7,297 research outputs found
The Correlation Function of Rich Clusters of Galaxies in CDM-like Models
We use ensembles of high-resolution CDM simulations to investigate the shape
and amplitude of the two point correlation function of rich clusters. The
standard scale-invariant CDM model with provides a poor description
of the clustering measured from the APM rich cluster redshift survey, which is
better fitted by models with more power at large scales. The amplitudes of the
rich cluster correlation functions measured from our models depend weakly on
cluster richness. Analytic calculations of the clustering of peaks in a
Gaussian density field overestimate the amplitude of the N-body cluster
correlation functions, but reproduce qualitatively the weak trend with cluster
richness. Our results suggest that the high amplitude measured for the
correlation function of richness class Abell clusters is either an
artefact arising from incompleteness in the Abell catalogue, or an indication
that the density perturbations in the early universe were very non-Gaussian.Comment: uuencoded compressed postscript ,MNRAS, in press, OUAST-93-1
Reconstruction of cosmological density and velocity fields in the Lagrangian Zel'dovich Approximation
We present a method for reconstructing cosmological densityn for and velocity
fields using the Lagrangian Zel'dovich formalism. . The method involves finding
the least action solution for straight line particle paths in an evolving
density field. Our starting point is the final, evolved density , so that we
are in effect carrying out the standard Zel'dovich Approximation based process
in reverse. Using a simple numerical algorithm we are able to minimise the
action for the trajectories of several million particles. We apply our method
to the evolved density taken from N-body simulations of different cold dark
matter dominated universes, testing both the prediction for the present day
velocity field and for the initial density field. The method is easy to apply,
reproduces the accuracy of the forward Zel'dovich Approximation, and also works
directly in redshift space with minimal modification.Comment: 13 pages with only 2 (out 9) figures. MNRAS in press. New Appendix
shows the relation between shell crossing and PIZA. A completed version with
all 9 figures available by anonymous ftp at
ftp://bessel.mps.ohio-state.edu/pub/racc/piza.ps.gz (USA) or
ftp://ftp-astro.physics.ox.ac.uk/pub/eg/piza3.ps.gz (UK
Weak lensing of the Lyman-alpha forest
The angular positions of quasars are deflected by the gravitational lensing
effect of foreground matter. The Lyman-alpha forest seen in the spectra of
these quasars is therefore also lensed. We propose that the signature of weak
gravitational lensing of the forest could be measured using similar techniques
that have been applied to the lensed Cosmic Microwave Background, and which
have also been proposed for application to spectral data from 21cm radio
telescopes. As with 21cm data, the forest has the advantage of spectral
information, potentially yielding many lensed "slices" at different redshifts.
We perform an illustrative idealized test, generating a high resolution angular
grid of quasars (of order arcminute separation), and lensing the
Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field.
We find that standard quadratic estimators can be used to reconstruct images of
the foreground mass distribution at z~1. There currently exists a wealth of Lya
forest data from quasar and galaxy spectral surveys, with smaller sightline
separations expected in the future. Lyman-alpha forest lensing is sensitive to
the foreground mass distribution at redshifts intermediate between CMB lensing
and galaxy shear, and avoids the difficulties of shape measurement associated
with the latter. With further refinement and application of mass reconstruction
techniques, weak gravitational lensing of the high redshift Lya forest may
become a useful new cosmological probe.Comment: 9 pages, 7 figures, submitted to MNRA
Personal propulsion unit Patent
Lightweight propulsion unit for movement of personnel and equipment across lunar surfac
Noise Estimates for Measurements of Weak Lensing from the Lyman-alpha Forest
We have proposed a method for measuring weak lensing using the Lyman-alpha
forest. Here we estimate the noise expected in weak lensing maps and power
spectra for different sets of observational parameters. We find that surveys of
the size and quality of the ones being done today and ones planned for the
future will be able to measure the lensing power spectrum at a source redshift
of z~2.5 with high precision and even be able to image the distribution of
foreground matter with high fidelity on degree scales. For example, we predict
that Lyman-alpha forest lensing measurement from the Dark Energy Spectroscopic
Instrument survey should yield the mass fluctuation amplitude with statistical
errors of 1.5%. By dividing the redshift range into multiple bins some
tomographic lensing information should be accessible as well. This would allow
for cosmological lensing measurements at higher redshift than are accessible
with galaxy shear surveys and correspondingly better constraints on the
evolution of dark energy at relatively early times.Comment: 8 pages, 8 figures, submitted to MNRA
Magnesium oxide doping reduces acoustic wave attenuation in lithium metatantalate and lithium metaniobate crystals
Single crystals of lithium metatantalate and lithium metaniobate, grown from melts having different stoichiometries and different amounts of magnesium oxide, show that doping lowers temperature-independent portion of attenuation of acoustic waves. Doped crystals possess optical properties well suited for electro-optical and photoelastic applications
Impacts of an active travel intervention with a cycling focus in a suburban context: One-year findings from an evaluation of Londonâs in-progress mini-Hollands programme
Background
More evidence is needed on the impacts of building infrastructure for walking and cycling. A knowledge gap and an implementation gap have been mutually reinforcing. This paper reports on a longitudinal study examining the impacts of the still in progress âmini-Hollands programmeâ, which seeks to transform local environments for walking and cycling, in three Outer London boroughs. Compared to Inner London, Outer London has low levels of cycling and low levels of walking, and is relatively car dependent.
Methods
We conducted a longitudinal study of 1712 individuals sampled from households in mini-Holland boroughs (intervention sample) and from non mini-Holland Outer London boroughs (control sample). The intervention sample was further divided, a priori, into those living in âhigh-dose neighbourhoodsâ, where substantial changes to the local walking and cycling infrastructure had been implemented, versus âlow-dose neighbourhoodsâ where such improvements had not (yet) been made. At both baseline (2016) and one-year follow-up (2017), we administered an online survey of travel behaviour and attitudes to transport and the local environment.
Results
One yearâs worth of interventions was associated with an increase in active travel among those living in areas defined as âhigh-doseâ neighbourhoods. Specifically, those in high-dose areas were 24% more likely to have done any past-week cycling at follow-up, compared to those living in non mini-Holland areas (95% CI, 2% to 52%). The mid-point estimate for increase in active travel (walking plus cycling) time for the same group was an additional 41.0âŻmin (95% CI 7.0, 75.0âŻmin). Positive changes in views about local environments were recorded in intervention areas, driven by a perceived improvement in cycling-related items. Controversy related to the interventions is expressed in a growth in perceptions that âtoo muchâ money is spent on cycling in intervention areas. However, intervention areas also saw a reduction in perceptions that âtoo littleâ money is spent (the latter view being common both at baseline and Wave 1 in control areas).
Conclusion
Overall, the findings here suggest that programme interventions, while controversial, are having a measurable and early impact on active travel behaviour and perceptions of the local cycling environment
The Peculiar Velocity Function of Galaxy Clusters
The peculiar velocity function of clusters of galaxies is determined using an
accurate sample of cluster velocities based on Tully-Fisher distances of Sc
galaxies (Giovanelli et al 1995b). In contrast with previous results based on
samples with considerably larger velocity uncertainties, the observed velocity
function does not exhibit a tail of high velocity clusters. The results
indicate a low probability of \,5\% of finding clusters with
one-dimensional velocities greater than 600 {\kms}. The root-mean-square
one-dimensional cluster velocity is 29328 {\kms}. The observed cluster
velocity function is compared with expectations from different cosmological
models. The absence of a high velocity tail in the observed function is most
consistent with a low mass-density (0.3) CDM model, and is
inconsistent at level with = 1.0 CDM and HDM models.
The root-mean-square one-dimensional cluster velocities in these models
correspond, respectively, to 314, 516, and 632 {\kms} (when convolved with the
observational uncertainties). Comparison with the observed RMS cluster velocity
of 29328 {\kms} further supports the low-density CDM model.Comment: revised version accepted for publication in ApJ Letters, 18 pages,
uuencoded PostScript with 3 figures included; complete paper available
through WWW at http://www.astro.princeton.edu/~library/prep.htm
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