8,767 research outputs found
The Parallelometer: a mechanical device to study curvature
A simple mechanical device is introduced, the parallelometer, that can be
used to measure curvatures of surfaces. The device can be used as a practical
illustration of parallel transport of a vector and to study Berry phase shift
when it is carried along a loop on the surface. Its connection to the Foucault
pendulum is discussed. The experimental results can be successfully compared
with the theoretical expectations. The experiment is inexpensive and
conceptually easy to perform and understand for a beginner
Scaling Evolution of Universal Dark-Matter Halo Density Profiles
Dark-matter halos show a universal density profile with a scaling such that
less massive systems are typically denser. This mass-density relation is well
described by a proportionality between the characteristic density of halos and
the mean cosmic density at halo formation time. It has recently been shown that
this proportionality could be the result of the following simple evolutionary
picture. Halos form in major mergers with essentially the same,
cosmogony-dependent, dimensionless profile, and then grow inside-outside, as a
consequence of accretion. Here we verify the consistency of this picture and
show that it predicts the correct zero point of the mass-density relation.Comment: 9 pages, 1 Table and 1 postscript figure, latex uses aaspp4.sty,
accepted for publication in The Astrophysical Journal Letter
The electron-phonon coupling strength at metal surfaces directly determined from the Helium atom scattering Debye-Waller factor
A new quantum-theoretical derivation of the elastic and inelastic scattering
probability of He atoms from a metal surface, where the energy and momentum
exchange with the phonon gas can only occur through the mediation of the
surface free-electron density, shows that the Debye-Waller exponent is directly
proportional to the electron-phonon mass coupling constant . The
comparison between the values of extracted from existing data on the
Debye-Waller factor for various metal surfaces and the values known
from literature indicates a substantial agreement, which opens the possibility
of directly extracting the electron-phonon coupling strength in quasi-2D
conducting systems from the temperature or incident energy dependence of the
elastic Helium atom scattering intensities.Comment: 14 pages, 2 figures, 1 tabl
Gaussian covariance matrices for anisotropic galaxy clustering measurements
Measurements of the redshift-space galaxy clustering have been a prolific
source of cosmological information in recent years. Accurate covariance
estimates are an essential step for the validation of galaxy clustering models
of the redshift-space two-point statistics. Usually, only a limited set of
accurate N-body simulations is available. Thus, assessing the data covariance
is not possible or only leads to a noisy estimate. Further, relying on
simulated realisations of the survey data means that tests of the cosmology
dependence of the covariance are expensive. With these points in mind, this
work presents a simple theoretical model for the linear covariance of
anisotropic galaxy clustering observations with synthetic catalogues.
Considering the Legendre moments (`multipoles') of the two-point statistics and
projections into wide bins of the line-of-sight parameter (`clustering
wedges'), we describe the modelling of the covariance for these anisotropic
clustering measurements for galaxy samples with a trivial geometry in the case
of a Gaussian approximation of the clustering likelihood. As main result of
this paper, we give the explicit formulae for Fourier and configuration space
covariance matrices. To validate our model, we create synthetic HOD galaxy
catalogues by populating the haloes of an ensemble of large-volume N-body
simulations. Using linear and non-linear input power spectra, we find very good
agreement between the model predictions and the measurements on the synthetic
catalogues in the quasi-linear regime.Comment: 17 pages, 16 figures, 3 tables; modified to match version accepted by
MNRA
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