4,520 research outputs found
Power spectra from an inflaton coupled to the Gauss-Bonnet term
We consider power-law inflation with a Gauss-Bonnet correction inspired by
string theory. We analyze the stability of cosmological perturbations and
obtain the allowed parameter space. We find that for GB-dominated inflation
ultra-violet instabilities of either scalar or tensor perturbations show up on
small scales. The Gauss-Bonnet correction with a positive (or negative)
coupling may lead to a reduction (or enhancement) of the tensor-to-scalar ratio
in the potential-dominated case. We place tight constraints on the model
parameters by making use of the WMAP 5-year data.Comment: 5 pages, 4 figures, RevTeX, references added, published versio
On the Nature and Location of the Microlenses
This paper uses the caustic crossing events in the microlens data sets to
explore the nature and location of the lenses. We conclude that the large
majority of lenses, whether they are luminous or dark, are likely to be
binaries. Further, we demonstrate that blending is an important feature of all
the data sets. An additional interpretation suggested by the data, that the
caustic crossing events along the directions to the Magellanic Clouds are due
to lenses located in the Clouds, implies that most of the LMC/SMC events to
date are due to lenses in the Magellanic Clouds. All of these conclusions can
be tested. If they are correct, a large fraction of lenses along the direction
to the LMC may be ordinary stellar binary systems, just as are the majority of
the lenses along the direction to the Bulge. Thus, a better understanding of
the larger-than-anticipated value derived for the Bulge optical depth may allow
us to better interpret the large value derived for the optical depth to the
LMC. Indeed, binarity and blending in the data sets may illuminate connections
among several other puzzles: the dearth of binary-source light curves, the
dearth of non-caustic-crossing perturbed binary-lens events, and the dearth of
obviously blended point-lens events.Comment: 15 pages, 2 figures. Submitted to the Astrophysical Journal Letters,
4 January 199
Characterizations of Morse quasi-geodesics via superlinear divergence and sublinear contraction
We introduce and begin a systematic study of sublinearly contracting
projections. We give two characterizations of Morse quasi-geodesics in an
arbitrary geodesic metric space. One is that they are sublinearly contracting;
the other is that they have completely superlinear divergence. We give a
further characterization of sublinearly contracting projections in terms of
projections of geodesic segments.Comment: 24 pages, 5 figures. v2: 22 pages, 5 figures. Correction in proof of
Thm 7.1. Proof of Prop 4.2 revised for improved clarity. Other minor changes
per referee comments. To appear in Documenta Mathematic
Negative curvature in graphical small cancellation groups
We use the interplay between combinatorial and coarse geometric versions of
negative curvature to investigate the geometry of infinitely presented
graphical small cancellation groups. In particular, we characterize
their 'contracting geodesics', which should be thought of as the geodesics that
behave hyperbolically.
We show that every degree of contraction can be achieved by a geodesic in a
finitely generated group. We construct the first example of a finitely
generated group containing an element that is strongly contracting with
respect to one finite generating set of and not strongly contracting with
respect to another. In the case of classical small cancellation
groups we give complete characterizations of geodesics that are Morse and that
are strongly contracting.
We show that many graphical small cancellation groups contain
strongly contracting elements and, in particular, are growth tight. We
construct uncountably many quasi-isometry classes of finitely generated,
torsion-free groups in which every maximal cyclic subgroup is hyperbolically
embedded. These are the first examples of this kind that are not subgroups of
hyperbolic groups.
In the course of our analysis we show that if the defining graph of a
graphical small cancellation group has finite components, then the
elements of the group have translation lengths that are rational and bounded
away from zero.Comment: 40 pages, 14 figures, v2: improved introduction, updated statement of
Theorem 4.4, v3: new title (previously: "Contracting geodesics in infinitely
presented graphical small cancellation groups"), minor changes, to appear in
Groups, Geometry, and Dynamic
Lattice simulation of a center symmetric three-dimensional effective theory for SU(2) Yang-Mills
We perform simulations of an effective theory of SU(2) Wilson lines in three
dimensions. Our action includes a kinetic term, the one-loop perturbative
potential for the Wilson line, a non-perturbative "fuzzy-bag" contribution and
spatial gauge fields. We determine the phase diagram of the theory and confirm
that, at moderately weak coupling, the non-perturbative term leads to
eigenvalue repulsion in a finite region above the deconfining phase transition.Comment: To appear in the proceedings of "Strong and Electroweak Matter",
Amsterdam, the Netherlands, August 26-29, Nucl. Phys. A, in prin
Terrestrial Planet Formation in Extra-Solar Planetary Systems
Terrestrial planets form in a series of dynamical steps from the solid
component of circumstellar disks. First, km-sized planetesimals form likely via
a combination of sticky collisions, turbulent concentration of solids, and
gravitational collapse from micron-sized dust grains in the thin disk midplane.
Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also
called "planetary embryos". Finally, full-sized terrestrial planets accrete
from protoplanets and planetesimals. This final stage of accretion lasts about
10-100 Myr and is strongly affected by gravitational perturbations from any gas
giant planets, which are constrained to form more quickly, during the 1-10 Myr
lifetime of the gaseous component of the disk. It is during this final stage
that the bulk compositions and volatile (e.g., water) contents of terrestrial
planets are set, depending on their feeding zones and the amount of radial
mixing that occurs. The main factors that influence terrestrial planet
formation are the mass and surface density profile of the disk, and the
perturbations from giant planets and binary companions if they exist. Simple
accretion models predicts that low-mass stars should form small, dry planets in
their habitable zones. The migration of a giant planet through a disk of rocky
bodies does not completely impede terrestrial planet growth. Rather, "hot
Jupiter" systems are likely to also contain exterior, very water-rich
Earth-like planets, and also "hot Earths", very close-in rocky planets. Roughly
one third of the known systems of extra-solar (giant) planets could allow a
terrestrial planet to form in the habitable zone.Comment: 19 pages, 5 figures. To appear in the proceedings of IAU Symposium
249: Exoplanets: Detection, Formation and Dynamics, held in Suzhou, China,
Oct 22-26 200
Tan(beta)-enhanced supersymmetric corrections to the anomalous magnetic moment of the muon
We report on a two-loop supersymmetric contribution to the magnetic moment
(g-2)_mu of the muon which is enhanced by two powers of tan(beta). This
contribution arises from a shift in the relation between the muon mass and
Yukawa coupling and can increase the supersymmetric contribution to (g-2)_mu
sizably. As a result, if the currently observed 3 sigma deviation between the
experimental and SM theory value of (g-2)_mu is analyzed within the Minimal
Supersymmetric Standard Model (MSSM), the derived constraints on the parameter
space are modified significantly: If (g-2)_mu is used to determine tan(beta) as
a function of the other MSSM parameters, our corrections decrease tan(beta) by
roughly 10% for tan(beta)=50.Comment: 4 pages, 1 figur
Dynamics and evaporation of defects in Mott-insulating clusters of boson pairs
Repulsively bound pairs of particles in a lattice governed by the
Bose-Hubbard model can form stable incompressible clusters of dimers
corresponding to finite-size n=2 Mott insulators. Here we study the dynamics of
hole defects in such clusters corresponding to unpaired particles which can
resonantly tunnel out of the cluster into the lattice vacuum. Due to bosonic
statistics, the unpaired particles have different effective mass inside and
outside the cluster, and "evaporation" of hole defects from the cluster
boundaries is possible only when their quasi-momenta are within a certain
transmission range. We show that quasi-thermalization of hole defects occurs in
the presence of catalyzing particle defects which thereby purify the Mott
insulating clusters. We study the dynamics of one-dimensional system using
analytical techniques and numerically exact t-DMRG simulations. We derive an
effective strong-interaction model that enables simulations of the system
dynamics for much longer times. We also discuss a more general case of two
bosonic species which reduces to the fermionic Hubbard model in the strong
interaction limit.Comment: 12 pages, 10 figures, minor update
Analysis of an Ultra-precision Positioning System and Parametrization of Its Structural Model for Error Compensation
Conventional compensation of position errors of machine tools relies only on measured values. Due to this principle it is not always possible to compensate the errors in time, especially dynamic ones. Moreover, the relevant control variables cannot always be measured directly. Thus, this approach proves to be insufficient for high precision applications. In this context, a model-based error prediction allows for minimal position errors. However, ultra-precision applications set high demands for the models' accuracy. This paper presents the design of an accurate and real time-capable structural model of an ultra-precision positioning system. The modeling method for the developed ultra-precision demonstrator is shown and the initial parameter identification is presented. © 2017 The Authors. Published by Elsevier B.V.DFG/FOR/184
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