2,089 research outputs found
Halo Mass Function and the Free Streaming Scale
The nature of structure formation around the particle free streaming scale is
still far from understood. Many attempts to simulate hot, warm, and cold dark
matter cosmologies with a free streaming cutoff have been performed with
cosmological particle-based simulations, but they all suffer from spurious
structure formation at scales below their respective free streaming scales --
i.e. where the physics of halo formation is most affected by free streaming. We
perform a series of high resolution numerical simulations of different WDM
models, and develop an approximate method to subtract artificial structures in
the measured halo mass function. The corrected measurements are then used to
construct and calibrate an extended Press-Schechter (EPS) model with sharp-
window function and adequate mass assignment. The EPS model gives accurate
predictions for the low redshift halo mass function of CDM and WDM models, but
it significantly under-predicts the halo abundance at high redshifts. By taking
into account the ellipticity of the initial patches and connecting the
characteristic filter scale to the smallest ellipsoidal axis, we are able to
eliminate this inconsistency and obtain an accurate mass function over all
redshifts and all dark matter particle masses covered by the simulations. As an
additional application we use our model to predict the microhalo abundance of
the standard neutralino-CDM scenario and we give the first quantitative
prediction of the mass function over the full range of scales of CDM structure
formation.Comment: 16 pages, 10 figures, published in MNRA
The detection and treatment of distance errors in kinematic analyses of stars
We present a new method for detecting and correcting systematic errors in the
distances to stars when both proper motions and line-of-sight velocities are
available. The method, which is applicable for samples of 200 or more stars
that have a significant extension on the sky, exploits correlations between the
measured U, V and W velocity components that are introduced by distance errors.
We deliver a formalism to describe and interpret the specific imprints of
distance errors including spurious velocity correlations and shifts of mean
motion in a sample. We take into account correlations introduced by measurement
errors, Galactic rotation and changes in the orientation of the velocity
ellipsoid with position in the Galaxy. Tests on pseudodata show that the method
is more robust and sensitive than traditional approaches to this problem. We
investigate approaches to characterising the probability distribution of
distance errors, in addition to the mean distance error, which is the main
theme of the paper. Stars with the most overestimated distances bias our
estimate of the overall distance scale, leading to the corrected distances
being slightly too small. We give a formula that can be used to correct for
this effect. We apply the method to samples of stars from the SEGUE survey,
exploring optimal gravity cuts, sample contamination, and correcting the used
distance relations.Comment: published in MNRAS 14 pages, 8 figures, 2 tables, corrected eq.(35),
minor editin
High-Dimensional Geometric Streaming in Polynomial Space
Many existing algorithms for streaming geometric data analysis have been
plagued by exponential dependencies in the space complexity, which are
undesirable for processing high-dimensional data sets. In particular, once
, there are no known non-trivial streaming algorithms for problems
such as maintaining convex hulls and L\"owner-John ellipsoids of points,
despite a long line of work in streaming computational geometry since [AHV04].
We simultaneously improve these results to bits of
space by trading off with a factor distortion. We
achieve these results in a unified manner, by designing the first streaming
algorithm for maintaining a coreset for subspace embeddings with
space and distortion. Our
algorithm also gives similar guarantees in the \emph{online coreset} model.
Along the way, we sharpen results for online numerical linear algebra by
replacing a log condition number dependence with a dependence,
answering a question of [BDM+20]. Our techniques provide a novel connection
between leverage scores, a fundamental object in numerical linear algebra, and
computational geometry.
For subspace embeddings, we give nearly optimal trade-offs between
space and distortion for one-pass streaming algorithms. For instance, we give a
deterministic coreset using space and
distortion for , whereas previous deterministic algorithms incurred a
factor in the space or the distortion [CDW18].
Our techniques have implications in the offline setting, where we give
optimal trade-offs between the space complexity and distortion of subspace
sketch data structures. To do this, we give an elementary proof of a "change of
density" theorem of [LT80] and make it algorithmic.Comment: Abstract shortened to meet arXiv limits; v2 fix statements concerning
online condition numbe
On Deterministic Sketching and Streaming for Sparse Recovery and Norm Estimation
We study classic streaming and sparse recovery problems using deterministic
linear sketches, including l1/l1 and linf/l1 sparse recovery problems (the
latter also being known as l1-heavy hitters), norm estimation, and approximate
inner product. We focus on devising a fixed matrix A in R^{m x n} and a
deterministic recovery/estimation procedure which work for all possible input
vectors simultaneously. Our results improve upon existing work, the following
being our main contributions:
* A proof that linf/l1 sparse recovery and inner product estimation are
equivalent, and that incoherent matrices can be used to solve both problems.
Our upper bound for the number of measurements is m=O(eps^{-2}*min{log n, (log
n / log(1/eps))^2}). We can also obtain fast sketching and recovery algorithms
by making use of the Fast Johnson-Lindenstrauss transform. Both our running
times and number of measurements improve upon previous work. We can also obtain
better error guarantees than previous work in terms of a smaller tail of the
input vector.
* A new lower bound for the number of linear measurements required to solve
l1/l1 sparse recovery. We show Omega(k/eps^2 + klog(n/k)/eps) measurements are
required to recover an x' with |x - x'|_1 <= (1+eps)|x_{tail(k)}|_1, where
x_{tail(k)} is x projected onto all but its largest k coordinates in magnitude.
* A tight bound of m = Theta(eps^{-2}log(eps^2 n)) on the number of
measurements required to solve deterministic norm estimation, i.e., to recover
|x|_2 +/- eps|x|_1.
For all the problems we study, tight bounds are already known for the
randomized complexity from previous work, except in the case of l1/l1 sparse
recovery, where a nearly tight bound is known. Our work thus aims to study the
deterministic complexities of these problems
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