6,872 research outputs found
On the ergoregion instability in rotating gravastars
The ergoregion instability is known to affect very compact objects that
rotate very rapidly and do not possess a horizon. We present here a detailed
analysis on the relevance of the ergoregion instability for the viability of
gravastars. Expanding on some recent results, we show that not all rotating
gravastars are unstable. Rather, stable models can be constructed also with
J/M^2 ~ 1, where J and M are the angular momentum and mass of the gravastar,
respectively. The genesis of gravastars is still highly speculative and
fundamentally unclear if not dubious. Yet, their existence cannot be ruled out
by invoking the ergoregion instability. For the same reason, not all
ultra-compact astrophysical objects rotating with J/M^2 <~ 1 are to be
considered necessarily black holes.Comment: 10 pages, 7 figure
Identifying the starting point of a spreading process in complex networks
When dealing with the dissemination of epidemics, one important question that
can be asked is the location where the contamination began. In this paper, we
analyze three spreading schemes and propose and validate an effective
methodology for the identification of the source nodes. The method is based on
the calculation of the centrality of the nodes on the sampled network,
expressed here by degree, betweenness, closeness and eigenvector centrality. We
show that the source node tends to have the highest measurement values. The
potential of the methodology is illustrated with respect to three theoretical
complex network models as well as a real-world network, the email network of
the University Rovira i Virgili
D-meson diffusion in hadronic matter
We present effective-field-theory results with unitarized interactions on the
D-meson transport coefficients in a gas populated by light mesons and baryons
at finite temperature and baryochemical potential. The Fokker-Planck equation
is used to compute the drag force, the relaxation time and the diffusion
coefficients of D mesons for collisions at FAIR. At finite baryochemical
potential, the combined effect of net baryonic density and sizable meson-baryon
interaction makes the D mesons to relax more efficiently than in the case at
zero baryochemical potential. We also describe the connection with the
quark-gluon plasma phase in adiabatic trajectories on the phase diagram at both
zero and finite baryochemical potential.Comment: 5 pages, 4 figures. Contribution to the FAIRNESS 2013 - Workshop for
young scientists with research interests focused on FAIR physics. 15-21
September 2013, Berlin (Germany
Preferential attachment growth model and nonextensive statistical mechanics
We introduce a two-dimensional growth model where every new site is located,
at a distance from the barycenter of the pre-existing graph, according to
the probability law , and is attached to
(only) one pre-existing site with a probability ; is the number of links of the site of the
pre-existing graph, and its distance to the new site). Then we
numerically determine that the probability distribution for a site to have
links is asymptotically given, for all values of , by , where is the function
naturally emerging within nonextensive statistical mechanics. The entropic
index is numerically given (at least for not too large) by , and the characteristic number of links by . The particular case belongs to the same
universality class to which the Barabasi-Albert model belongs. In addition to
this, we have numerically studied the rate at which the average number of links
increases with the scaled time ; asymptotically, , the exponent being close to for , and zero otherwise.
The present results reinforce the conjecture that the microscopic dynamics of
nonextensive systems typically build (for instance, in Gibbs -space for
Hamiltonian systems) a scale-free network.Comment: 5 pages including 5 figures (the original colored figures 1 and 5a
can be asked directly to the authors
Black-hole horizons as probes of black-hole dynamics I: post-merger recoil in head-on collisions
The understanding of strong-field dynamics near black-hole horizons is a
long-standing and challenging prob- lem in general relativity. Recent advances
in numerical relativity and in the geometric characterization of black- hole
horizons open new avenues into the problem. In this first paper in a series of
two, we focus on the analysis of the recoil occurring in the merger of binary
black holes, extending the analysis initiated in [1] with Robinson- Trautman
spacetimes. More specifically, we probe spacetime dynamics through the
correlation of quantities defined at the black-hole horizon and at null
infinity. The geometry of these hypersurfaces responds to bulk gravitational
fields acting as test screens in a scattering perspective of spacetime
dynamics. Within a 3 + 1 approach we build an effective-curvature vector from
the intrinsic geometry of dynamical-horizon sections and correlate its
evolution with the flux of Bondi linear momentum at large distances. We employ
this setup to study numerically the head-on collision of nonspinning black
holes and demonstrate its validity to track the qualita- tive aspects of recoil
dynamics at infinity. We also make contact with the suggestion that the
antikick can be described in terms of a "slowness parameter" and how this can
be computed from the local properties of the horizon. In a companion paper [2]
we will further elaborate on the geometric aspects of this approach and on its
relation with other approaches to characterize dynamical properties of
black-hole horizons.Comment: final version published on PR
Heating triangle singularities in heavy ion collisions
We predict that triangle singularities of hadron spectroscopy are strongly
affected in heavy ion collisions. To do it we examine various effects of finite
temperature on the triangle loop yielding the singularity within the hadron
phase. Pion-containing triangles can be enhanced by exchanging them with the
medium, but in other cases, especially with heavy-quark hadrons, known thermal
effects over the particles mass and width can quickly reduce the singularity:
at temperatures of about 150 MeV, below the transition to a quark-gluon plasma,
even by two orders of magnitude. It appears that peaks seen in central heavy
ion collisions are more likely to be hadrons than rescattering effects unless
perhaps if a pion is involved in the triangle. The medium then acts as a
spectroscopic filter.Comment: 5 pages, 7 plot
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