262 research outputs found
Distances in Inhomogeneous Cosmological Models
Distances play important roles in cosmological observations, especially in gravitational lens systems, but there is a problem in determining distances because they are defined in terms of light propagation, which is influenced gravitationally by the inhomogeneities in the universe. In this paper we first give the basic optical relations and the definitions of different distances in inhomogeneous universes. Next we show how the observational relations depend quantitatively on the distances. Finally, we give results for the frequency distribution of different distances and the shear effect on distances obtained using various methods of numerical simulation
Extensions of operator algebras I
We transcribe a portion of the theory of extensions of C*-algebras to general
operator algebras. We also include several new general facts about
approximately unital ideals in operator algebras and the C*-algebras which they
generate
Optimizing future imaging survey of galaxies to confront dark energy and modified gravity models
We consider the extent to which future imaging surveys of galaxies can
distinguish between dark energy and modified gravity models for the origin of
the cosmic acceleration. Dynamical dark energy models may have similar
expansion rates as models of modified gravity, yet predict different growth of
structure histories. We parameterize the cosmic expansion by the two
parameters, and , and the linear growth rate of density fluctuations
by Linder's , independently. Dark energy models generically predict
, while the DGP model . To determine
if future imaging surveys can constrain within 20 percent (or
), we perform the Fisher matrix analysis for a weak lensing
survey such as the on-going Hyper Suprime-Cam (HSC) project. Under the
condition that the total observation time is fixed, we compute the Figure of
Merit (FoM) as a function of the exposure time \texp. We find that the
tomography technique effectively improves the FoM, which has a broad peak
around \texp\simeq {\rm several}\sim 10 minutes; a shallow and wide survey is
preferred to constrain the parameter. While
cannot be achieved by the HSC weak-lensing survey alone, one can improve the
constraints by combining with a follow-up spectroscopic survey like WFMOS
and/or future CMB observations.Comment: 18 pages, typos correcte
Testing the reliability of weak lensing cluster detections
We study the reliability of dark-matter halo detections with three different
linear filters applied to weak-lensing data. We use ray-tracing in the multiple
lens-plane approximation through a large cosmological simulation to construct
realizations of cosmic lensing by large-scale structures between redshifts zero
and two. We apply the filters mentioned above to detect peaks in the
weak-lensing signal and compare them with the true population of dark matter
halos present in the simulation. We confirm the stability and performance of a
filter optimized for suppressing the contamination by large-scale structure. It
allows the reliable detection of dark-matter halos with masses above a few
times 1e13 M_sun/h with a fraction of spurious detections below ~10%. For
sources at redshift two, 50% of the halos more massive than ~7e13 M_sun/h are
detected, and completeness is reached at ~2e14 M_sun/h.Comment: 14 pages, 13 figures, accepted on A&
Dark energy constraints and correlations with systematics from CFHTLS weak lensing, SNLS supernovae Ia and WMAP5
We combine measurements of weak gravitational lensing from the CFHTLS-Wide
survey, supernovae Ia from CFHT SNLS and CMB anisotropies from WMAP5 to obtain
joint constraints on cosmological parameters, in particular, the dark energy
equation of state parameter w. We assess the influence of systematics in the
data on the results and look for possible correlations with cosmological
parameters.
We implement an MCMC algorithm to sample the parameter space of a flat CDM
model with a dark-energy component of constant w. Systematics in the data are
parametrised and included in the analysis. We determine the influence of
photometric calibration of SNIa data on cosmological results by calculating the
response of the distance modulus to photometric zero-point variations. The weak
lensing data set is tested for anomalous field-to-field variations and a
systematic shape measurement bias for high-z galaxies.
Ignoring photometric uncertainties for SNLS biases cosmological parameters by
at most 20% of the statistical errors, using supernovae only; the parameter
uncertainties are underestimated by 10%. The weak lensing field-to-field
variance pointings is 5%-15% higher than that predicted from N-body
simulations. We find no bias of the lensing signal at high redshift, within the
framework of a simple model. Assuming a systematic underestimation of the
lensing signal at high redshift, the normalisation sigma_8 increases by up to
8%. Combining all three probes we obtain -0.10<1+w<0.06 at 68% confidence
(-0.18<1+w<0.12 at 95%), including systematic errors. Systematics in the data
increase the error bars by up to 35%; the best-fit values change by less than
0.15sigma. [Abridged]Comment: 14 pages, 10 figures. Revised version, matches the one to be
published in A&A. Modifications have been made corresponding to the referee's
suggestions, including reordering of some section
Nominal Logic Programming
Nominal logic is an extension of first-order logic which provides a simple
foundation for formalizing and reasoning about abstract syntax modulo
consistent renaming of bound names (that is, alpha-equivalence). This article
investigates logic programming based on nominal logic. We describe some typical
nominal logic programs, and develop the model-theoretic, proof-theoretic, and
operational semantics of such programs. Besides being of interest for ensuring
the correct behavior of implementations, these results provide a rigorous
foundation for techniques for analysis and reasoning about nominal logic
programs, as we illustrate via examples.Comment: 46 pages; 19 page appendix; 13 figures. Revised journal submission as
of July 23, 200
Information Loss in Coarse Graining of Polymer Configurations via Contact Matrices
Contact matrices provide a coarse grained description of the configuration
omega of a linear chain (polymer or random walk) on Z^n: C_{ij}(omega)=1 when
the distance between the position of the i-th and j-th step are less than or
equal to some distance "a" and C_{ij}(omega)=0 otherwise. We consider models in
which polymers of length N have weights corresponding to simple and
self-avoiding random walks, SRW and SAW, with "a" the minimal permissible
distance. We prove that to leading order in N, the number of matrices equals
the number of walks for SRW, but not for SAW. The coarse grained Shannon
entropies for SRW agree with the fine grained ones for n <= 2, but differs for
n >= 3.Comment: 18 pages, 2 figures, latex2e Main change: the introduction is
rewritten in a less formal way with the main results explained in simple
term
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