466 research outputs found
Cosmology with Weak Lensing Surveys
Weak gravitational lensing surveys measure the distortion of the image of
distant sources due to the deflections of light rays by the fluctuations of the
gravitational potential along the line of sight. Since they probe the
non-linear matter power spectrum itself at medium redshift such surveys are
complimentary to both galaxy surveys (which follow stellar light) and cosmic
microwave background observations (which probe the linear regime at high
redshift). Ongoing CMB experiments such as WMAP and the future Planck satellite
mission will measure the standard cosmological parameters with unprecedented
accuracy. The focus of attention will then shift to understanding the nature of
dark matter and vacuum energy: several recent studies suggest that lensing is
the best method for constraining the dark energy equation of state. During the
next 5 year period ongoing and future weak lensing surveys such as the Joint
Dark Energy Mission (JDEM, e.g. SNAP) or the Large-aperture Synoptic Survey
Telescope (LSST) will play a major role in advancing our understanding of the
universe in this direction. In this review article we describe various aspects
of weak lensing surveys and how they can help us in understanding our universe.Comment: 15 pages, review article to appear in 2005 Triennial Issue of Phil.
Trans.
Short-Term Impacts of 2017 Western North American Wildfires On Meteorology, the Atmosphere\u27s Energy Budget, and Premature Mortality
Western North American fires have been increasing in magnitude and severity over the last few decades. The complex coupling of fires with the atmospheric energy budget and meteorology creates short-term feedbacks on regional weather altering the amount of pollution to which Americans are exposed. Using a combination of model simulations and observations, this study shows that the severe fires in the summer of 2017 increased atmospheric aerosol concentrations leading to a cooling of the air at the surface, reductions in sensible heat fluxes, and a lowering of the planetary boundary layer height over land. This combination of lower-boundary layer height and increased aerosol pollution from the fires reduces air quality. We estimate that from start of August to end of October 2017, ~400 premature deaths occurred within the western US as a result of short-term exposure to elevated PM2.5 from fire smoke. As North America confronts a warming climate with more fires the short-term climate and pollution impacts of increased fire activity should be assessed within policy aimed to minimize impacts of climate change on society
Constraining the expansion history of the universe from the red shift evolution of cosmic shear
We present a quantitative analysis of the constraints on the total equation
of state parameter that can be obtained from measuring the red shift evolution
of the cosmic shear. We compare the constraints that can be obtained from
measurements of the spin two angular multipole moments of the cosmic shear to
those resulting from the two dimensional and three dimensional power spectra of
the cosmic shear. We find that if the multipole moments of the cosmic shear are
measured accurately enough for a few red shifts the constraints on the dark
energy equation of state parameter improve significantly compared to those that
can be obtained from other measurements.Comment: 17 pages, 4 figure
Distribution function approach to redshift space distortions
We develop a phase space distribution function approach to redshift space
distortions (RSD), in which the redshift space density can be written as a sum
over velocity moments of the distribution function. These moments are density
weighted and their lowest orders are density, momentum density, and stress
energy density. The series expansion is convergent on large scales. We perform
an expansion of these velocity moments into helicity modes, which are
eigenmodes under rotation around the axis of Fourier mode direction,
generalizing the scalar, vector, tensor decomposition of perturbations to an
arbitrary order. We show that only equal helicity moments correlate and derive
the angular dependence of the individual contributions to the redshift space
power spectrum in terms of angle mu between wave vector and line of sight. We
show that the dominant term of mu^2 dependence on large scales is the
cross-correlation between the density and scalar part of momentum density,
which can be related to the time derivative of the matter power spectrum.
Additional terms contributing and dominating on small scales are the vector
part of momentum density-momentum density correlations, the energy
density-density correlations, and the scalar part of anisotropic stress
density-density correlations. Similarly, we identify 7 terms contributing to
mu^4 dependence. Some of the advantages of the distribution function approach
are that the series expansion converges on large scales and remains valid in
multi-stream situations. We finish with a brief discussion of implications for
RSD in galaxies relative to dark matter, highlighting the issue of scale
dependent bias of velocity moments correlators.Comment: 12 page
Distribution function approach to redshift space distortions. Part IV: perturbation theory applied to dark matter
We develop a perturbative approach to redshift space distortions (RSD) using
the phase space distribution function approach and apply it to the dark matter
redshift space power spectrum and its moments. RSD can be written as a sum over
density weighted velocity moments correlators, with the lowest order being
density, momentum density and stress energy density. We use standard and
extended perturbation theory (PT) to determine their auto and cross
correlators, comparing them to N-body simulations. We show which of the terms
can be modeled well with the standard PT and which need additional terms that
include higher order corrections which cannot be modeled in PT. Most of these
additional terms are related to the small scale velocity dispersion effects,
the so called finger of god (FoG) effects, which affect some, but not all, of
the terms in this expansion, and which can be approximately modeled using a
simple physically motivated ansatz such as the halo model. We point out that
there are several velocity dispersions that enter into the detailed RSD
analysis with very different amplitudes, which can be approximately predicted
by the halo model. In contrast to previous models our approach systematically
includes all of the terms at a given order in PT and provides a physical
interpretation for the small scale dispersion values. We investigate RSD power
spectrum as a function of \mu, the cosine of the angle between the Fourier mode
and line of sight, focusing on the lowest order powers of \mu and multipole
moments which dominate the observable RSD power spectrum. Overall we find
considerable success in modeling many, but not all, of the terms in this
expansion.Comment: 37 pages, 13 figures, published in JCA
Unpacking the ‘Emergent Farmer’ Concept in Agrarian Reform:Evidence from Livestock Farmers in South Africa
South Africa has historically perpetuated a dual system of freehold commercial and communal subsistence farming. To bridge these extremes, agrarian reform policies have encouraged the creation of a class of ‘emergent’, commercially oriented farmers. However, these policies consider ‘emergent’ farmers as a homogeneous group of land reform beneficiaries, with limited appreciation of the class differences between them, and do little to support the rise of a ‘middle’ group of producers able to bridge that gap. This article uses a case study of livestock farmers in Eastern Cape Province to critique the ‘emergent farmer’ concept. The authors identify three broad categories of farmers within the emergent livestock sector: a large group who, despite having accessed private farms, remain effectively subsistence farmers; a smaller group of small/medium-scale commercial producers who have communal farming origins and most closely approximate to ‘emergent’ farmers; and an elite group of large-scale, fully commercialized farmers, whose emergence has been facilitated primarily by access to capital and a desire to invest in alternative business ventures. On this basis the authors suggest that current agrarian reform policies need considerable refocusing if they are to effectively facilitate the emergence of a ‘middle’ group of smallholder commercial farmers from communal systems
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
A step towards testing general relativity using weak gravitational lensing and redshift surveys
Using the linear theory of perturbations in General Relativity, we express a
set of consistency relations that can be observationally tested with current
and future large scale structure surveys. We then outline a stringent
model-independent program to test gravity on cosmological scales. We illustrate
the feasibility of such a program by jointly using several observables like
peculiar velocities, galaxy clustering and weak gravitational lensing. After
addressing possible observational or astrophysical caveats like galaxy bias and
redshift uncertainties, we forecast in particular how well one can predict the
lensing signal from a cosmic shear survey using an over-lapping galaxy survey.
We finally discuss the specific physics probed this way and illustrate how
gravity models would fail such a test.Comment: 12 pages, 10 figure
Diffuse light and galaxy interactions in the core of nearby clusters
The kinematics of the diffuse light in the densest regions of the nearby
clusters can be unmasked using the planetary nebulae (PNs) as probes of the
stellar motions. The position-velocity diagrams around the brightest cluster
galaxies (BCGs) identify the relative contributions from the outer halos and
the intracluster light (ICL), defined as the light radiated by the stars
floating in the cluster potential. The kinematics of the ICL can then be used
to asses the dynamical status of the nearby cluster cores and to infer their
formation histories. The cores of the Virgo and Coma are observed to be far
from equilibrium, with mergers currently on-going, while the ICL properties in
the Fornax and Hydra clusters show the presence of sub-components being
accreted in their cores, but superposed to an otherwise relaxed population of
stars. Finally the comparison of the observed ICL properties with those
predicted from Lambda-CDM simulations indicates a qualitative agreement and
provides insights on the ICL formation. Both observations and simulations
indicate that BCG halos and ICL are physically distinct components, with the
``hotter" ICL dominating at large radial distances from the BCGs halos as the
latter become progressively fainter.Comment: 14 pages, 5 figures. Invited review to appear in the proceedings of
"Galaxies and their masks" eds. Block, D.L., Freeman, K.C. and Puerari, I.,
2010, Springer (New York
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
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