2,151 research outputs found
Holographic predictions for cosmological 3-point functions
We present the holographic predictions for cosmological 3-point correlators,
involving both scalar and tensor modes, for a universe which started in a
non-geometric holographic phase. Holographic formulae relate the cosmological
3-point functions to stress tensor correlation functions of a holographically
dual three-dimensional non-gravitational QFT. We compute these correlators at
1-loop order for a theory containing massless scalars, fermions and gauge
fields, and present an extensive analysis of the constraints due to Ward
identities showing that they uniquely determine the correlators up to a few
constants. We define shapes for all cosmological bispectra and compare the
holographic shapes to the slow-roll ones, finding that some are distinguishable
while others, perhaps surprisingly, are not.Comment: 51pp; 4 fig
Holographic Non-Gaussianity
We investigate the non-Gaussianity of primordial cosmological perturbations
within our recently proposed holographic description of inflationary universes.
We derive a holographic formula that determines the bispectrum of cosmological
curvature perturbations in terms of correlation functions of a holographically
dual three-dimensional non-gravitational quantum field theory (QFT). This
allows us to compute the primordial bispectrum for a universe which started in
a non-geometric holographic phase, using perturbative QFT calculations.
Strikingly, for a class of models specified by a three-dimensional
super-renormalisable QFT, the primordial bispectrum is of exactly the
factorisable equilateral form with f_nl^eq=5/36, irrespective of the details of
the dual QFT. A by-product of this investigation is a holographic formula for
the three-point function of the trace of the stress-energy tensor along general
holographic RG flows, which should have applications outside the remit of this
work.Comment: 42 pages, 2 figs, published versio
The Holographic Universe
We present a holographic description of four-dimensional single-scalar
inflationary universes in terms of a three-dimensional quantum field theory.
The holographic description correctly reproduces standard inflationary
predictions in their regime of applicability. In the opposite case, wherein
gravity is strongly coupled at early times, we propose a holographic
description in terms of perturbative QFT and present models capable of
satisfying the current observational constraints while exhibiting a
phenomenology distinct from standard inflation. This provides a qualitatively
new method for generating a nearly scale-invariant spectrum of primordial
cosmological perturbations.Comment: 20 pages, 5 figs; extended version of arXiv:0907.5542 including
background material and detailed derivations. To appear in Proceedings of 1st
Mediterranean Conference on Classical and Quantum Gravit
The holographic fluid dual to vacuum Einstein gravity
We present an algorithm for systematically reconstructing a solution of the
(d+2)-dimensional vacuum Einstein equations from a (d+1)-dimensional fluid,
extending the non-relativistic hydrodynamic expansion of Bredberg et al in
arXiv:1101.2451 to arbitrary order. The fluid satisfies equations of motion
which are the incompressible Navier-Stokes equations, corrected by specific
higher derivative terms. The uniqueness and regularity of this solution is
established to all orders and explicit results are given for the bulk metric
and the stress tensor of the dual fluid through fifth order in the hydrodynamic
expansion. We establish the validity of a relativistic hydrodynamic description
for the dual fluid, which has the unusual property of having a vanishing
equilibrium energy density. The gravitational results are used to identify
transport coefficients of the dual fluid, which also obeys an interesting and
exact constraint on its stress tensor. We propose novel Lagrangian models which
realise key properties of the holographic fluid.Comment: 31 pages; v2: references added and minor improvements, published
versio
Many-worlds interpretation of quantum theory and mesoscopic anthropic principle
We suggest to combine the Anthropic Principle with Many-Worlds Interpretation
of Quantum Theory. Realizing the multiplicity of worlds it provides an
opportunity of explanation of some important events which are assumed to be
extremely improbable. The Mesoscopic Anthropic Principle suggested here is
aimed to explain appearance of such events which are necessary for emergence of
Life and Mind. It is complementary to Cosmological Anthropic Principle
explaining the fine tuning of fundamental constants. We briefly discuss various
possible applications of Mesoscopic Anthropic Principle including the Solar
Eclipses and assembling of complex molecules. Besides, we address the problem
of Time's Arrow in the framework of Many-World Interpretation. We suggest the
recipe for disentangling of quantities defined by fundamental physical laws and
by an anthropic selection.Comment: 11 page
Advanced practice in radiotherapy across Europe: stakeholders’ perceptions of implementation and evolution
Introduction Adapting radiotherapy services with workforce innovation using skills-mix or task-shifting optimises resources, supporting current and future demands. Advanced practitioners (APs) work at a different level of practice (beyond initial registration) across four pillars: clinical practice, leadership and management, education, and research. There is limited cross-country research on the advanced therapeutic radiographers/radiation therapists (TR/RTTs), particularly in Europe. This study aimed to investigate European radiotherapy stakeholders’ perceptions regarding current and future advanced practice (AP). Methods From June to September 2022, one-to-one online semi-structured interviews were conducted in English, and audio and video were recorded. Full verbatim audio files were independently transcribed and checked by interviewer and interviewees. Braun and Clarke's seven steps guided the thematic analysis (using NVivo). Results Thirty-three interviewees working or studying in 16 European countries represented practitioners (n=14), managers (n=6), educators (n=4), professional bodies (n=4), students (n=3), and regulators (n=2). Four overarching themes emerged: “AP drivers and outcomes”, “AP challenges vs enablers”, “Current vs future AP”, “Becoming and being advanced practitioner”. Participants identified research as the neglected AP pillar due to a lack of protected time, limited staff skills, no research culture, no funding, workload, and clinical priorities. Interviewees highlighted the importance of consistency in job titles, harmonisation of education models and curricula, definition of AP requirements, and support for all AP pillars through job plans and workforce planning. Conclusion Neither the profession nor education of TR/RTTs are harmonised across Europe, which is highly reflected in advanced-level practice. Advanced TR/RTTs should work across all pillars, including research, and these should be embedded in master's programmes, including leadership. Implications for practice This study highlights a policy gap in the education and practice of APs in radiotherapy
The clustering of polarity reversals of the geomagnetic field
Often in nature the temporal distribution of inhomogeneous stochastic point
processes can be modeled as a realization of renewal Poisson processes with a
variable rate. Here we investigate one of the classical examples, namely the
temporal distribution of polarity reversals of the geomagnetic field. In spite
of the commonly used underlying hypothesis, we show that this process strongly
departs from a Poisson statistics, the origin of this failure stemming from the
presence of temporal clustering. We find that a Levy statistics is able to
reproduce paleomagnetic data, thus suggesting the presence of long-range
correlations in the underlying dynamo process.Comment: 4 pages, in press on PRL (31 march 2006?
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