22,752 research outputs found
BPS Skyrme neutron stars in generalized gravity
We study the coupling of nuclear matter described by the BPS Skyrme model to
generalized gravity. Concretely, we consider the Starobinsky model which
provides the leading-order correction to the Einstein-Hilbert action. Static
solutions describing neutron stars are found both for the full field theory and
for the mean-field approximation. We always consider the full Starobinsky model
in the nonperturbative approach, using appropriately generalized shooting
methods for the numerical neutron star calculations. Many of our results are
similar to previous investigations of neutron stars for the Starobinsky model
using other models of nuclear matter, but there are some surprizing
discrepancies. The "Newtonian mass" relevant for the surface redshift, e.g.,
results larger than the ADM mass in our model, in contrast to other
investigations. This difference is related to the particularly high stiffness
of nuclear matter described by the BPS Skyrme model and offers an interesting
possibility to distinguish different models of nuclear matter within
generalized gravity.Comment: LaTex, 28 pages, 13 figures; v2: minor change
Dynamical phase coexistence: A simple solution to the "savanna problem"
We introduce the concept of 'dynamical phase coexistence' to provide a simple
solution for a long-standing problem in theoretical ecology, the so-called
"savanna problem". The challenge is to understand why in savanna ecosystems
trees and grasses coexist in a robust way with large spatio-temporal
variability. We propose a simple model, a variant of the Contact Process (CP),
which includes two key extra features: varying external
(environmental/rainfall) conditions and tree age. The system fluctuates locally
between a woodland and a grassland phase, corresponding to the active and
absorbing phases of the underlying pure contact process. This leads to a highly
variable stable phase characterized by patches of the woodland and grassland
phases coexisting dynamically. We show that the mean time to tree extinction
under this model increases as a power-law of system size and can be of the
order of 10,000,000 years in even moderately sized savannas. Finally, we
demonstrate that while local interactions among trees may influence tree
spatial distribution and the order of the transition between woodland and
grassland phases, they do not affect dynamical coexistence. We expect dynamical
coexistence to be relevant in other contexts in physics, biology or the social
sciences.Comment: 8 pages, 7 figures. Accepted for publication in Journal of
Theoretical Biolog
Time scale competition leading to fragmentation and recombination transitions in the coevolution of network and states
We study the co-evolution of network structure and node states in a model of
multiple state interacting agents. The system displays two transitions, network
recombination and fragmentation, governed by time scales that emerge from the
dynamics. The recombination transition separates a frozen configuration,
composed by disconnected network components whose agents share the same state,
from an active configuration, with a fraction of links that are continuously
being rewired. The nature of this transition is explained analytically as the
maximum of a characteristic time. The fragmentation transition, that appears
between two absorbing frozen phases, is an anomalous order-disorder transition,
governed by a crossover between the time scales that control the structure and
state dynamics.Comment: 5 pages, 5 figures, figures 2 and 4 changed, tile changed, to be
published in PR
Analytical Solution of the Voter Model on Disordered Networks
We present a mathematical description of the voter model dynamics on
heterogeneous networks. When the average degree of the graph is
the system reaches complete order exponentially fast. For , a finite
system falls, before it fully orders, in a quasistationary state in which the
average density of active links (links between opposite-state nodes) in
surviving runs is constant and equal to , while an
infinite large system stays ad infinitum in a partially ordered stationary
active state. The mean life time of the quasistationary state is proportional
to the mean time to reach the fully ordered state , which scales as , where is the number of nodes of the
network, and is the second moment of the degree distribution. We find
good agreement between these analytical results and numerical simulations on
random networks with various degree distributions.Comment: 20 pages, 8 figure
Gluon Saturation and Black Hole Criticality
We discuss the recent proposal in hep-th/0611312 where it was shown that the
critical anomalous dimension associated to the onset of non-linear effects in
the high energy limit of QCD coincides with the critical exponent governing the
radius of the black hole formed in the spherically symmetric collapse of a
massless scalar field. We argue that a new essential ingredient in this mapping
between gauge theory and gravity is continuous self-similarity, not present in
the scalar field case but in the spherical collapse of a perfect fluid with
barotropic equation of state. We identify this property with geometric scaling,
present in DIS data at small values of Bjorken x. We also show that the
Choptuik exponent in dimension five tends to the QCD critical value in the
traceless limit of the energy momentum tensor.Comment: Talk given at 12th International Conference on Elastic and
Diffractive Scattering: Forward Physics and QCD, Hamburg, DESY, Germany,
21-25 May 200
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