2,388 research outputs found
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
Tests of Lorentz and CPT symmetry with hadrons and nuclei
We apply chiral-perturbation-theory techniques to the QCD sector of the Lorentz and CPT violating standard-model extension. We derive the effective Lagrangian in terms of pions and nucleons for a selected set of dimension-five operators involving quarks and gluons. This derivation is based on chiral-symmetry properties of the operators, as well as on their behaviour under C,P, and T transformations. We consider the power counting rules and apply the heavy-baryon approach to account for the large nucleon mass. Having obtained the relevant Lorentz-violating contributions to the pion-nucleon Lagrangian, we proceed to derive the particle and anti-particle Hamiltonian, from which we obtain the Lorentz-violating contribution to comagnetometer experiments. This allows us to place stringent limits on some of the parameters. For some other parameters we find that the best bounds will come from nucleon nucleon interactions, and we derive the relevant nucleon-nucleon potential. These considerations imply possible new opportunities for spin-precession experiments involving for example the deuteron.Portuguese Foundation for Science and Technology (FCT) [SFRH/BPD/101403/2014]; program POPH/FS
Orbital Cargo Transfer Simulation
Future space operations will require transfers of a large variety of cargo under both intravehicular and extravehicular conditions. In order to determine the techniques, human factor considerations, assistive devices, package limitations, training procedures, and so forth related to the cargo transfer problem, extensive ground-based simulation is required.
To date, several zero- and reduced-gravity simulation techniques have been developed and utilized. All of these techniques have both limitations and definite areas of application. Two of these techniques, water-immersion and zero-g aircraft, are considered usable for cargo transfer simulation. However, the results being obtained using the techniques differ substantially. The reasons for disagreement are to be found in the limitations of the techniques and how they are considered.
The purpose of this paper is to provide a comparison of zero-g aircraft .and water-immersion simulation, and to discuss various techniques which can be used to minimize the limitations associated with water immersion
Dirichlet boundary value problem for Chern-Simons modified gravity
Chern-Simons modified gravity comprises the Einstein-Hilbert action and a
higher-derivative interaction containing the Chern-Pontryagin density. We
derive the analog of the Gibbons-Hawking-York boundary term required to render
the Dirichlet boundary value problem well-defined. It turns out to be a
boundary Chern-Simons action for the extrinsic curvature. We address
applications to black hole thermodynamics.Comment: 4 pages, revtex4, v2: added Refs., made one statement stronger, added
footnote and added paragraph on single field inflatio
Three Questions on Lorentz Violation
We review the basics of the two most widely used approaches to Lorentz
violation - the Stardard Model Extension and Noncommutative Field Theory - and
discuss in some detail the example of the modified spectrum of the synchrotron
radiation. Motivated by touching upon such a fundamental issue as Lorentz
symmetry, we ask three questions: What is behind the search for Lorentz
violation? Is String Theory a physical theory? Is there an alternative to
Supersymmetry?Comment: 16 pages; invited luecture at DICE2006 - Piombino, Italy - September
200
Probabilistic Fréchet means for time varying persistence diagrams
© 2015, Institute of Mathematical Statistics. All rights reserved.In order to use persistence diagrams as a true statistical tool, it would be very useful to have a good notion of mean and variance for a set of diagrams. In [23], Mileyko and his collaborators made the first study of the properties of the Fréchet mean in (D<inf>p</inf>, W<inf>p</inf>), the space of persistence diagrams equipped with the p-th Wasserstein metric. In particular, they showed that the Fréchet mean of a finite set of diagrams always exists, but is not necessarily unique. The means of a continuously-varying set of diagrams do not themselves (necessarily) vary continuously, which presents obvious problems when trying to extend the Fréchet mean definition to the realm of time-varying persistence diagrams, better known as vineyards. We fix this problem by altering the original definition of Fréchet mean so that it now becomes a probability measure on the set of persistence diagrams; in a nutshell, the mean of a set of diagrams will be a weighted sum of atomic measures, where each atom is itself a persistence diagram determined using a perturbation of the input diagrams. This definition gives for each N a map (D<inf>p</inf>)<sup>N</sup>→ℙ(D<inf>p</inf>). We show that this map is Hölder continuous on finite diagrams and thus can be used to build a useful statistic on vineyards
Modified dispersion relations and the response of the rotating Unruh-DeWitt detector
We study the response of a rotating monopole detector that is coupled to a
massless scalar field which is described by a non-linear dispersion relation in
flat spacetime. Since it does not seem to be possible to evaluate the response
of the rotating detector analytically, we resort to numerical computations.
Interestingly, unlike the case of the uniformly accelerated detector that has
been considered recently, we find that defining the transition probability rate
of the rotating detector poses no difficulties. Further, we show that the
response of the rotating detector can be computed {\it exactly}\vee (albeit,
numerically) even when it is coupled to a field that is governed by a
non-linear dispersion relation. We also discuss the response of the rotating
detector in the presence of a cylindrical boundary on which the scalar field is
constrained to vanish. While super-luminal dispersion relations hardly affect
the standard results, we find that sub-luminal dispersion relations can lead to
relatively large modifications.Comment: V1: 11 pages, 3 figures; V2: 12 pages, 3 figures, considerably
revised, we now calculate the detector response exactly; V3: 14 pages, 3
figures, minor changes, to appear in Phys. Rev.
Exponential Mixing for a Stochastic PDE Driven by Degenerate Noise
We study stochastic partial differential equations of the reaction-diffusion
type. We show that, even if the forcing is very degenerate (i.e. has not full
rank), one has exponential convergence towards the invariant measure. The
convergence takes place in the topology induced by a weighted variation norm
and uses a kind of (uniform) Doeblin condition.Comment: 10 pages, 1 figur
Gauge-invariant perturbation theory for trans-Planckian inflation
The possibility that the scale-invariant inflationary spectrum may be
modified due to the hidden assumptions about the Planck scale physics -- dubbed
as trans-Planckian inflation -- has received considerable attention. To mimic
the possible trans-Planckian effects, among various models, modified dispersion
relations have been popular in the literature. In almost all the earlier
analyzes, unlike the canonical scalar field driven inflation, the
trans-Planckian effects are introduced to the scalar/tensor perturbation
equations in an ad hoc manner -- without calculating the stress-tensor of the
cosmological perturbations from the covariant Lagrangian. In this work, we
perform the gauge-invariant cosmological perturbations for the single-scalar
field inflation with the Jacobson-Corley dispersion relation by computing the
fluctuations of all the fields including the unit time-like vector field which
defines a preferred rest frame. We show that: (i) The non-linear effects
introduce corrections only to the perturbed energy density. The corrections to
the energy density vanish in the super-Hubble scales. (ii) The scalar
perturbations, in general, are not purely adiabatic. (iii) The equation of
motion of the Mukhanov-Sasaki variable corresponding to the inflaton field is
different than those presumed in the earlier analyzes. (iv) The tensor
perturbation equation remains unchanged. We perform the classical analysis for
the resultant system of equations and also compute the power-spectrum of the
scalar perturbations in a particular limit. We discuss the implications of our
results and compare with the earlier results.Comment: 19 Pages, Revtex4; V2 Final version, To appear in Phys. Rev. D., 1
figure and references adde
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