369,965 research outputs found
Impact of string and monopole-type junctions on domain wall dynamics: implications for dark energy
We investigate the potential role of string and monopole-type junctions in
the frustration of domain wall networks using a velocity-dependent one-scale
model for the characteristic velocity, , and the characteristic length, ,
of the network. We show that, except for very special network configurations,
v^2 \lsim (HL)^2 \lsim (\rho_\sigma + \rho_\mu)/\rho_m where is the
Hubble parameter and , and are the average
density of domain walls, strings and monopole-type junctions. We further show
that if domain walls are to provide a significant contribution to the dark
energy without generating exceedingly large CMB temperature fluctuations then,
at the present time, the network must have a characteristic length L_0 \lsim
10 \Omega_{\sigma 0}^{-2/3} {\rm kpc} and a characteristic velocity v_0 \lsim
10^{-5} \Omega_{\sigma 0}^{-2/3} where and is the critical density. In order to satisfy these
constraints with , would have to be at
least 10 orders of magnitude larger than , which would be in
complete disagreement with observations. This result provides very strong
additional support for the conjecture that no natural frustration mechanism,
which could lead to a significant contribution of domain walls to the dark
energy budget, exists.Comment: 4 pages, 1 figur
The Immunity of Polymer-Microemulsion Networks
The concept of network immunity, i.e., the robustness of the network
connectivity after a random deletion of edges or vertices, has been
investigated in biological or communication networks. We apply this concept to
a self-assembling, physical network of microemulsion droplets connected by
telechelic polymers, where more than one polymer can connect a pair of
droplets. The gel phase of this system has higher immunity if it is more likely
to survive (i.e., maintain a macroscopic, connected component) when some of the
polymers are randomly degraded. We consider the distribution of the
number of polymers between a pair of droplets, and show that gel immunity
decreases as the variance of increases. Repulsive interactions
between the polymers decrease the variance, while attractive interactions
increase the variance, and may result in a bimodal .Comment: Corrected typo
Storing cycles in Hopfield-type networks with pseudoinverse learning rule: admissibility and network topology
Cyclic patterns of neuronal activity are ubiquitous in animal nervous
systems, and partially responsible for generating and controlling rhythmic
movements such as locomotion, respiration, swallowing and so on. Clarifying the
role of the network connectivities for generating cyclic patterns is
fundamental for understanding the generation of rhythmic movements. In this
paper, the storage of binary cycles in neural networks is investigated. We call
a cycle admissible if a connectivity matrix satisfying the cycle's
transition conditions exists, and construct it using the pseudoinverse learning
rule. Our main focus is on the structural features of admissible cycles and
corresponding network topology. We show that is admissible if and only
if its discrete Fourier transform contains exactly nonzero
columns. Based on the decomposition of the rows of into loops, where a
loop is the set of all cyclic permutations of a row, cycles are classified as
simple cycles, separable or inseparable composite cycles. Simple cycles contain
rows from one loop only, and the network topology is a feedforward chain with
feedback to one neuron if the loop-vectors in are cyclic permutations
of each other. Composite cycles contain rows from at least two disjoint loops,
and the neurons corresponding to the rows in from the same loop are
identified with a cluster. Networks constructed from separable composite cycles
decompose into completely isolated clusters. For inseparable composite cycles
at least two clusters are connected, and the cluster-connectivity is related to
the intersections of the spaces spanned by the loop-vectors of the clusters.
Simulations showing successfully retrieved cycles in continuous-time
Hopfield-type networks and in networks of spiking neurons are presented.Comment: 48 pages, 3 figure
Network Models of Quantum Percolation and Their Field-Theory Representations
We obtain the field-theory representations of several network models that are
relevant to 2D transport in high magnetic fields. Among them, the simplest one,
which is relevant to the plateau transition in the quantum Hall effect, is
equivalent to a particular representation of an antiferromagnetic SU(2N) () spin chain. Since the later can be mapped onto a ,
sigma model, and since recent numerical analyses of the
corresponding network give a delocalization transition with ,
we conclude that the same exponent is applicable to the sigma model
Gauge Defect Networks in Two-Dimensional CFT
An interpretation of the gauge anomaly of the two-dimensional multi-phase
sigma model is presented in terms of an obstruction to the existence of a
topological defect network implementing a local trivialisation of the gauged
sigma model.Comment: 8 pages; The article is the author's contribution to the Proceedings
of the XXIX International Colloquium on Group-Theoretical Methods in Physics
(20-26 August 2012, Tianjin, China
First-passage phenomena in hierarchical networks
In this paper we study Markov processes and related first passage problems on
a class of weighted, modular graphs which generalize the Dyson hierarchical
model. In these networks, the coupling strength between two nodes depends on
their distance and is modulated by a parameter . We find that, in the
thermodynamic limit, ergodicity is lost and the "distant" nodes can not be
reached. Moreover, for finite-sized systems, there exists a threshold value for
such that, when is relatively large, the inhomogeneity of the
coupling pattern prevails and "distant" nodes are hardly reached. The same
analysis is carried on also for generic hierarchical graphs, where interactions
are meant to involve -plets () of nodes, finding that ergodicity is
still broken in the thermodynamic limit, but no threshold value for is
evidenced, ultimately due to a slow growth of the network diameter with the
size
Distance Oracles for Time-Dependent Networks
We present the first approximate distance oracle for sparse directed networks
with time-dependent arc-travel-times determined by continuous, piecewise
linear, positive functions possessing the FIFO property.
Our approach precomputes approximate distance summaries from
selected landmark vertices to all other vertices in the network. Our oracle
uses subquadratic space and time preprocessing, and provides two sublinear-time
query algorithms that deliver constant and approximate
shortest-travel-times, respectively, for arbitrary origin-destination pairs in
the network, for any constant . Our oracle is based only on
the sparsity of the network, along with two quite natural assumptions about
travel-time functions which allow the smooth transition towards asymmetric and
time-dependent distance metrics.Comment: A preliminary version appeared as Technical Report ECOMPASS-TR-025 of
EU funded research project eCOMPASS (http://www.ecompass-project.eu/). An
extended abstract also appeared in the 41st International Colloquium on
Automata, Languages, and Programming (ICALP 2014, track-A
Cosmological Evolution of Global Monopoles
We investigate the cosmological evolution of global monopoles in the
radiation dominated (RD) and matter dominated (MD) universes by numerically
solving field equations of scalar fields. It is shown that the global monopole
network relaxes into the scaling regime, unlike the gauge monopole network. The
number density of global monopoles is given by during the RD era and during the MD
era. Thus, we have confirmed that density fluctuations produced by global
monopoles become scale invariant and are given by during the RD (MD) era, where is the breaking
scale of the symmetry.Comment: 6 pages, 2 figures, to appear in Phys. Rev. D (R
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