9,217 research outputs found
Dynamical variables in Gauge-Translational Gravity
Assuming that the natural gauge group of gravity is given by the group of
isometries of a given space, for a maximally symmetric space we derive a model
in which gravity is essentially a gauge theory of translations. Starting from
first principles we verify that a nonlinear realization of the symmetry
provides the general structure of this gauge theory, leading to a simple choice
of dynamical variables of the gravity field corresponding, at first order, to a
diagonal matrix, whereas the non-diagonal elements contribute only to higher
orders.Comment: 15 page
Universal Conductance Distributions in the Crossover between Diffusive and Localization Regimes
The full distribution of the conductance in quasi-one-dimensional
wires with rough surfaces is analyzed from the diffusive to the localization
regime. In the crossover region, where the statistics is dominated by only one
or two eigenchannels, the numerically obtained P(G) is found to be independent
of the details of the system with the average conductance as the only
scaling parameter. For < e^2/h, P(G) is given by an essentially
``one-sided'' log-normal distribution. In contrast, for e^2/h <= 2e^2/h,
the shape of P(G) remarkable agrees with those predicted by random matrix
theory for two fluctuating transmission eigenchannels.Comment: Accepted for publication in Phys. Rev. Let
The role of translational invariance in non linear gauge theories of gravity
The internal structure of the tetrads in a Poincar\'e non linear gauge theory
of gravity is considered. Minkowskian coordinates becomes dynamical degrees of
freedom playing the role of Goldstone bosons of the translations. A critical
length allowing a covariant expansion similar to the weak field approach is
deduced, the zeroth order metric being maximally symmetric (Minkowskian in some
cases).Comment: 17 pages, LaTe
Perturbation expansion for 2-D Hubbard model
We develop an efficient method to calculate the third-order corrections to
the self-energy of the hole-doped two-dimensional Hubbard model in space-time
representation. Using the Dyson equation we evaluate the renormalized spectral
function in various parts of the Brillouin zone and find significant
modifications with respect to the second-order theory even for rather small
values of the coupling constant U. The spectral function becomes unphysical for
, where W is the half-width of the conduction band. Close to the
Fermi surface and for U<W, the single-particle spectral weight is reduced in a
finite energy interval around the Fermi energy. The increase of U opens a gap
between the occupied and unoccupied parts of the spectral function.Comment: 17 pages, 11 Postscript figures, Phys. Rev. B, accepte
Stationary axisymmetric exteriors for perturbations of isolated bodies in general relativity, to second order
Perturbed stationary axisymmetric isolated bodies, e.g. stars, represented by
a matter-filled interior and an asymptotically flat vacuum exterior joined at a
surface where the Darmois matching conditions are satisfied, are considered.
The initial state is assumed to be static. The perturbations of the matching
conditions are derived and used as boundary conditions for the perturbed Ernst
equations in the exterior region. The perturbations are calculated to second
order. The boundary conditions are overdetermined: necessary and sufficient
conditions for their compatibility are derived. The special case of
perturbations of spherical bodies is given in detail.Comment: RevTeX; 32 pp. Accepted by Phys. Rev. D. Added references and extra
comments in introductio
Generalized scattering-matrix approach for magneto-optics in periodically patterned multilayer systems
We present here a generalization of the scattering-matrix approach for the
description of the propagation of electromagnetic waves in nanostructured
magneto-optical systems. Our formalism allows us to describe all the key
magneto-optical effects in any configuration in periodically patterned
multilayer structures. The method can also be applied to describe periodic
multilayer systems comprising materials with any type of optical anisotropy. We
illustrate the method with the analysis of a recent experiment in which the
transverse magneto-optical Kerr effect was measured in a Fe film with a
periodic array of subwavelength circular holes. We show, in agreement with the
experiments, that the excitation of surface plasmon polaritons in this system
leads to a resonant enhancement of the transverse magneto-optical Kerr effect.Comment: 12 pages, 6 figures, submitted to Physical Review
Picking on the family: disrupting android malware triage by forcing misclassification
Machine learning classification algorithms are widely applied to different malware analysis problems because of their proven abilities to learn from examples and perform relatively well with little human input. Use cases include the labelling of malicious samples according to families during triage of suspected malware. However, automated algorithms are vulnerable to attacks. An attacker could carefully manipulate the sample to force the algorithm to produce a particular output. In this paper we discuss one such attack on Android malware classifiers. We design and implement a prototype tool, called IagoDroid, that takes as input a malware sample and a target family, and modifies the sample to cause it to be classified as belonging to this family while preserving its original semantics. Our technique relies on a search process that generates variants of the original sample without modifying their semantics. We tested IagoDroid against RevealDroid, a recent, open source, Android malware classifier based on a variety of static features. IagoDroid successfully forces misclassification for 28 of the 29 representative malware families present in the DREBIN dataset. Remarkably, it does so by modifying just a single feature of the original malware. On average, it finds the first evasive sample in the first search iteration, and converges to a 100% evasive population within 4 iterations. Finally, we introduce RevealDroid*, a more robust classifier that implements several techniques proposed in other adversarial learning domains. Our experiments suggest that RevealDroid* can correctly detect up to 99% of the variants generated by IagoDroid
Discovery of a massive supercluster system at
Superclusters are the largest relatively isolated systems in the cosmic web.
Using the SDSS BOSS survey we search for the largest superclusters in the
redshift range .
We generate a luminosity-density field smoothed over
to detect the large-scale over-density regions. Each individual over-density
region is defined as single supercluster in the survey. We define the
superclusters in the way that they are comparable with the superclusters found
in the SDSS main survey.
We found a system we call the BOSS Great Wall (BGW), which consists of two
walls with diameters 186 and 173 Mpc, and two other major superclusters
with diameters of 64 and 91 Mpc. As a whole, this system consists of
830 galaxies with the mean redshift 0.47. We estimate the total mass to be
approximately . The morphology of the
superclusters in the BGW system is similar to the morphology of the
superclusters in the Sloan Great Wall region.
The BGW is one of the most extended and massive system of superclusters yet
found in the Universe.Comment: 4 pages, accepted as a letter in A&
Enhanced Inflation in the Dirac-Born-Infeld framework
We consider the Einstein equations within the DBI scenario for a spatially
flat Friedmann-Robertson-Walker (FRW) spacetime without a cosmological
constant. We derive the inflationary scenario by applying the symmetry
transformations which preserve the form of the Friedmann and conservation
equations. These form invariance transformations generate a symmetry group
parametrized by the Lorentz factor \ga. We explicitly obtain an inflationary
scenario by the cooperative effect of adding energy density into the Friedmann
equation. For the case of a constant Lorentz factor, and under the slow roll
assumption, we find the transformation rules for the scalar and tensor power
spectra of perturbations as well as their ratio under the action of the form
invariance symmetry group. Within this case and due to its relevance for the
inflationary paradigm, we find the general solution of the dynamical equations
for a DBI field driven by an exponential potential and show a broad set of
inflationary solutions. The general solution can be split into three subsets
and all these behave asymptotically as a power law solution at early and at
late times.Comment: 9 pages, revtex 4.
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