246,283 research outputs found
A Covariant Information-Density Cutoff in Curved Space-Time
In information theory, the link between continuous information and discrete
information is established through well-known sampling theorems. Sampling
theory explains, for example, how frequency-filtered music signals are
reconstructible perfectly from discrete samples. In this Letter, sampling
theory is generalized to pseudo-Riemannian manifolds. This provides a new set
of mathematical tools for the study of space-time at the Planck scale: theories
formulated on a differentiable space-time manifold can be completely equivalent
to lattice theories. There is a close connection to generalized uncertainty
relations which have appeared in string theory and other studies of quantum
gravity.Comment: 4 pages, RevTe
Bayesian cosmic density field inference from redshift space dark matter maps
We present a self-consistent Bayesian formalism to sample the primordial
density fields compatible with a set of dark matter density tracers after
cosmic evolution observed in redshift space. Previous works on density
reconstruction did not self-consistently consider redshift space distortions or
included an additional iterative distortion correction step. We present here
the analytic solution of coherent flows within a Hamiltonian Monte Carlo
posterior sampling of the primordial density field. We test our method within
the Zel'dovich approximation, presenting also an analytic solution including
tidal fields and spherical collapse on small scales using augmented Lagrangian
perturbation theory. Our resulting reconstructed fields are isotropic and their
power spectra are unbiased compared to the true one defined by our mock
observations. Novel algorithmic implementations are introduced regarding the
mass assignment kernels when defining the dark matter density field and
optimization of the time step in the Hamiltonian equations of motions. Our
algorithm, dubbed barcode, promises to be specially suited for analysis of the
dark matter cosmic web down to scales of a few Megaparsecs. This large scale
structure is implied by the observed spatial distribution of galaxy clusters
--- such as obtained from X-ray, SZ or weak lensing surveys --- as well as that
of the intergalactic medium sampled by the Lyman alpha forest or perhaps even
by deep hydrogen intensity mapping. In these cases, virialized motions are
negligible, and the tracers cannot be modeled as point-like objects. It could
be used in all of these contexts as a baryon acoustic oscillation
reconstruction algorithm.Comment: 34 pages, 25 figures, 1 table. Submitted to MNRAS. Accompanying code
at https://github.com/egpbos/barcod
Wavelet-based density estimation for noise reduction in plasma simulations using particles
For given computational resources, the accuracy of plasma simulations using
particles is mainly held back by the noise due to limited statistical sampling
in the reconstruction of the particle distribution function. A method based on
wavelet analysis is proposed and tested to reduce this noise. The method, known
as wavelet based density estimation (WBDE), was previously introduced in the
statistical literature to estimate probability densities given a finite number
of independent measurements. Its novel application to plasma simulations can be
viewed as a natural extension of the finite size particles (FSP) approach, with
the advantage of estimating more accurately distribution functions that have
localized sharp features. The proposed method preserves the moments of the
particle distribution function to a good level of accuracy, has no constraints
on the dimensionality of the system, does not require an a priori selection of
a global smoothing scale, and its able to adapt locally to the smoothness of
the density based on the given discrete particle data. Most importantly, the
computational cost of the denoising stage is of the same order as one time step
of a FSP simulation. The method is compared with a recently proposed proper
orthogonal decomposition based method, and it is tested with three particle
data sets that involve different levels of collisionality and interaction with
external and self-consistent fields
Elastic properties of a tungsten-silver composite by reconstruction and computation
We statistically reconstruct a three-dimensional model of a tungsten-silver
composite from an experimental two-dimensional image. The effective Young's
modulus () of the model is computed in the temperature range 25-1060^o C
using a finite element method. The results are in good agreement with
experimental data. As a test case, we have reconstructed the microstructure and
computed the moduli of the overlapping sphere model. The reconstructed and
overlapping sphere models are examples of bi-continuous (non-particulate)
media. The computed moduli of the models are not generally in good agreement
with the predictions of the self-consistent method. We have also evaluated
three-point variational bounds on the Young's moduli of the models using the
results of Beran, Molyneux, Milton and Phan-Thien. The measured data were close
to the upper bound if the properties of the two phases were similar ().Comment: 23 Pages, 12 Figure
Bias deconstructed: Unravelling the scale dependence of halo bias using real space measurements
We explore the scale dependence of halo bias using real space
cross-correlation measurements in N-body simulations and in Pinocchio, an
algorithm based on Lagrangian Perturbation Theory. Recent work has shown how to
interpret such real space measurements in terms of k-dependent bias in Fourier
space, and how to remove the k-dependence to reconstruct the k-independent
peak-background split halo bias parameters. We compare our reconstruction of
the linear bias, which requires no free parameters, with previous estimates
from N-body simulations which were obtained directly in Fourier space at large
scales, and find very good agreement. Our reconstruction of the quadratic bias
is similarly parameter-free, although in this case there are no previous
Fourier space measurements to compare with. Our analysis of N-body simulations
explicitly tests the predictions of the excursion set peaks (ESP) formalism of
Paranjape et al. (2013) for the scale dependence of bias; we find that the ESP
predictions accurately describe our measurements. In addition, our measurements
in Pinocchio serve as a useful, successful consistency check between Pinocchio
and N-body simulations that is not accessible to traditional measurements.Comment: 13 pages, 9 figures; v3 -- Matches published versio
Electronic charge reconstruction of doped Mott insulators in multilayered nanostructures
Dynamical mean-field theory is employed to calculate the electronic charge
reconstruction of multilayered inhomogeneous devices composed of semi-infinite
metallic lead layers sandwiching barrier planes of a strongly correlated
material (that can be tuned through the metal-insulator Mott transition). The
main focus is on barriers that are doped Mott insulators, and how the
electronic charge reconstruction can create well-defined Mott insulating
regions in a device whose thickness is governed by intrinsic materials
properties, and hence may be able to be reproducibly made.Comment: 9 pages, 8 figure
- …