1,781 research outputs found
Wilson loops, geometric operators and fermions in 3d group field theory
Group field theories whose Feynman diagrams describe 3d gravity with a
varying configuration of Wilson loop observables and 3d gravity with volume
observables at each vertex are defined. The volume observables are created by
the usual spin network grasping operators which require the introduction of
vector fields on the group. We then use this to define group field theories
that give a previously defined spin foam model for fermion fields coupled to
gravity, and the simpler quenched approximation, by using tensor fields on the
group. The group field theory naturally includes the sum over fermionic loops
at each order of the perturbation theory.Comment: 13 pages, many figures, uses psfra
Neutron Stars as Dark Matter Probes
We examine whether the accretion of dark matter onto neutron stars could ever
have any visible external effects. Captured dark matter which subsequently
annihilates will heat the neutron stars, although it seems the effect will be
too small to heat close neutron stars at an observable rate whilst those at the
galactic centre are obscured by dust. Non-annihilating dark matter would
accumulate at the centre of the neutron star. In a very dense region of dark
matter such as that which may be found at the centre of the galaxy, a neutron
star might accrete enough to cause it to collapse within a period of time less
than the age of the Universe. We calculate what value of the stable dark
matter-nucleon cross section would cause this to occur for a large range of
masses.Comment: 8 pages, 7 figure
Radion Assisted Gauge Inflation
We propose an extension to the recently proposed extranatural or gauge
inflation scenario in which the radius modulus field around which the Wilson
loop is wrapped assists inflation as it shrinks. We discuss how this might lead
to more generic initial conditions for inflation.Comment: 10 pages, 2 figure
Observables in 3d spinfoam quantum gravity with fermions
We study expectation values of observables in three-dimensional spinfoam
quantum gravity coupled to Dirac fermions. We revisit the model introduced by
one of the authors and extend it to the case of massless fermionic fields. We
introduce observables, analyse their symmetries and the corresponding proper
gauge fixing. The Berezin integral over the fermionic fields is performed and
the fermionic observables are expanded in open paths and closed loops
associated to pure quantum gravity observables. We obtain the vertex amplitudes
for gauge-invariant observables, while the expectation values of gauge-variant
observables, such as the fermion propagator, are given by the evaluation of
particular spin networks.Comment: 32 pages, many diagrams, uses psfrag
Aspectos computacionais da implementaçao da formulaçao diferencial aplicada à análise reológica de barragens de concreto
Neste artigo sáo apresentadas as formulaçóes integral e diferencial para a modelizacão matemática do problema reológico do concreto, assim como a correspondencia entre as duas formulaçoes. Sáo então apresentados exemplos numéricos visando uma comparacáo da eficiencia computacional de algoritmos baseados nos dois tipos de modelizaçao.Peer Reviewe
"I make it real by putting it into words": Negotiating the Subjective, Mutable, and Interpersonal in Virginia Woolf’s "A Sketch of the Past"
Posthumously published in her autobiographical collection, Moments of Being, Virginia Woolf’s unfinished essay, “A Sketch of the Past” is largely a meta-autobiography, as Woolf works through not only her formative memories, but the implications of representing those memories in a work of life writing. Integral to Woolf’s memories and conception of the self are her transcendent “shocks” of experience, elsewhere called “moments of being.” Nearly every scholarly essay on “A Sketch" engages with and attempts to define these shocks in relation to Woolf’s writing practises. However, there has been no scholarly work published on the shock as it relates to the “memoir writer’s difficulties” Woolf enumerates at the beginning of her essay (65). These difficulties can be grouped into three general categories: representation of subjectivity, the possible movement and change, and negotiation between the personal and interpersonal. Taking into consideration the unfinished nature of the text, it becomes not just a meta-autobiography, but also a text caught in the midst of creation, illuminating the relationship between learning about the self and the practise of representing that self in writing. Thus, in this paper I analyze “A Sketch” for its treatment of shocks and their relationship to autobiography. In conversation with other Woolf scholars, I attempt to detail the shock as a site of contention between unity and stasis and multiplicity and movement, two sets of oppositional states which underlie the majority of Woolf’s reservations about the autobiographical genre. Ultimately, I demonstrate that the shock as represented in “A Sketch” incites the movement and interpersonal connection that are necessary to an understanding of multiple selves and the ever-changing world we are all unified within, and that are reflected in the continued research on Woolf’s unfinished and ambiguous autobiographical text
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
An effective field theory for matter coupled to three-dimensional quantum
gravity was recently derived in the context of spinfoam models in
hep-th/0512113. In this paper, we show how this relates to group field theories
and generalized matrix models. In the first part, we realize that the effective
field theory can be recasted as a matrix model where couplings between matrices
of different sizes can occur. In a second part, we provide a family of
classical solutions to the three-dimensional group field theory. By studying
perturbations around these solutions, we generate the dynamics of the effective
field theory. We identify a particular case which leads to the action of
hep-th/0512113 for a massive field living in a flat non-commutative space-time.
The most general solutions lead to field theories with non-linear redefinitions
of the momentum which we propose to interpret as living on curved space-times.
We conclude by discussing the possible extension to four-dimensional spinfoam
models.Comment: 17 pages, revtex4, 1 figur
Dependability of Wireless Sensor Networks
As wireless sensor networks (WSNs) are becoming ever more prevalent, the runtime characteristics of these networks are becoming an increasing issue. Commonly, external sources of interference make WSNs behave in a different manner to that expected from within simplistic simulations, resulting in the need to use additional systems which monitor the state of the network. Despite dependability of WSNs being an increasingly important issue, there are still only a limited number of works within this specific field, with the majority of works focusing on ensuring that specific devices are operational, not the application as a whole. This work instead aims to look at the dependability of WSNs from an application-centric view, taking into account the possible ways in which the application may fail and using the application's requirements to focus on assuring dependability
Effective action and semiclassical limit of spin foam models
We define an effective action for spin foam models of quantum gravity by
adapting the background field method from quantum field theory. We show that
the Regge action is the leading term in the semi-classical expansion of the
spin foam effective action if the vertex amplitude has the large-spin
asymptotics which is proportional to an exponential function of the vertex
Regge action. In the case of the known three-dimensional and four-dimensional
spin foam models this amounts to modifying the vertex amplitude such that the
exponential asymptotics is obtained. In particular, we show that the ELPR/FK
model vertex amplitude can be modified such that the new model is finite and
has the Einstein-Hilbert action as its classical limit. We also calculate the
first-order and some of the second-order quantum corrections in the
semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure
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