10,422 research outputs found
StaRMAP - A second order staggered grid method for spherical harmonics moment equations of radiative transfer
We present a simple method to solve spherical harmonics moment systems, such
as the the time-dependent and equations, of radiative transfer.
The method, which works for arbitrary moment order , makes use of the
specific coupling between the moments in the equations. This coupling
naturally induces staggered grids in space and time, which in turn give rise to
a canonical, second-order accurate finite difference scheme. While the scheme
does not possess TVD or realizability limiters, its simplicity allows for a
very efficient implementation in Matlab. We present several test cases, some of
which demonstrate that the code solves problems with ten million degrees of
freedom in space, angle, and time within a few seconds. The code for the
numerical scheme, called StaRMAP (Staggered grid Radiation Moment
Approximation), along with files for all presented test cases, can be
downloaded so that all results can be reproduced by the reader.Comment: 28 pages, 7 figures; StaRMAP code available at
http://www.math.temple.edu/~seibold/research/starma
Towards reconstructing the quantum effective action of gravity
Starting from a parameterisation of the quantum effective action for gravity
we calculate correlation functions for observable quantities. The resulting
templates allow to reverse-engineer the couplings describing the effective
dynamics from the correlation functions. Applying this new formalism to the
autocorrelation function of spatial volume fluctuations measured within the
Causal Dynamical Triangulations program suggests that the corresponding quantum
effective action consists of the Einstein-Hilbert action supplemented by a
non-local interaction term. We expect that our matching-template formalism can
be adapted to a wide range of quantum gravity programs allowing to bridge the
gap between the fundamental formulation and observable low-energy physics.Comment: 6 pages, 1 figure; v2: reference update+clarification; v3: matches
published versio
Efficient Bayesian estimation of Markov model transition matrices with given stationary distribution
Direct simulation of biomolecular dynamics in thermal equilibrium is
challenging due to the metastable nature of conformation dynamics and the
computational cost of molecular dynamics. Biased or enhanced sampling methods
may improve the convergence of expectation values of equilibrium probabilities
and expectation values of stationary quantities significantly. Unfortunately
the convergence of dynamic observables such as correlation functions or
timescales of conformational transitions relies on direct equilibrium
simulations. Markov state models are well suited to describe both, stationary
properties and properties of slow dynamical processes of a molecular system, in
terms of a transition matrix for a jump process on a suitable discretiza- tion
of continuous conformation space. Here, we introduce statistical estimation
methods that allow a priori knowledge of equilibrium probabilities to be
incorporated into the estimation of dynamical observables. Both, maximum
likelihood methods and an improved Monte Carlo sampling method for reversible
transition ma- trices with fixed stationary distribution are given. The
sampling approach is applied to a toy example as well as to simulations of the
MR121-GSGS-W peptide, and is demonstrated to converge much more rapidly than a
previous approach in [F. Noe, J. Chem. Phys. 128, 244103 (2008)]Comment: 15 pages, 8 figure
On The Dimension of The Virtually Cyclic Classifying Space of a Crystallographic Group
In this paper we construct a model for the classifying space, BVCG, of a
crystallographic group G of rank n relative to the family VC of
virtually-cyclic subgroups of G. The model is used to show that there exists no
other model for the virtually-cyclic classifying space of G with dimension less
than vcd(G)+1, where vcd(G) denotes the virtual cohomological dimension of G.
In addition, the dimension of our construction realizes this limit.Comment: 10 page
Resolving Spacetime Singularities within Asymptotic Safety
A key incentive of quantum gravity is the removal of spacetime singularities
plaguing the classical theory. We compute the non-perturbative
momentum-dependence of a specific structure function within the gravitational
asymptotic safety program which encodes the quantum corrections to the graviton
propagator for momenta above the Planck scale. The resulting quantum corrected
Newtonian potential approaches a constant negative value as the distance
between the two point masses goes to zero, thereby removing the classical
singularity. The generic nature of the underlying mechanism suggests that it
will remain operative in the context of black hole and cosmic singularities.Comment: v2: some improvements and clarifications; version accepted for
publication in PR
Differentiating supersymmetric models with right sneutrino and neutralino dark matter
We perform a detailed analysis of dark matter signals of supersymmetric
models containing an extra gauge group. We investigate scenarios
in which either the right sneutrino or the lightest neutralino are
phenomenologically acceptable dark matter candidates and we explore the
parameter spaces of different supersymmetric realisations featuring an extra
. We impose consistency with low energy observables, with known
mass limits for the superpartners and bosons, as well as with Higgs
boson signal strengths, and we moreover verify that predictions for the
anomalous magnetic moment of the muon agree with the experimental value and
require that the dark matter candidate satisfies the observed relic density and
direct and indirect dark matter detection constraints. For the case where the
sneutrino is the dark matter candidate, we find distinguishing characteristics
among different mixing angles. If the neutralino is the lightest
supersymmetric particle, its mass is heavier than that of the light sneutrino
in scenarios where the latter is a dark matter candidate, the parameter space
is less restricted and differentiation between models is more difficult. We
finally comment on the possible collider tests of these models.Comment: 21 pages, 11 figures, version accepted by PR
Real-Time Recommendation of Streamed Data
This tutorial addressed two trending topics in the field of recommender systems research, namely A/B testing and real-time recommendations of streamed data. Focusing on the news domain, participants learned how to benchmark the performance of stream-based recommendation algorithms in a live recommender system and in a simulated environment
Holonomy Spin Foam Models: Definition and Coarse Graining
We propose a new holonomy formulation for spin foams, which naturally extends
the theory space of lattice gauge theories. This allows current spin foam
models to be defined on arbitrary two-complexes as well as to generalize
current spin foam models to arbitrary, in particular finite groups. The
similarity with standard lattice gauge theories allows to apply standard coarse
graining methods, which for finite groups can now be easily considered
numerically. We will summarize other holonomy and spin network formulations of
spin foams and group field theories and explain how the different
representations arise through variable transformations in the partition
function. A companion paper will provide a description of boundary Hilbert
spaces as well as a canonical dynamic encoded in transfer operators.Comment: 36 pages, 12 figure
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