26,794 research outputs found
String Evolution with Friction
We study the effects of friction on the scaling evolution of string networks
in condensed matter and cosmological contexts. We derive a generalized
`one-scale' model with the string correlation length and velocity as
dynamical variables. In non-relativistic systems, we obtain a well-known
law, showing that loop production is important. For
electroweak cosmic strings, we show transient damped epoch scaling with
(or, in the matter era, ). A low initial
density implies an earlier period with . For GUT strings, the
approach to linear scaling is faster than previously estimated.Comment: 8 pages, uuencoded gziped .ps file. Paper submitted to Phys. Rev.
Let
Effects of Inflation on a Cosmic String Loop Population
We study the evolution of simple cosmic string loop solutions in an
inflationary universe. We show, for the particular case of circular loops, that
periodic solutions do exist in a de Sitter universe, below a critical loop
radius . On the other hand, larger loops freeze in comoving
coordinates, and we explicitly show that they can survive more -foldings of
inflation than point-like objects. We discuss the implications of these
findings for the survival of realistic cosmic string loops during inflation,
and for the general characteristics of post-inflationary cosmic string
networks. We also consider the analogous solutions for domain walls, in which
case the critical radius is .Comment: 5 pages, 5 figures, accepted for publication in Physical Review
Contribution of domain wall networks to the CMB power spectrum
We use three domain wall simulations from the radiation era to the late time
dark energy domination era based on the PRS algorithm to calculate the
energy-momentum tensor components of domain wall networks in an expanding
universe. Unequal time correlators in the radiation, matter and cosmological
constant epochs are calculated using the scaling regime of each of the
simulations. The CMB power spectrum of a network of domain walls is determined.
The first ever quantitative constraint for the domain wall surface tension is
obtained using a Markov chain Monte Carlo method; an energy scale of domain
walls of 0.93 MeV, which is close but below the Zel'dovich bound, is
determined.Comment: Submitted to Physics Letters
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
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DEveloping a Complex Intervention for DEteriorating Patients using Theoretical Modelling (DECIDE study): study protocol
AIM: To develop a theory-based complex intervention (targeting nursing staff), to enhance enablers and overcome barriers to enacting expected behaviour when monitoring patients and responding to abnormal vital signs that signal deterioration.
DESIGN: A mixed method design including structured observations on hospital wards, field notes, brief, un-recorded interviews and semi-structured interviews to inform the development of an intervention to enhance practice.
METHODS: Semi-structured interviews will be conducted with nursing staff using a topic guide informed by the Theoretical Domains Framework. Semi-structured interviews will be transcribed verbatim and coded deductively into the 14 Theoretical Domains Framework domains and then inductively into 'belief statements'. Priority domains will be identified and mapped to appropriate behaviour change techniques. Intervention content and mode of delivery (how behaviour change techniques are operationalised) will be developed using nominal groups, during which participants (clinicians) will rank behaviour change techniques /mode of delivery combinations according to acceptability and feasibility. Findings will be synthesised to develop an intervention manual.
DISCUSSION: Despite being a priority for clinicians, researchers and policymakers for two decades, 'sub-optimal care' of the deteriorating ward patient persists. Existing interventions have been largely educational (i.e., targeting assumed knowledge deficits) with limited evidence that they change staff behaviour. Staff behaviour when monitoring and responding to abnormal vital signs is likely influenced by a range of mediators that includes barriers and enablers.
IMPACT: Systematically applying theory and evidence-based methods, will result in the specification of an intervention which is more likely to result in behaviour change and can be tested empirically in future research. This article is protected by copyright. All rights reserved
Topological Defects in Contracting Universes
We study the behaviour and consequences of cosmic string networks in
contracting universes. They approximately behave during the collapse phase as a
radiation fluids. Scaling solutions describing this are derived and tested
against high-resolution numerical simulations. A string network in a
contracting universe, together with the gravitational radiation it generates,
can affect the dynamics of the universe both locally and globally, and be an
important source of radiation, entropy and inhomogeneity. We discuss possible
implications for bouncing and cyclic models.Comment: Shorter version of astro-ph/0206287. To appear in Phys. Rev. Let
Probing dark energy beyond with CODEX
Precision measurements of nature's fundamental couplings and a first
measurement of the cosmological redshift drift are two of the key targets for
future high-resolution ultra-stable spectrographs such as CODEX. Being able to
do both gives CODEX a unique advantage, allowing it to probe dynamical dark
energy models (by measuring the behavior of their equation of state) deep in
the matter era and thereby testing classes of models that would otherwise be
difficult to distinguish from the standard CDM paradigm. We illustrate
this point with two simple case studies.Comment: 4 pages, 4 figures; submitted to Phys. Rev.
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