3,136 research outputs found
Anisotropic Pressures at Ultra-stiff Singularities and the Stability of Cyclic Universes
We show that the inclusion of simple anisotropic pressures stops the
isotropic Friedmann universe being a stable attractor as an initial or final
singularity is approached when pressures can exceed the energy density. This
shows that the situation with isotropic pressures, studied earlier in the
context of cyclic and ekpyrotic cosmologies, is not generic, and Kasner-like
behaviour occurs when simple pressure anisotropies are present. We find all the
asymptotic behaviours and determine the dynamics when the anisotropic principal
pressures are proportional to the density. We expect distortions and
anisotropies to be significantly amplified through a simple cosmological bounce
in cyclic or ekpyrotic cosmologies when ultra-stiff pressures are present.Comment: 18 pages, 2 figure
Singularity-free cosmological solutions in quadratic gravity
We study a general field theory of a scalar field coupled to gravity through
a quadratic Gauss-Bonnet term . The coupling function has
the form , where is a positive integer. In the absence of
the Gauss-Bonnet term, the cosmological solutions for an empty universe and a
universe dominated by the energy-momentum tensor of a scalar field are always
characterized by the occurrence of a true cosmological singularity. By
employing analytical and numerical methods, we show that, in the presence of
the quadratic Gauss-Bonnet term, for the dual case of even , the set of
solutions of the classical equations of motion in a curved FRW background
includes singularity-free cosmological solutions. The singular solutions are
shown to be confined in a part of the phase space of the theory allowing the
non-singular solutions to fill the rest of the space. We conjecture that the
same theory with a general coupling function that satisfies certain criteria
may lead to non-singular cosmological solutions.Comment: Latex, 25 pages, 6 figures, some explanatory sentences and Comments
added, version to appear in Physical Review
Structure and stability of the Lukash plane-wave spacetime
We study the vacuum, plane-wave Bianchi spacetimes described by
the Lukash metric. Combining covariant with orthonormal frame techniques, we
describe these models in terms of their irreducible kinematical and geometrical
quantities. This covariant description is used to study analytically the
response of the Lukash spacetime to linear perturbations. We find that the
stability of the vacuum solution depends crucially on the background shear
anisotropy. The stronger the deviation from the Hubble expansion, the more
likely the overall linear instability of the model. Our analysis addresses
rotational, shear and Weyl curvature perturbations and identifies conditions
sufficient for the linear growth of these distortions.Comment: Revised version, references added. To appear in Class. Quantum Gra
Proximity coherence for chip-multiprocessors
Many-core architectures provide an efficient way of harnessing the growing numbers of transistors available in modern fabrication processes; however, the parallel programs run on these platforms are increasingly limited by the energy and latency costs of communication. Existing designs provide a functional communication layer but do not necessarily implement the most efficient solution for chip-multiprocessors, placing limits on the performance of these complex systems. In an era of increasingly power limited silicon design, efficiency is now a primary concern that motivates designers to look again at the challenge of cache coherence.
The first step in the design process is to analyse the communication behaviour of parallel benchmark suites such as Parsec and SPLASH-2. This thesis presents work detailing the sharing patterns observed when running the full benchmarks on a simulated 32-core x86 machine. The results reveal considerable locality of shared data accesses between threads with consecutive operating system assigned thread IDs. This pattern, although of little consequence in a multi-node system, corresponds to strong physical locality of shared data between adjacent cores on a chip-multiprocessor platform.
Traditional cache coherence protocols, although often used in chip-multiprocessor designs, have been developed in the context of older multi-node systems. By redesigning coherence protocols to exploit new patterns such as the physical locality of shared data, improving the efficiency of communication, specifically in chip-multiprocessors, is possible. This thesis explores such a design â Proximity Coherence â a novel scheme in which L1 load misses are optimistically forwarded to nearby caches via new dedicated links rather than always being indirected via a directory structure.EPSRC DTA research scholarshi
Towards a High Energy Theory for the Higgs Phase of Gravity
Spontaneous Lorentz violation due to a time-dependent expectation value for a
massless scalar has been suggested as a method for dynamically generating dark
energy. A natural candidate for the scalar is a Goldstone boson arising from
the spontaneous breaking of a U(1) symmetry. We investigate the low-energy
effective action for such a Goldstone boson in a general class of models
involving only scalars, proving that if the scalars have standard kinetic terms
then at the {\em classical} level the effective action does not have the
required features for spontaneous Lorentz violation to occur asymptotically in an expanding FRW universe. Then we study the large limit of
a renormalizable field theory with a complex scalar coupled to massive
fermions. In this model an effective action for the Goldstone boson with the
properties required for spontaneous Lorentz violation can be generated.
Although the model has shortcomings, we feel it represents progress towards
finding a high energy completion for the Higgs phase of gravity.Comment: 20 pages, 5 figures;fixed typos and added reference
Solving the Flatness and Quasi-flatness Problems in Brans-Dicke Cosmologies with a Varying Light Speed
We define the flatness and quasi-flatness problems in cosmological models. We
seek solutions to both problems in homogeneous and isotropic Brans-Dicke
cosmologies with varying speed of light. We formulate this theory and find
perturbative, non-perturbative, and asymptotic solutions using both numerical
and analytical methods. For a particular range of variations of the speed of
light the flatness problem can be solved. Under other conditions there exists a
late-time attractor with a constant value of \Omega that is smaller than, but
of order, unity. Thus these theories may solve the quasi-flatness problem, a
considerably more challenging problem than the flatness problem. We also
discuss the related \Lambda and quasi-\Lambda problem in these theories. We
conclude with an appraisal of the difficulties these theories may face.Comment: 21 pages, 6 figure
An anti-Schwarzshild solution: wormholes and scalar-tensor solutions
We investigate a static solution with an hyperbolic nature, characterised by
a pseudo-spherical foliation of space. This space-time metric can be perceived
as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs
to the class of static vacuum solutions termed "a degenerate static solution of
class A". In the present work we review its fundamental features, discuss the
existence of generalised wormholes, and derive its extension to scalar-tensor
gravity theories in general.Comment: 3 pages, contribution to the proceedings of the Spanish Relativity
Meeting-ERE200
Inhomogenized sudden future singularities
We find that sudden future singularities may also appear in spatially
inhomogeneous Stephani models of the universe. They are temporal pressure
singularities and may appear independently of the spatial finite density
singularities already known to exist in these models. It is shown that the main
advantage of the homogeneous sudden future singularities which is the
fulfillment of the strong and weak energy conditions may not be the case for
inhomogeneous models.Comment: REVTEX 4, 5 pages, no figures, a discussion of the most general case
include
Cosmology in scalar tensor theory and asymptotically de-Sitter Universe
We have investigated the cosmological scenarios with a four dimensional
effective action which is connected with multidimensional, supergravity and
string theories. The solution for the scale factor is such that initially
universe undergoes a decelerated expansion but in late times it enters into the
accelerated expansion phase. Infact, it asymptotically becomes a de-Sitter
universe. The dilaton field in our model is a decreasing function of time and
it becomes a constant in late time resulting the exit from the scalar tensor
theory to the standard Einstein's gravity. Also the dilaton field results the
existence of a positive cosmological constant in late times.Comment: 7 pages, Revtex Style, 6 Postscript figure
The Isotropy of Compact Universes
We discuss the problem of the stability of the isotropy of the universe in
the space of ever-expanding spatially homogeneous universes with a compact
spatial topology. The anisotropic modes which prevent isotropy being
asymptotically stable in Bianchi-type universes with non-compact
topologies are excluded by topological compactness. Bianchi type and type
universes with compact topologies must be exactly isotropic. In the
flat case we calculate the dynamical degrees of freedom of Bianchi-type and
universes with compact 3-spaces and show that type solutions
are more general than type solutions for systems with perfect fluid,
although the type models are more general than type in the vacuum
case. For particular topologies the 4-velocity of any perfect fluid is required
to be non-tilted. Various consequences for the problems of the isotropy,
homogeneity, and flatness of the universe are discussed.Comment: 22 pages in LaTeX2e with the amsmath packag
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