models beyond slow-roll
If the expansion of the early Universe was not close to de Sitter, the
statistical imprints of the primordial density perturbation on the cosmic
microwave background can be quite different from those derived in slow-roll
inflation. In this paper we study the inflationary signatures of all
single-field models which are free of ghost-like instabilities. We allow for a
rapid change of the Hubble parameter and the speed of sound of scalar
fluctuations, in a way that is compatible with a nearly scale-invariant
spectrum of perturbations, as supported by current cosmological observations.
Our results rely on the scale-invariant approximation, which is different from
the standard slow-roll approximation. We obtain the propagator of scalar
fluctuations and compute the bispectrum, keeping next-order corrections
proportional to the deviation of the spectral index from unity. These theories
offer an explicit example where the shape and scale-dependences of the
bispectrum are highly non-trivial whenever slow-roll is not a good
approximation.Comment: v1: 36 pages, including tables, appendices and references. v2:
abstract improved, references added, minor clarifications throughout the
text; matches version published in JCA
On primordial trispectrum from exchanging scalar modes in general multiple field inflationary models
We make an complementary investigation of the primordial trispectrum from
exchanging intermediate scalar modes in multi-field inflation models with
generalized kinetic terms. Together with the calculation of irreducible
contributions to the primordial trispectrum in Ref.[103], we give the full
leading-order primordial trispectrum in generalized multi-field models.Comment: 15 pages, 1 figure; v2 references adde
N-body simulations of gravitational dynamics
We describe the astrophysical and numerical basis of N-body simulations, both
of collisional stellar systems (dense star clusters and galactic centres) and
collisionless stellar dynamics (galaxies and large-scale structure). We explain
and discuss the state-of-the-art algorithms used for these quite different
regimes, attempt to give a fair critique, and point out possible directions of
future improvement and development. We briefly touch upon the history of N-body
simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu
- …