50,651 research outputs found
Minkowski-type and Alexandrov-type theorems for polyhedral herissons
Classical H.Minkowski theorems on existence and uniqueness of convex
polyhedra with prescribed directions and areas of faces as well as the
well-known generalization of H.Minkowski uniqueness theorem due to
A.D.Alexandrov are extended to a class of nonconvex polyhedra which are called
polyhedral herissons and may be described as polyhedra with injective spherical
image.Comment: 19 pages, 8 figures, LaTeX 2.0
Fundamental measure theory for mixtures of parallel hard cubes. II. Phase behavior of the one-component fluid and of the binary mixture
A previously developed fundamental measure fucntional [J. Chem. Phys.
vol.107, 6379 (1997)] is used to study the phase behavior of a system of
parallel hard cubes. The single-component fluid exhibits a continuous
transition to a solid with an anomalously large density of vacancies. The
binary mixture has a demixing transition for edge-length ratios below 0.1.
Freezing in this mixture reveals that at least the phase rich in large cubes
lies in the region where the uniform fluid is unstable, hence suggesting a
fluid-solid phase separation. A method is develop to study very asymmetric
binary mixtures by taking the limit of zero size ratio (scaling the density and
fugacity of the solvent as appropriate) in the semi-grand ensemble where the
chemical potential of the solvent is fixed. With this procedure the mixture is
exactly mapped onto a one-component fluid of parallel adhesive hard cubes. At
any density and solvent fugacity the large cubes are shown to collapse into a
close-packed solid. Nevertheless the phase diagram contains a large
metastability region with fluid and solid phases. Upon introduction of a slight
polydispersity in the large cubes the system shows the typical phase diagram of
a fluid with an isostructural solid-solid transition (with the exception of a
continuous freezing). Consequences about the phase behavior of binary mixtures
of hard core particles are then drawn.Comment: 14 pages, 6 eps figures, uses revtex, amstex, epsfig, and multicol
style file
Steerable wavelet analysis of CMB structures alignment
This paper reviews the application of a novel methodology for analysing the
isotropy of the universe by probing the alignment of local structures in the
CMB. The strength of the proposed methodology relies on the steerable wavelet
filtering of the CMB signal. One the one hand, the filter steerability renders
the computation of the local orientation of the CMB features affordable in
terms of computation time. On the other hand, the scale-space nature of the
wavelet filtering allows to explore the alignment of the local structures at
different scales, probing possible different phenomena. We present the WMAP
first-year data analysis recently performed by the same authors (Wiaux et al.),
where an extremely significant anisotropy was found. In particular, a preferred
plane was detected, having a normal direction with a northern end position
close to the northern end of the CMB dipole axis. In addition, a most preferred
direction was found in that plane, with a northern end direction very close to
the north ecliptic pole. This result synthesised for the first time previously
reported anomalies identified in the direction of the dipole and the ecliptic
poles axes. In a forthcoming paper (Vielva et al.), we have extended our
analysis to the study of individual frequency maps finding first indications
for discarding foregrounds as the origin of the anomaly. We have also tested
that the preferred orientations are defined by structures homogeneously
distributed in the sky, rather than from localised regions. We have also
analysed the WMAP 3-year data, finding the same anomaly pattern, although at a
slightly lower significance level.Comment: 14 pages, 8 figures. Proceedings of the Fundamental Physics With CMB
workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review
Alignment and signed-intensity anomalies in WMAP data
Significant alignment and signed-intensity anomalies of local features of the
cosmic microwave background (CMB) are detected on the three-year WMAP data,
through a decomposition of the signal with steerable wavelets on the sphere.
Firstly, an alignment analysis identifies two mean preferred planes in the sky,
both with normal axes close to the CMB dipole axis. The first plane is defined
by the directions toward which local CMB features are anomalously aligned. A
mean preferred axis is also identified in this plane, located very close to the
ecliptic poles axis. The second plane is defined by the directions anomalously
avoided by local CMB features. This alignment anomaly provides further insight
on recent results (Wiaux et al. 2006). Secondly, a signed-intensity analysis
identifies three mean preferred directions in the southern galactic hemisphere
with anomalously high or low temperature of local CMB features: a cold spot
essentially identified with a known cold spot (Vielva et al. 2004), a second
cold spot lying very close to the southern end of the CMB dipole axis, and a
hot spot lying close to the southern end of the ecliptic poles axis. In both
analyses, the anomalies are observed at wavelet scales corresponding to angular
sizes around 10 degress on the celestial sphere, with global significance
levels around 1%. Further investigation reveals that the alignment and
signed-intensity anomalies are only very partially related. Instrumental noise,
foreground emissions, as well as some form of other systematics, are strongly
rejected as possible origins of the detections. An explanation might still be
envisaged in terms of a global violation of the isotropy of the Universe,
inducing an intrinsic statistical anisotropy of the CMB.Comment: 12 pages, 7 figures. Accepted for publication in MNRAS. Small changes
made (including the new subsection 3.4) to match the final versio
Increase of the Energy Necessary to Probe Ultraviolet Theories Due to the Presence of a Strong Magnetic Field
We use the gauge gravity correspondence to study the renormalization group
flow of a double trace fermionic operator in a quark-gluon plasma subject to
the influence of a strong magnetic field and compare it with the results for
the case at zero temperature and no magnetic field, where the flow between two
fixed points is observed. Our results show that the energy necessary to access
the physics of the ultraviolet theory increases with the intensity of the
magnetic field under which the processes happen. We provide arguments to
support that this increase is scheme independent, and to exhibit further
evidence we do a very simple calculation showing that the dimensional reduction
expected in the gauge theory in this scenario is effective up to an energy
scale that grows with the strength of such a background field. We also show
that independently of the renormalization scheme, the coupling of the double
trace operators in the ultraviolet fixed point increases with the intensity of
the background field. These effects combined can change both, the processes
that are expected to be involved in a collision experiment at a given energy
and the azimuthal anisotropy of the measurements resulting of them.Comment: 23 pages, 10 figures. Added section about renormalization scheme
independenc
Experimental velocity fields and forces for a cylinder penetrating into a granular medium
We present here a detailed granular flow characterization together with force
measurements for the quasi-bidimensional situation of a horizontal cylinder
penetrating vertically at a constant velocity in dry granular matter between
two parallel glass walls. In the velocity range studied here, the drag force on
the cylinder does not depend on the velocity V_0 and is mainly proportional to
the cylinder diameter d. Whereas the force on the cylinder increases with its
penetration depth, the granular velocity profile around the cylinder is found
stationary with fluctuations around a mean value leading to the granular
temperature profile. Both mean velocity profile and temperature profile exhibit
strong localization near the cylinder. The mean flow perturbation induced by
the cylinder decreases exponentially away from the cylinder on a characteristic
length \lambda, that is mainly governed by the cylinder diameter for large
enough cylinder/grain size ratio d/d_g: \lambda ~ d/4 + 2d_g. The granular
temperature exhibits a constant plateau value T_0 in a thin layer close to the
cylinder of extension \delta_{T_0} ~ \lambda/2 and decays exponentially far
away with a characteristic length \lambda_T of a few grain diameters (\lambda_T
~ 3d_g). The granular temperature plateau T_0 that scales as (V_0^2 d_g/d) is
created by the flow itself from the balance between the "granular heat"
production by the shear rate V_0/\lambda over \delta_{T_0} close to the
cylinder and the granular dissipation far away
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