6,790 research outputs found
Computing General Relativistic effects from Newtonian N-body simulations: Frame dragging in the post-Friedmann approach
We present the first calculation of an intrinsically relativistic quantity in
fully non-linear cosmolog- ical large-scale structure studies. Traditionally,
non-linear structure formation in standard {\Lambda}CDM cosmology is studied
using N-body simulations, based on Newtonian gravitational dynamics on an
expanding background. When one derives the Newtonian regime in a way that is a
consistent ap- proximation to the Einstein equations, a gravito-magnetic vector
potential - giving rise to frame dragging - is present in the metric in
addition to the usual Newtonian scalar potential. At leading order, this vector
potential does not affect the matter dynamics, thus it can be computed from
Newtonian N-body simulations. We explain how we compute the vector potential
from simulations in {\Lambda}CDM and examine its magnitude relative to the
scalar potential. We also discuss some possible observable effects.Comment: 5 pages, 3 figur
Latest evidence for a late time vacuum -- geodesic CDM interaction
We perform a reconstruction of the coupling function between vacuum energy
and geodesic cold dark matter using the latest observational data. We bin the
interaction in seventeen redshift bins but use a correlation prior to prevent
rapid, unphysical oscillations in the coupling function. This prior also serves
to eliminate any dependence of the reconstruction on the binning method. We use
two different forms of the correlation prior, finding that both give similar
results for the reconstruction of the dark matter -- dark energy interaction.
Calculating the Bayes factor for each case, we find no meaningful evidence for
deviation from the null interacting case, i.e. CDM, in our
reconstruction.Comment: 14 pages, 7 figures. Version 2 matches published version in Physics
of the Dark Universe (Figure 2 updated to better show H0 and sigma 8
tensions, additional discussion of results added in section 4.1
The central structure of Broad Absorption Line QSOs: observational characteristics in the cm-mm wavelength domain
Accounting for ~20% of the total QSO population, Broad Absorption Line QSOs
are still an unsolved problem in the AGN context. They present wide troughs in
the UV spectrum, due to material with velocities up to 0.2 c toward the
observer. The two models proposed in literature try to explain them as a
particular phase of the evolution of QSOs or as normal QSOs, but seen from a
particular line of sight.
We built a statistically complete sample of Radio-Loud BAL QSOs, and carried
out an observing campaign to piece together the whole spectrum in the cm
wavelength domain, and highlight all the possible differences with respect to a
comparison sample of Radio-Loud non-BAL QSOs. VLBI observations at high angular
resolution have been performed, to study the pc-scale morphology of these
objects. Finally, we tried to detect a possible dust component with
observations at mm-wavelengths.
Results do not seem to indicate a young age for all BAL QSOs. Instead a
variety of orientations and morphologies have been found, constraining the
outflows foreseen by the orientation model to have different possible angles
with respect to the jet axis
Emotional Reactions to the Perception of Risk in the Pompeii Archaeological Park
The assessment of perceived risk by people is extremely important for safety and security management. Each person is based on the opinion of others to make a choice and the Internet represents the place where these opinions are mostly researched, found and reviewed. Social networks have a decisive impact: 92% of consumers say they have more trust in social media reviews than in any other form of advertising. For this reason, Opinion Mining and Sentiment Analysis have found interesting applications in the most diverse context, among which the most innovative is certainly represented by public safety and security. Security managers can use the perceptions expressed by people to discover the unexpected and potential weaknesses of a controlled environment or otherwise the risk and security perception of people that sometimes can be very different from real level of risk and security of a given site. Since the perceptions are the result of mostly unconscious elaborations, it is necessary to go deeper and to search for the emotions, triggered by the sensorial stimuli, that determine them. The objective of this paper is to study the perception of risk within the Pompeii Archaeological Park, giving emphasis to the emotional components, using the semantic analysis of the textual contents present in Twitter.Peer reviewe
The Speciality Index as invariant indicator in the BKL Mixmaster Dynamics
The speciality index, which has been mainly used in Numerical Relativity for
studying gravitational waves phenomena as an indicator of the special or
non-special Petrov type character of a spacetime, is applied here in the
context of Mixmaster cosmology, using the Belinski-Khalatnikov-Lifshitz map.
Possible applications for the associated chaotic dynamics are discussed
Are braneworlds born isotropic?
It has recently been suggested that an isotropic singularity may be a generic
feature of brane cosmologies, even in the inhomogeneous case. Using the
covariant and gauge-invariant approach we present a detailed analysis of linear
perturbations of the isotropic model which is a past attractor in
the phase space of homogeneous Bianchi models on the brane. We find that for
matter with an equation of state parameter , the dimensionless
variables representing generic anisotropic and inhomogeneous perturbations
decay as , showing that the model is asymptotically stable
in the past. We conclude that brane universes are born with isotropy naturally
built-in, contrary to standard cosmology. The observed large-scale homogeneity
and isotropy of the universe can therefore be explained as a consequence of the
initial conditions if the brane-world paradigm represents a description of the
very early universe.Comment: Changed to match published versio
Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium
The amorphous iron-germanium system (a-FexGe1-x) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of a-FexGe1-x is well explained by the Stoner model for Fe concentrations x above the onset of magnetic order around x=0.4, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-3d states sufficiently to polarize the electronic structure despite k being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity ÏxyAH relative to crystalline FeGe; this ÏxyAH is compared to density-functional theory calculations of the anomalous Hall conductivity to resolve its underlying mechanisms. The intrinsic mechanism, typically understood as the Berry curvature integrated over occupied k states but shown here to be equivalent to the density of curvature integrated over occupied energies in aperiodic materials, dominates the anomalous Hall conductivity of a-FexGe1-x (0.38â€xâ€0.61). The density of curvature is the sum of spin-orbit correlations of local orbital states and can hence be calculated with no reference to k space. This result and the accompanying Stoner-like model for the intrinsic anomalous Hall conductivity establish a unified understanding of the underlying physics of the anomalous Hall effect in both crystalline and disordered systems
Two-parameter non-linear spacetime perturbations: gauge transformations and gauge invariance
An implicit fundamental assumption in relativistic perturbation theory is
that there exists a parametric family of spacetimes that can be Taylor expanded
around a background. The choice of the latter is crucial to obtain a manageable
theory, so that it is sometime convenient to construct a perturbative formalism
based on two (or more) parameters. The study of perturbations of rotating stars
is a good example: in this case one can treat the stationary axisymmetric star
using a slow rotation approximation (expansion in the angular velocity Omega),
so that the background is spherical. Generic perturbations of the rotating star
(say parametrized by lambda) are then built on top of the axisymmetric
perturbations in Omega. Clearly, any interesting physics requires non-linear
perturbations, as at least terms lambda Omega need to be considered. In this
paper we analyse the gauge dependence of non-linear perturbations depending on
two parameters, derive explicit higher order gauge transformation rules, and
define gauge invariance. The formalism is completely general and can be used in
different applications of general relativity or any other spacetime theory.Comment: 22 pages, 3 figures. Minor changes to match the version appeared in
Classical and Quantum Gravit
Are Simple Real Pole Solutions Physical?
We consider exact solutions generated by the inverse scattering technique,
also known as the soliton transformation. In particular, we study the class of
simple real pole solutions. For quite some time, those solutions have been
considered interesting as models of cosmological shock waves. A coordinate
singularity on the wave fronts was removed by a transformation which induces a
null fluid with negative energy density on the wave front. This null fluid is
usually seen as another coordinate artifact, since there seems to be a general
belief that that this kind of solution can be seen as the real pole limit of
the smooth solution generated with a pair of complex conjugate poles in the
transformation. We perform this limit explicitly, and find that the belief is
unfounded: two coalescing complex conjugate poles cannot yield a solution with
one real pole. Instead, the two complex conjugate poles go to a different
limit, what we call a ``pole on a pole''. The limiting procedure is not unique;
it is sensitive to how quickly some parameters approach zero. We also show that
there exists no improved coordinate transformation which would remove the
negative energy density. We conclude that negative energy is an intrinsic part
of this class of solutions.Comment: 13 pages, 3 figure
Study on active wheelset steering from the perspective of wheel wear evolution
Active wheelset steering promises an attractive costâbenefit ratio and is highly likely to be implemented in the future. Previous studies investigated the steering effect simply through a wear index like wear number. However, a wear index cannot predict how much the material removal on wheels can be reduced, and it is unable to reveal the wear pattern. This paper builds an iterative wear model to predict wheel wear evaluation under the presence of an active steering system. Three active steering schemes are proposed, and they are compared in terms of wheel wear evolution. To quantify the steering effectiveness, two factors are created to, respectively, evaluate economic impacts and satisfaction of three steering schemes. Finally, a simplified method based on traditional wear indices is compared with the established iterative wear calculation method to examine the applicability and tolerance of the simplified method
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