385 research outputs found
Targeting Conservation Investments in Heterogeneous Landscapes: A distance function approach and application to watershed management
To achieve a given level of an environmental amenity at least cost, decision-makers must integrate information about spatially variable biophysical and economic conditions. Although the biophysical attributes that contribute to supplying an environmental amenity are often known, the way in which these attributes interact to produce the amenity is often unknown. Given the difficulty in converting multiple attributes into a unidimensional physical measure of an environmental amenity (e.g., habitat quality), analyses in the academic literature tend to use a single biophysical attribute as a proxy for the environmental amenity (e.g., species richness). A narrow focus on a single attribute, however, fails to consider the full range of biophysical attributes that are critical to the supply of an environmental amenity. Drawing on the production efficiency literature, we introduce an alternative conservation targeting approach that relies on distance functions to cost-efficiently allocate conservation funds across a spatially heterogeneous landscape. An approach based on distance functions has the advantage of not requiring a parametric specification of the amenity function (or cost function), but rather only requiring that the decision-maker identify important biophysical and economic attributes. We apply the distance-function approach empirically to an increasingly common, but little studied, conservation initiative: conservation contracting for water quality objectives. The contract portfolios derived from the distance-function application have many desirable properties, including intuitive appeal, robust performance across plausible parametric amenity measures, and the generation of ranking measures that can be easily used by field practitioners in complex decision-making environments that cannot be completely modeled. Working Paper # 2002-01
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Theoretical studies of chemisorption and surface reactions on nickel and silicon
The research is part of a theoretical program on the structure of molecules adsorbed on solid surfaces and dissociative chemisorption which emphasis on transition metal substrates and electronic materials. An embedding theory for treating chemisorption on metals is further developed and applied to the reaction of hydrocarbon fragments and hydrogen coadsorbed on nickel, the dissociation of water on nickel, and the system H/Ni, NH{sub 3}/Ni, and C{sub 6}H{sub 6}/Ni. The main emphasis of the work is on the energetics of adsorption as a function of surface site, the potential energy for adsorbate motion between surface sites and the energetics of surface reactions. Equilibrium geometries, vibrational frequencies and ionization potentials are also calculated. Preliminary work on the description of Si(100) surfaces has been completed in preparation of H/Si and N/Si adsorption studies. Studies of {pi} bonding on Si(111) were also completed and work was begun on the Auger ionization of F/Si. The original form of the embedding theory has been extended to make use of an effective potential representation of the bulk electrons interacting with the embedded surface region. Effective core potentials for Fe have been developed
Unified dark energy models : a phenomenological approach
A phenomenological approach is proposed to the problem of universe
accelerated expansion and of the dark energy nature. A general class of models
is introduced whose energy density depends on the redshift in such a way
that a smooth transition among the three main phases of the universe evolution
(radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type
Ia Supernovae and the angular size - redshift relation for compact and
ultracompact radio structures to test whether the model is in agreement with
astrophysical observation and to constrain its main parameters. Although
phenomenologically motivated, the model may be straightforwardly interpreted as
a two fluids scenario in which the quintessence is generated by a suitably
chosen scalar field potential. On the other hand, the same model may also be
read in the context of unified dark energy models or in the framework of
modified Friedmann equation theories.Comment: 12 pages, 10 figures, accepted for publication on Physical Review
Circular orbits of corotating binary black holes: comparison between analytical and numerical results
We compare recent numerical results, obtained within a ``helical Killing
vector'' (HKV) approach, on circular orbits of corotating binary black holes to
the analytical predictions made by the effective one body (EOB) method (which
has been recently extended to the case of spinning bodies). On the scale of the
differences between the results obtained by different numerical methods, we
find good agreement between numerical data and analytical predictions for
several invariant functions describing the dynamical properties of circular
orbits. This agreement is robust against the post-Newtonian accuracy used for
the analytical estimates, as well as under choices of resummation method for
the EOB ``effective potential'', and gets better as one uses a higher
post-Newtonian accuracy. These findings open the way to a significant
``merging'' of analytical and numerical methods, i.e. to matching an EOB-based
analytical description of the (early and late) inspiral, up to the beginning of
the plunge, to a numerical description of the plunge and merger. We illustrate
also the ``flexibility'' of the EOB approach, i.e. the possibility of
determining some ``best fit'' values for the analytical parameters by
comparison with numerical data.Comment: Minor revisions, accepted for publication in Phys. Rev. D, 19 pages,
6 figure
The Hamiltonian formulation of General Relativity: myths and reality
A conventional wisdom often perpetuated in the literature states that: (i) a
3+1 decomposition of space-time into space and time is synonymous with the
canonical treatment and this decomposition is essential for any Hamiltonian
formulation of General Relativity (GR); (ii) the canonical treatment
unavoidably breaks the symmetry between space and time in GR and the resulting
algebra of constraints is not the algebra of four-dimensional diffeomorphism;
(iii) according to some authors this algebra allows one to derive only spatial
diffeomorphism or, according to others, a specific field-dependent and
non-covariant four-dimensional diffeomorphism; (iv) the analyses of Dirac
[Proc. Roy. Soc. A 246 (1958) 333] and of ADM [Arnowitt, Deser and Misner, in
"Gravitation: An Introduction to Current Research" (1962) 227] of the canonical
structure of GR are equivalent. We provide some general reasons why these
statements should be questioned. Points (i-iii) have been shown to be incorrect
in [Kiriushcheva et al., Phys. Lett. A 372 (2008) 5101] and now we thoroughly
re-examine all steps of the Dirac Hamiltonian formulation of GR. We show that
points (i-iii) above cannot be attributed to the Dirac Hamiltonian formulation
of GR. We also demonstrate that ADM and Dirac formulations are related by a
transformation of phase-space variables from the metric to lapse
and shift functions and the three-metric , which is not canonical. This
proves that point (iv) is incorrect. Points (i-iii) are mere consequences of
using a non-canonical change of variables and are not an intrinsic property of
either the Hamilton-Dirac approach to constrained systems or Einstein's theory
itself.Comment: References are added and updated, Introduction is extended,
Subsection 3.5 is added, 83 pages; corresponds to the published versio
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
Cosmological parameters from SDSS and WMAP
We measure cosmological parameters using the three-dimensional power spectrum
P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in
combination with WMAP and other data. Our results are consistent with a
``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt,
tensor modes or massive neutrinos. Adding SDSS information more than halves the
WMAP-only error bars on some parameters, tightening 1 sigma constraints on the
Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter
density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on
neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when
dropping prior assumptions about curvature, neutrinos, tensor modes and the
equation of state. Our results are in substantial agreement with the joint
analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive
consistency check with independent redshift survey data and analysis
techniques. In this paper, we place particular emphasis on clarifying the
physical origin of the constraints, i.e., what we do and do not know when using
different data sets and prior assumptions. For instance, dropping the
assumption that space is perfectly flat, the WMAP-only constraint on the
measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to
t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running
tilt, neutrino mass and equation of state in the list of free parameters, many
constraints are still quite weak, but future cosmological measurements from
SDSS and other sources should allow these to be substantially tightened.Comment: Minor revisions to match accepted PRD version. SDSS data and ppt
figures available at http://www.hep.upenn.edu/~max/sdsspars.htm
Assessing Big-Bang Nucleosynthesis
Systematic uncertainties in the light-element abundances and their evolution
make a rigorous statistical assessment difficult. However, using Bayesian
methods we show that the following statement is robust: the predicted and
measured abundances are consistent with 95\% credibility only if the
baryon-to-photon ratio is between and
and the number of light neutrino species is less than 3.9. Our analysis
suggests that the He abundance may have been systematically underestimated.Comment: 7 pages, LaTeX(2.09), 6 postscript figures (attached). A postscript
version with figures can be found at
ftp://astro.uchicago.edu/pub/astro/copi/assessing_BBN . (See the README file
for details
Dark Matter and Fundamental Physics with the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is a project for a next-generation
observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy,
currently in its design phase, and foreseen to be operative a few years from
now. Several tens of telescopes of 2-3 different sizes, distributed over a
large area, will allow for a sensitivity about a factor 10 better than current
instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few
tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the
following study, we investigate the prospects for CTA to study several science
questions that influence our current knowledge of fundamental physics. Based on
conservative assumptions for the performance of the different CTA telescope
configurations, we employ a Monte Carlo based approach to evaluate the
prospects for detection. First, we discuss CTA prospects for cold dark matter
searches, following different observational strategies: in dwarf satellite
galaxies of the Milky Way, in the region close to the Galactic Centre, and in
clusters of galaxies. The possible search for spatial signatures, facilitated
by the larger field of view of CTA, is also discussed. Next we consider
searches for axion-like particles which, besides being possible candidates for
dark matter may also explain the unexpectedly low absorption by extragalactic
background light of gamma rays from very distant blazars. Simulated
light-curves of flaring sources are also used to determine the sensitivity to
violations of Lorentz Invariance by detection of the possible delay between the
arrival times of photons at different energies. Finally, we mention searches
for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
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