3,690 research outputs found
Topological analysis of scalar fields with outliers
Given a real-valued function defined over a manifold embedded in
, we are interested in recovering structural information about
from the sole information of its values on a finite sample . Existing
methods provide approximation to the persistence diagram of when geometric
noise and functional noise are bounded. However, they fail in the presence of
aberrant values, also called outliers, both in theory and practice.
We propose a new algorithm that deals with outliers. We handle aberrant
functional values with a method inspired from the k-nearest neighbors
regression and the local median filtering, while the geometric outliers are
handled using the distance to a measure. Combined with topological results on
nested filtrations, our algorithm performs robust topological analysis of
scalar fields in a wider range of noise models than handled by current methods.
We provide theoretical guarantees and experimental results on the quality of
our approximation of the sampled scalar field
Search for domain wall dark matter with atomic clocks on board global positioning system satellites
Cosmological observations indicate that 85% of all matter in the Universe is
dark matter (DM), yet its microscopic composition remains a mystery. One
hypothesis is that DM arises from ultralight quantum fields that form
macroscopic objects such as topological defects. Here we use GPS as a ~ 50,000
km aperture DM detector to search for such defects in the form of domain walls.
GPS navigation relies on precision timing signals furnished by atomic clocks
hosted on board GPS satellites. As the Earth moves through the galactic DM
halo, interactions with topological defects could cause atomic clock glitches
that propagate through the GPS satellite constellation at galactic velocities ~
300 km/s. Mining 16 years of archival GPS data, we find no evidence for DM in
the form of domain walls at our current sensitivity level. This allows us to
improve the limits on certain quadratic scalar couplings of domain wall DM to
standard model particles by several orders of magnitude.Comment: 7 pages (main text), and 12 pages for Supplementary Information. v3:
Update titl
Urban identity through quantifiable spatial attributes: coherence and dispersion of local identity through the automated comparative analysis of building block plans
This analysis investigates whether and to what degree quantifiable spatial attrib-utes, as expressed in plan representations, can capture elements related to the ex-perience of spatial identity. By combining different methods of shape and spatial analysis it attempts to quantify spatial attributes, predominantly derived from plans, in order to illustrate patterns of interrelations between spaces through an ob-jective automated process. The study focuses on the scale of the urban block as the basic modular unit for the formation of urban configurations and the issue of spa-tial identity is perceived through consistency and differentiation within and amongst urban neighbourhoods
Is there Correlation between Fine Structure and Dark Energy Cosmic Dipoles?
We present a detailed analysis (including redshift tomography) of the cosmic
dipoles in the Keck+VLT quasar absorber and in the Union2 SnIa samples. We show
that the fine structure constant cosmic dipole obtained through the Keck+VLT
quasar absorber sample at level is anomalously aligned with the
corresponding dark energy dipole obtained through the Union2 sample at
level. The angular separation between the two dipole directions is
. We use Monte Carlo simulations to find the
probability of obtaining the observed dipole magnitudes with the observed
alignment, in the context of an isotropic cosmological model with no
correlation between dark energy and fine structure constant . We find
that this probability is less than one part in . We propose a simple
physical model (extended topological quintessence) which naturally predicts a
spherical inhomogeneous distribution for both dark energy density and fine
structure constant values. The model is based on the existence of a recently
formed giant global monopole with Hubble scale core which also couples
non-minimally to electromagnetism. Aligned dipole anisotropies would naturally
emerge for an off-centre observer for both the fine structure constant and for
dark energy density. This model smoothly reduces to \lcdm for proper limits of
its parameters. Two predictions of this model are (a) a correlation between the
existence of strong cosmic electromagnetic fields and the value of and
(b) the existence of a dark flow on Hubble scales due to the repulsive gravity
of the global defect core (`Great Repulser') aligned with the dark energy and
dipoles. The direction of the dark flow is predicted to be towards the
spatial region of lower accelerating expansion. Existing data about the dark
flow are consistent with this prediction.Comment: 14 pages 11 figures (two column revtex). Aceepted in Phys. Rev. D (to
appear). Significant extensions (mostly on section 4 on the theoretical
model), added references, corrected typos. The data, updated mathematica and
C program files used for the numerical analysis may be downloaded from
http://leandros.physics.uoi.gr/defsdipole
Spectral comparison of large urban graphs
The spectrum of an axial graph is proposed as a means for comparison between spaces,
particularly for measuring between very large and complex graphs. A number of methods have
been used in recent years for comparative analysis within large sets of urban areas, both to
investigate properties of specific known types of street network or to propose a taxonomy of urban
morphology based on an analytical technique. In many cases, a single or small range of predefined,
scalar measures such as metric distance, integration, control or clustering coefficient have
been used to compare the graphs. While these measures are well understood theoretically, their
low dimensionality determines the range of observations that can ultimately be drawn from the data.
Spectral analysis consists of a high dimensional vector representing each space, between which
metric distance may be measured to indicate the overall difference between two spaces, or
subspaces may be extracted to correspond to certain features. It is used for comparison of entire
urban graphs, to determine similarities (and differences) in their overall structure.
Results are shown of a comparison of 152 cities distributed around the world. The clustering of
cities of similar properties in a high dimensional space is discussed. Principal and nonlinear
components of the data set indicate significant correlations in the graph similarities between cities
and their proximity to one another, suggesting that cultural features based on location are evident in
the city form and that these can be quantified by the proposed method. Results of classification
tests show that a city’s location can be estimated based purely on its form.
The high dimensionality of the spectra is beneficial for its utility in data-mining applications that can
draw correlations with other data sets such as land use information. It is shown how further
processing by supervised learning allows the extraction of relevant features. A methodological
comparison is also drawn with statistical studies that use a strong correlation between human
genetic markers and geographical location of populations to derive detailed reconstructions of
prehistoric migration. Thus, it is suggested that the method may be utilised for mapping the transfer
of cultural memes by measuring comparison between cities
Instanton dominance over at low momenta from lattice QCD simulations at , and
We report on an instanton-based analysis of the gluon Green functions in the
Landau gauge for low momenta; in particular we use lattice results for
in the symmetric momentum subtraction scheme () for
large-volume lattice simulations. We have exploited quenched gauge field
configurations, , with both Wilson and tree-level Symanzik improved
actions, and unquenched ones with and dynamical flavors
(domain wall and twisted-mass fermions, respectively).
We show that the dominance of instanton correlations on the low-momenta gluon
Green functions can be applied to the determination of phenomenological
parameters of the instanton liquid and, eventually, to a determination of the
lattice spacing.
We furthermore apply the Gradient Flow to remove short-distance fluctuations.
The Gradient Flow gets rid of the QCD scale, , and reveals
that the instanton prediction extents to large momenta. For those gauge field
configurations free of quantum fluctuations, the direct study of topological
charge density shows the appearance of large-scale lumps that can be identified
as instantons, giving access to a direct study of the instanton density and
size distribution that is compatible with those extracted from the analysis of
the Green functions.Comment: Proceedings of the 35th International Symposium on Lattice Field
Theory, Granada, Spai
Search for transient ultralight dark matter signatures with networks of precision measurement devices using a Bayesian statistics method
We analyze the prospects of employing a distributed global network of
precision measurement devices as a dark matter and exotic physics observatory.
In particular, we consider the atomic clocks of the Global Positioning System
(GPS), consisting of a constellation of 32 medium-Earth orbit satellites
equipped with either Cs or Rb microwave clocks and a number of Earth-based
receiver stations, some of which employ highly-stable H-maser atomic clocks.
High-accuracy timing data is available for almost two decades. By analyzing the
satellite and terrestrial atomic clock data, it is possible to search for
transient signatures of exotic physics, such as "clumpy" dark matter and dark
energy, effectively transforming the GPS constellation into a 50,000km aperture
sensor array. Here we characterize the noise of the GPS satellite atomic
clocks, describe the search method based on Bayesian statistics, and test the
method using simulated clock data. We present the projected discovery reach
using our method, and demonstrate that it can surpass the existing constrains
by several order of magnitude for certain models. Our method is not limited in
scope to GPS or atomic clock networks, and can also be applied to other
networks of precision measurement devices.Comment: See also Supplementary Information located in ancillary file
Intrinsic geometry of collider events and nearest neighbour based weighted filtration
Collider observations have mainly been studied on an event-by-event basis,
leveraging several kinematic techniques. However, the intrinsic topological
imprints of the ensemble of new physics events can be strikingly different from
the SM background ensemble. Traditional topological data analysis (TDA) is
known for its stability against small perturbations. However, a plethora of
rich information encoded in the clustering of ensembles is often lost due to
the unweighted filtration of simplicial complexes. Taking a singlet extended
model as an example, this work illustrates the rich global properties
associated with the so-called distance-to-measure (DTM) filtration on Alpha
complexes using weights determined from k-nearest neighbours.Comment: 1 table, 7 figure
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