2,854 research outputs found
Inflation over the hill
We calculate the power spectrum of curvature perturbations when the inflaton
field is rolling over the top of a local maximum of a potential. We show that
the evolution of the field can be decomposed into a late-time attractor, which
is identified as the slow roll solution, plus a rapidly decaying non-slow roll
solution, corresponding to the field rolling ``up the hill'' to the maximum of
the potential. The exponentially decaying transient solution can map to an
observationally relevant range of scales because the universe is also expanding
exponentially. We consider the two branches separately and we find that they
are related through a simple transformation of the slow roll parameter
and they predict identical power spectra. We generalize this approach to the
case where the inflaton field is described by both branches simultaneously and
find that the mode equation can be solved exactly at all times. Even though the
slow roll parameter is evolving rapidly during the transition from the
transient solution to the late-time attractor solution, the resultant power
spectrum is an exact power-law spectrum. Such solutions may be useful for
model-building on the string landscape.Comment: 11 pages, 1 figure (V3: Version accepted by PRD, title changed by
journal
A Hamilton-Jacobi approach to non-slow-roll inflation
I describe a general approach to characterizing cosmological inflation
outside the standard slow-roll approximation, based on the Hamilton-Jacobi
formulation of scalar field dynamics. The basic idea is to view the equation of
state of the scalar field matter as the fundamental dynamical variable, as
opposed to the field value or the expansion rate. I discuss how to formulate
the equations of motion for scalar and tensor fluctuations in situations where
the assumption of slow roll is not valid. I apply the general results to the
simple case of inflation from an ``inverted'' polynomial potential, and to the
more complicated case of hybrid inflation.Comment: 21 pages, RevTeX (minor revisions to match published version
Exceptional Indices
Recently a prescription to compute the superconformal index for all theories
of class S was proposed. In this paper we discuss some of the physical
information which can be extracted from this index. We derive a simple
criterion for the given theory of class S to have a decoupled free component
and for it to have enhanced flavor symmetry. Furthermore, we establish a
criterion for the "good", the "bad", and the "ugly" trichotomy of the theories.
After interpreting the prescription to compute the index with non-maximal
flavor symmetry as a residue calculus we address the computation of the index
of the bad theories. In particular we suggest explicit expressions for the
superconformal index of higher rank theories with E_n flavor symmetry, i.e. for
the Hilbert series of the multi-instanton moduli space of E_n.Comment: 33 pages, 11 figures, v2: minor correction
Life test, 8000-hour, of an electron bombardment mercury ion thruster system for SERT 2 Final report
Operational performance of electron bombardment mercury ion thruster system during life testing in space simulation for SERT
Investigation of shock waves in explosive blasts using fibre optic pressure sensors
The published version of this article may be accessed at the link below. Copyright @ IOP Publishing, 2006.We describe miniature all-optical pressure sensors, fabricated by wafer etching techniques, less than 1 mm(2) in overall cross-section with rise times in the mu s regime and pressure ranges typically 900 kPa (9 bar). Their performance is suitable for experimental studies of the pressure-time history for test models exposed to shocks initiated by an explosive charge. The small size and fast response of the sensors promises higher quality data than has been previously available from conventional electrical sensors, with potential improvements to numerical models of blast effects. Results from blast tests are presented in which up to six sensors were multiplexed, embedded within test models in a range of orientations relative to the shock front.Support from the UK Engineering&Physical
Sciences Research Council and Dstl Fort Halstead through the MoD Joint Grants Scheme are acknowledged. WN MacPherson is supported by an EPSRC Advanced Research Fellowship
Medication communication between nurses and doctors for paediatric acute care: An ethnographic study
AIMS AND OBJECTIVES: To examine how communication between nurses and doctors occurred for managing medications in inpatient paediatric settings. BACKGROUND: Communication between health professionals influences medication incidents' occurrence and safe care. DESIGN: An ethnographic study was undertaken. METHODS: Semi-structured interviews, observations and focus groups were conducted in three clinical areas of an Australian tertiary paediatric hospital. Data were transcribed verbatim and thematically analysed using the Medication Communication Model. RESULTS: The actual communication act revealed professionals' commitment to effective medication management and the influence of professional identities on medication communication. Nurses and doctors were dedicated to providing safe, effective medication therapy for children, within their scope of practice and perceived role responsibilities. Most nurses and junior doctors used tentative language in their communication while senior doctors tended to use direct language. Irrespective of language style, nurses actively engaged with doctors to promote patients' needs. Yet, the medical hierarchical structure, staffing and attendant expectations influenced communication for medication management, causing frustration among nurses and doctors. Doctors' lack of verbal communication of documented changes to medication orders particularly troubled nurses. Nurses persisted in their efforts to acquire appropriate orders for safe medication administration to paediatric patients. CONCLUSIONS: Collaborative practice between nurses and doctors involved complex, symbiotic relationships. Their dedication to providing safe medication therapy to paediatric patients facilitated effective medication management. At times, shortcomings in inter-disciplinary communication impacted on potential and actual medication incidents. RELEVANCE TO CLINICAL PRACTICE: Understanding of the complexities affecting medication communication between nurses and doctors helps to ensure inter-professional respect for each other's roles and inherent demands. Interdisciplinary education delivered in health care organisations would facilitate greater clarity in communication related to medications. Encouraging the use of concise, clear words in communication would help to promote improved understanding between parties, and accuracy and efficacy of medication management. This article is protected by copyright. All rights reserved
Completing Natural Inflation
If the inflaton is a pseudo-scalar axion, the axion shift symmetry can
protect the flatness of its potential from too large radiative corrections.
This possibility, known as natural inflation, requires an axion scale which is
greater than the (reduced) Planck scale. It is unclear whether such a high
value is compatible with an effective field theoretical description, and if the
global axionic symmetry survives quantum gravity effects. We propose a
mechanism which provides an effective large axion scale, although the original
one is sub-Planckian. The mechanism is based on the presence of two axions,
with a potential provided by two anomalous gauge groups. The effective large
axion scale is due to an almost exact symmetry between the couplings of the
axions to the anomalous groups. We also comment on a possible implementation in
heterotic string theory.Comment: 9 pages, 1 figur
On a modular property of N=2 superconformal theories in four dimensions
In this note we discuss several properties of the Schur index of N=2
superconformal theories in four dimensions. In particular, we study modular
properties of this index under SL(2,Z) transformations of its parameters.Comment: 23 page, 2 figure
Ribbons on the CBR Sky: A Powerful Test of a Baryon Symmetric Universe
If the Universe consists of domains of matter and antimatter, annihilations
at domain interfaces leave a distinctive imprint on the Cosmic Background
Radiation (CBR) sky. The signature is anisotropies in the form of long, thin
ribbons of width , separated by angle where L is the characteristic domain size, and
y-distortion parameter . Such a pattern could potentially be
detected by the high-resolution CBR anisotropy experiments planned for the next
decade, and such experiments may finally settle the question of whether or not
our Hubble volume is baryon symmetric.Comment: LaTeX, 10 pages, 4 figures in epsf. Revised version corrects a couple
of relevant mistake
Cascade Failure in a Phase Model of Power Grids
We propose a phase model to study cascade failure in power grids composed of
generators and loads. If the power demand is below a critical value, the model
system of power grids maintains the standard frequency by feedback control. On
the other hand, if the power demand exceeds the critical value, an electric
failure occurs via step out (loss of synchronization) or voltage collapse. The
two failures are incorporated as two removal rules of generator nodes and load
nodes. We perform direct numerical simulation of the phase model on a
scale-free network and compare the results with a mean-field approximation.Comment: 7 pages, 2 figure
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