184 research outputs found
What is General Relativity?
General relativity is a set of physical and geometric principles, which lead
to a set of (Einstein) field equations that determine the gravitational field,
and to the geodesic equations that describe light propagation and the motion of
particles on the background. But open questions remain, including: What is the
scale on which matter and geometry are dynamically coupled in the Einstein
equations? Are the field equations valid on small and large scales? What is the
largest scale on which matter can be coarse grained while following a geodesic
of a solution to Einstein's equations? We address these questions. If the field
equations are causal evolution equations, whose average on cosmological scales
is not an exact solution of the Einstein equations, then some simplifying
physical principle is required to explain the statistical homogeneity of the
late epoch Universe. Such a principle may have its origin in the dynamical
coupling between matter and geometry at the quantum level in the early
Universe. This possibility is hinted at by diverse approaches to quantum
gravity which find a dynamical reduction to two effective dimensions at high
energies on one hand, and by cosmological observations which are beginning to
strongly restrict the class of viable inflationary phenomenologies on the
other. We suggest that the foundational principles of general relativity will
play a central role in reformulating the theory of spacetime structure to meet
the challenges of cosmology in the 21st century.Comment: 18 pages. Invited article for Physica Scripta Focus issue on 21st
Century Frontiers. v2: Appendix amended, references added. v3: Small
corrections, references added, matches published versio
Observational Challenges for the Standard FLRW Model
We summarise some of the main observational challenges for the standard
Friedmann-Lemaitre-Robertson-Walker cosmological model and describe how results
recently presented in the parallel session `Large--scale Structure and
Statistics' (DE3) at the `Fourteenth Marcel Grossman Meeting on General
Relativity' are related to these challenges.Comment: 17 pages; references added. Matches published version in Int. J. Mod.
Phys. D; Report on Parallel Session DE3 of MG1
Kundt spacetimes as solutions of topologically massive gravity
We obtain new solutions of topologically massive gravity. We find the general
Kundt solutions, which in three dimensions are spacetimes admitting an
expansion-free null geodesic congruence. The solutions are generically of
algebraic type II, but special cases are types III, N or D. Those of type D are
the known spacelike-squashed AdS_3 solutions, and of type N are the known AdS
pp-waves or new solutions. Those of types II and III are the first known
solutions of these algebraic types. We present explicitly the Kundt solutions
that are CSI spacetimes, for which all scalar polynomial curvature invariants
are constant, whereas for the general case we reduce the field equations to a
series of ordinary differential equations. The CSI solutions of types II and
III are deformations of spacelike-squashed AdS_3 and the round AdS_3,
respectively.Comment: 30 pages. This material has come from splitting v1 of arXiv:0906.3559
into 2 separate papers. v2: minor changes
Generalized Teleparallel de Sitter geometries
Theories of gravity based on teleparallel geometries are characterized by the
torsion, which is a function of the coframe, derivatives of the coframe, and a
zero curvature and metric compatible spin connection. The appropriate notion of
a symmetry in a teleparallel geometry is that of an affine symmetry. Due to the
importance of the de Sitter geometry and Einstein spaces within general
relativity, we shall describe teleparallel de Sitter geometries and discuss
their possible generalizations. In particular, we shall analyse a class of
Einstein teleparallel geometries which have a 4-dimensional Lie algebra of
affine symmetries, and display two one-parameter families of explicit exact
solutions.Comment: 25 pages, no figure, Submitted to EPJ
Food miles: time for a re-think?
PurposeThe purpose of this paper is to test the efficacy of the concept of food miles that has proved so popular with the public as a means of assessing the sustainability of produce.Design/methodology/approachThis paper uses data from a UK major food importer and retailer to correlate carbon emissions from transport, and transport‐related storage, with food miles by creating farm‐specific mode‐weighted emission factors.FindingsThe correlation is found to be poor for a wide range of products and locations and it is clear that the mode of transport is as important as the distance, with sourcing from parts of the Mediterranean resulting in emissions greater than those from the Americas.Practical implicationsIt is concluded that it is difficult to justify the use of food miles when attempting to influence purchasing behaviour. Because of this result, processes and tools have been developed that relay information on true transport‐related carbon emissions to customers and bulk purchasers that allow them to make informed decisions.Originality/valueThis paper questions the value of using the concept of food miles as a driving force for changing purchasing behaviour by either the customer or the purchasing department of a retailer.</jats:sec
The reliability of inverse modelling for the wide scale characterization of the thermal properties of buildings
The reduction of energy use in buildings is a major component of greenhouse gas mitigation policy and requires knowledge of the fabric and the occupant behaviour. Hence there has been a longstanding desire to use automatic means to identify these. Smart metres and the internet-of-things have the potential to do this. This paper describes a study where the ability of inverse modelling to identify building parameters is evaluated for 6 monitored real and 1000 simulated buildings. It was found that low-order models provide good estimates of heat transfer coefficients and internal temperatures if heating, electricity use and CO2 concentration are measured during the winter period. This implies that the method could be used with a small number of cheap sensors and enable the accurate assessment of buildings’ thermal properties, and therefore the impact of any suggested retrofit. This has the potential to be transformative for the energy efficiency industry.</p
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Restriction Spectrum Imaging Differentiates True Tumor Progression From Immune-Mediated Pseudoprogression: Case Report of a Patient With Glioblastoma.
Immunotherapy is increasingly used in the treatment of glioblastoma (GBM), with immune checkpoint therapy gaining in popularity given favorable outcomes achieved for other tumors. However, immune-mediated (IM)-pseudoprogression is common, remains poorly characterized, and renders conventional imaging of little utility when evaluating for treatment response. We present the case of a 64-year-old man with GBM who developed pathologically proven IM-pseudoprogression after initiation of a checkpoint inhibitor, and who subsequently developed true tumor progression at a distant location. Based on both qualitative and quantitative analysis, we demonstrate that an advanced diffusion-weighted imaging (DWI) technique called restriction spectrum imaging (RSI) can differentiate IM-pseudoprogression from true progression even when conventional imaging, including standard DWI/apparent diffusion coefficient (ADC), is not informative. These data complement existing literature supporting the ability of RSI to estimate tumor cellularity, which may help to resolve complex diagnostic challenges such as the identification of IM-pseudoprogression
Accessing the thermal mass above suspended ceilings via a perimeter gap:A CFD study of naturally ventilated spaces
Accessing the thermal mass above suspended ceilings via a perimeter gap:A CFD study of naturally ventilated spaces
Cosmology with positive and negative exponential potentials
We present a phase-plane analysis of cosmologies containing a scalar field
with an exponential potential
where and may be positive or negative. We show that
power-law kinetic-potential scaling solutions only exist for sufficiently flat
() negative
potentials. The latter correspond to a class of ever-expanding cosmologies with
negative potential. However we show that these expanding solutions with a
negative potential are to unstable in the presence of ordinary matter, spatial
curvature or anisotropic shear, and generic solutions always recollapse to a
singularity. Power-law kinetic-potential scaling solutions are the late-time
attractor in a collapsing universe for steep negative potentials (the ekpyrotic
scenario) and stable against matter, curvature or shear perturbations.
Otherwise kinetic-dominated solutions are the attractor during collapse (the
pre big bang scenario) and are only marginally stable with respect to
anisotropic shear.Comment: 8 pages, latex with revtex, 9 figure
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