20,909 research outputs found
Wide Angle Redshift Distortions Revisited
We explore linear redshift distortions in wide angle surveys from the point
of view of symmetries. We show that the redshift space two-point correlation
function can be expanded into tripolar spherical harmonics of zero total
angular momentum . The
coefficients of the expansion are analogous to the 's of
the angular power spectrum, and express the anisotropy of the redshift space
correlation function. Moreover, only a handful of are
non-zero: the resulting formulae reveal a hidden simplicity comparable to
distant observer limit. The depend on spherical Bessel
moments of the power spectrum and . In the plane parallel
limit, the results of \cite{Kaiser1987} and \cite{Hamilton1993} are recovered.
The general formalism is used to derive useful new expressions. We present a
particularly simple trigonometric polynomial expansion, which is arguably the
most compact expression of wide angle redshift distortions. These formulae are
suitable to inversion due to the orthogonality of the basis functions. An
alternative Legendre polynomial expansion was obtained as well. This can be
shown to be equivalent to the results of \cite{SzalayEtal1998}. The simplicity
of the underlying theory will admit similar calculations for higher order
statistics as well.Comment: 6 pages, 1 figure, ApJL submitte
Serving children: the impact of poverty on children's experiences of services
This study arose from the identification of a gap in knowledge and corresponding need for the development of a better contemporary understanding of children's experiences of poverty. Focusing on children aged 10 - 14 years, the study aimed to provide a perspective on the lives of children and young people affected by poverty in Scotland through comparing the experiences of children living in poverty with those more economically advantaged
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Transport and ageing: Extending quality of life for older people via public and private transport
Accessible public transport and the independence that comes with car driving are generally thought to be linked to quality of life in old age. However, there has been almost no research on this topic in the UK. This study by researchers from the University of Paisley and the University of East London used a multi-method approach to explore the relationship between quality of life and access to public and private transport. The study also examined the extent to which the transport needs of older people are taken into account by transport professionals
Determination of the Baryon Density from Large Scale Galaxy Redshift Surveys
We estimate the degree to which the baryon density, , can be
determined from the galaxy power spectrum measured from large scale galaxy
redshift surveys, and in particular, the Sloan Digital Sky Survey. A high
baryon density will cause wiggles to appear in the power spectrum, which should
be observable at the current epoch. We assume linear theory on scales and do not include the effects of redshift distortions, evolution,
or biasing. With an optimum estimate of to ,
the uncertainties in are roughly 0.07 and 0.016 in flat
and open () cosmological models, respectively. This result
suggests that it should be possible to test for consistency with big bang
nucleosynthesis estimates of if we live in an open universe.Comment: 23 Pages, 10 Postscript figure
Karhunen-Loeve eigenvalue problems in cosmology: how should we tackle large data sets?
Since cosmology is no longer "the data-starved science", the problem of how
to best analyze large data sets has recently received considerable attention,
and Karhunen-Loeve eigenvalue methods have been applied to both galaxy redshift
surveys and Cosmic Microwave Background (CMB) maps. We present a comprehensive
discussion of methods for estimating cosmological parameters from large data
sets, which includes the previously published techniques as special cases. We
show that both the problem of estimating several parameters jointly and the
problem of not knowing the parameters a priori can be readily solved by adding
an extra singular value decomposition step.
It has recently been argued that the information content in a sky map from a
next generation CMB satellite is sufficient to measure key cosmological
parameters (h, Omega, Lambda, etc) to an accuracy of a few percent or better -
in principle. In practice, the data set is so large that both a brute force
likelihood analysis and a direct expansion in signal-to-noise eigenmodes will
be computationally unfeasible. We argue that it is likely that a Karhunen-Loeve
approach can nonetheless measure the parameters with close to maximal accuracy,
if preceded by an appropriate form of quadratic "pre-compression".
We also discuss practical issues regarding parameter estimation from present
and future galaxy redshift surveys, and illustrate this with a generalized
eigenmode analysis of the IRAS 1.2 Jy survey optimized for measuring
beta=Omega^{0.6}/b using redshift space distortions.Comment: 15 pages, with 5 figures included. Substantially expanded with worked
COBE examples for e.g. the multiparameter case. Available from
http://www.sns.ias.edu/~max/karhunen.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/karhunen.html (faster from Europe) or
from [email protected]
Beyond the plane-parallel and Newtonian approach: Wide-angle redshift distortions and convergence in general relativity
We extend previous analyses of wide-angle correlations in the galaxy power
spectrum in redshift space to include all general relativistic effects. These
general relativistic corrections to the standard approach become important on
large scales and at high redshifts, and they lead to new terms in the
wide-angle correlations. We show that in principle the new terms can produce
corrections of nearly 10 % on Gpc scales over the usual Newtonian
approximation. General relativistic corrections will be important for future
large-volume surveys such as SKA and Euclid, although the problem of cosmic
variance will present a challenge in observing this.Comment: 14 pages, 5 figures; Typo in equation 5 corrected; results unaffecte
Specialization of strategies and herding behavior of trading firms in a financial market
The understanding of complex social or economic systems is an important
scientific challenge. Here we present a comprehensive study of the Spanish
Stock Exchange showing that most financial firms trading in that market are
characterized by a resulting strategy and can be classified in groups of firms
with different specialization. Few large firms overally act as trending firms
whereas many heterogeneous firm act as reversing firms. The herding properties
of these two groups are markedly different and consistently observed over a
four-year period of trading.Comment: 8 pages, 5 figure
The ALTO project at IPN Orsay
In order to probe neutron rich radioactive noble gases produced by
photo-fission, a PARRNe1 experiment (Production d'Atomes Radioactifs Riches en
Neutrons) has been carried out at CERN. The incident electron beam of 50 MeV
was delivered by the LIL machine: LEP Injector Linac. The experiment allowed to
compare under the same conditions two production methods of radioactive noble
gases: fission induced by fast neutrons and photo-fission. The obtained results
show that the use of the electrons is a promising mode to get intense neutron
rich ion beams. Thereafter, the success of this photo-fission experiment, a
conceptual design for the installation at IPN Orsay of a 50 MeV electron
accelerator close to the PARRNe-2 device has been worked out: ALTO Project.
This work has started within a collaboration between IPNO, LAL and CERN groups.Comment: 14 pages, pdf file, International School-Seminar on Heavy-Ion Physics
7 (2002
Measuring the galaxy power spectrum with future redshift surveys
Precision measurements of the galaxy power spectrum P(k) require a data
analysis pipeline that is both fast enough to be computationally feasible and
accurate enough to take full advantage of high-quality data. We present a
rigorous discussion of different methods of power spectrum estimation, with
emphasis on the traditional Fourier method, the linear (Karhunen-Loeve; KL),
and quadratic data compression schemes, showing in what approximations they
give the same result. To improve speed, we show how many of the advantages of
KL data compression and power spectrum estimation may be achieved with a
computationally faster quadratic method. To improve accuracy, we derive
analytic expressions for handling the integral constraint, since it is crucial
that finite volume effects are accurately corrected for on scales comparable to
the depth of the survey. We also show that for the KL and quadratic techniques,
multiple constraints can be included via simple matrix operations, thereby
rendering the results less sensitive to galactic extinction and mis-estimates
of the radial selection function. We present a data analysis pipeline that we
argue does justice to the increases in both quality and quantity of data that
upcoming redshift surveys will provide. It uses three analysis techniques in
conjunction: a traditional Fourier approach on small scales, a pixelized
quadratic matrix method on large scales and a pixelized KL eigenmode analysis
to probe anisotropic effects such as redshift-space distortions.Comment: Major revisions for clarity. Matches accepted ApJ version. 23 pages,
with 2 figs included. Color figure and links at
http://www.sns.ias.edu/~max/galpower.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/galpower.html (faster from Europe) or
from [email protected]
Ground-plane screening of Coulomb interactions in two-dimensional systems: How effectively can one two-dimensional system screen interactions in another?
The use of a nearby metallic ground-plane to limit the range of the Coulomb
interactions between carriers is a useful approach in studying the physics of
two-dimensional (2D) systems. This approach has been used to study Wigner
crystallization of electrons on the surface of liquid helium, and most
recently, the insulating and metallic states of semiconductor-based
two-dimensional systems. In this paper, we perform calculations of the
screening effect of one 2D system on another and show that a 2D system is at
least as effective as a metal in screening Coulomb interactions. We also show
that the recent observation of the reduced effect of the ground-plane when the
2D system is in the metallic regime is due to intralayer screening.Comment: 14 pages, 7 figures Accepted in PR
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