2,360 research outputs found
Loop Quantum Cosmology and the Fine Structure Constant
The cosmological implications of introducing a variation to the fine
structure `constant', \alpha are examined within the context of Loop Quantum
Cosmology. The evolution of \alpha is described using the model introduced by
Bekenstein, Sandvik, Barrow and Magueijo, in which a ghost scalar field
produces the variation. The dynamics of the system are examined in flat and
closed cosmological settings. Matter consisting of the scalar field and
radiation are examined with a thermodynamically motivated coupling between the
two, which can lead to a series of bounces induced by both the negative density
effects of the ghost field and the loop effects.Comment: 10 pages, 5 figure
Minimal Coupling and Attractors
The effects of minimally coupling a gravity to matter on a flat
Robertson-Walker geometry are explored. Particular attention is paid to the
evolution of the symplectic structure and the Liouville measure it defines. We
show that the rescaling freedom introduced by choice of fiducial cell leads to
a symmetry between dynamical trajectories, which together with the Liouville
measure provides a natural volume weighting explanation for the generic
existence of attractors.Comment: 11 page
First-order action and Euclidean quantum gravity
We show that the on-shell path integral for asymptotically flat Euclidean
spacetimes can be given in the first-order formulation of general relativity,
without assuming the boundary to be isometrically embedded in Euclidean space
and without adding infinite counter-terms. For illustrative examples of our
approach, we evaluate the first-order action for the four-dimensional Euclidean
Schwarzschild and NUT-charged spacetimes to derive the corresponding on-shell
partition functions, and show that the correct thermodynamic quantities for the
solutions are reproduced.Comment: 8 pages; v2: references added; minor corrections; v3: typos corrected
in eqns (20) and (21); v4: substantially revised; addition of NUT-charged
spacetimes; to appear in Classical and Quantum Gravit
Probability of Inflation in Loop Quantum Cosmology
Inflationary models of the early universe provide a natural mechanism for the
formation of large scale structure. This success brings to forefront the
question of naturalness: Does a sufficiently long slow roll inflation occur
generically or does it require a careful fine tuning of initial parameters? In
recent years there has been considerable controversy on this issue. In
particular, for a quadratic potential, Kofman, Linde and Mukhanov have argued
that the probability of inflation with at least 65 e-foldings is close to one,
while Gibbons and Turok have argued that this probability is suppressed by a
factor of ~ \10^{-85}. We first clarify that such dramatically different
predictions can arise because the required measure on the space of solutions is
intrinsically ambiguous in general relativity. We then show that this ambiguity
can be naturally resolved in loop quantum cosmology (LQC) because the big bang
is replaced by a big bounce and the bounce surface can be used to introduce the
structure necessary to specify a satisfactory measure.
The second goal of the paper is to present a detailed analysis of the
inflationary dynamics of LQC using analytical and numerical methods. By
combining this information with the measure on the space of solutions, we
address a sharper question than those investigated in the literature: What is
the probability of a sufficiently long slow roll inflation WHICH IS COMPATIBLE
WITH THE SEVEN YEAR WMAP DATA? We show that the probability is very close to 1.
The material is so organized that cosmologists who may be more interested in
the inflationary dynamics in LQC than in the subtleties associated with
measures can skip that material without loss of continuity.Comment: 34 pages, 3 figure
A Homogeneous Model of Spinfoam Cosmology
We examine spinfoam cosmology by use of a simple graph adapted to homogeneous
cosmological models. We calculate dynamics in the isotropic limit, and provide
the framework for the aniostropic case. The dynamical behaviour is calculating
transition amplitudes between holomorphic coherent states on a single node
graph. The resultant dynamics is peaked on solutions which have no support on
the zero volume state, indicating that big bang type singularities are avoided
within such models.Comment: 22 pages, 4 figure
Inflationary Attractors and their Measures
Several recent misconceptions about the measure problem in inflation and the
nature of inflationary attractors are addressed. We show that within the
Hamiltonian system of flat Friedmann-Lema\^itre-Robertson-Walker cosmology
coupled to a massive scalar field, the focussing of the Liouville measure on
attractor solutions is brought about by a spread in a gauge degree of freedom -
the spatial volume. Using this we show how the Liouville measure formulated on
a surface of constant Hubble rate induces a probability distribution function
on surfaces of other Hubble rates, and the attractor behaviour is seen through
the focussing of this function on a narrow range of physical observables. One
can conclude then that standard techniques from Hamiltonian dynamics suffice to
provide a satisfactory description of attractor solutions and the measure
problem.
Updated to match published version.Comment: 8 pages, 1 figur
A Hamiltonian Formulation of the BKL Conjecture
The Belinskii, Khalatnikov and Lifshitz conjecture \cite{bkl1} posits that on
approach to a space-like singularity in general relativity the dynamics are
well approximated by `ignoring spatial derivatives in favor of time
derivatives.' In \cite{ahs1} we examined this idea from within a Hamiltonian
framework and provided a new formulation of the conjecture in terms of
variables well suited to loop quantum gravity. We now present the details of
the analytical part of that investigation. While our motivation came from
quantum considerations, thanks to some of its new features, our formulation
should be useful also for future analytical and numerical investigations within
general relativity.Comment: 26 pages, 3 figures. Two references added. Minor typos corrected. To
appear in PR
Using multimedia to enhance the accessibility of the learning environment for disabled students: reflections from the Skills for Access project
As educators' awareness of their responsibilities towards ensuring the accessibility of the learning environment to disabled students increases, significant debate surrounds the implications of accessibility requirements on educational multimedia. There would appear to be widespread concern that the fundamental principles of creating accessible webâbased materials seem at odds with the creative and innovative use of multimedia to support learning and teaching, as well as concerns over the time and cost of providing accessibility features that can hold back resource development and application. Yet, effective use of multimedia offers a way of enhancing the accessibility of the learning environment for many groups of disabled students. Using the development of âSkills for Accessâ, a web resource supporting the dual aims of creating optimally accessible multimedia for learning, as an example, the attitudinal, practical and technical challenges facing the effective use of multimedia as an accessibility aid in a learning environment will be explored. Reasons why a holistic approach to accessibility may be the most effective in ensuring that multimedia reaches its full potential in enabling and supporting students in learning, regardless of any disability they may have, will be outlined and discussed
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