68 research outputs found
Gradient instability for w < -1
We show that in single scalar field models of the dark energy with equations
of state satisfying , the effective Lagrangian for
fluctuations about the homogeneous background has a wrong sign spatial kinetic
term. In most cases, spatial gradients are ruled out by microwave background
observations. The sign of is not connected to the sign of the time
derivative kinetic term in the effective Lagrangian.Comment: revtex4, 8 pages, 1 figure. v2: corrected typo in Eq. 16, added
references and a paragraph on quintessence models; v3: reordering of
references. To appear in Phys. Lett.
Answering a Basic Objection to Bang/Crunch Holography
The current cosmic acceleration does not imply that our Universe is basically
de Sitter-like: in the first part of this work we argue that, by introducing
matter into *anti-de Sitter* spacetime in a natural way, one may be able to
account for the acceleration just as well. However, this leads to a Big Crunch,
and the Euclidean versions of Bang/Crunch cosmologies have [apparently]
disconnected conformal boundaries. As Maldacena and Maoz have recently
stressed, this seems to contradict the holographic principle. In the second
part we argue that this "double boundary problem" is a matter not of geometry
but rather of how one chooses a conformal compactification: if one chooses to
compactify in an unorthodox way, then the appearance of disconnectedness can be
regarded as a *coordinate effect*. With the kind of matter we have introduced
here, namely a Euclidean axion, the underlying compact Euclidean manifold has
an unexpectedly non-trivial topology: it is in fact one of the 75 possible
underlying manifolds of flat compact four-dimensional Euclidean spaces.Comment: 29 pages, 3 figures, added references and comparison with "cyclic"
cosmology, JHEP versio
Elasticity and Petri nets
Digital electronic systems typically use synchronous clocks and primarily assume fixed duration of their operations to simplify the design process. Time elastic systems can be constructed either by replacing the clock with communication handshakes (asynchronous version) or by augmenting the clock with a synchronous version of a handshake (synchronous version). Time elastic systems can tolerate static and dynamic changes in delays (asynchronous case) or latencies (synchronous case) of operations that can be used for modularity, ease of reuse and better power-delay trade-off. This paper describes methods for the modeling, performance analysis and optimization of elastic systems using Marked Graphs and their extensions capable of describing behavior with early evaluation. The paper uses synchronous elastic systems (aka latency-tolerant systems) for illustrating the use of Petri nets, however, most of the methods can be applied without changes (except changing the delay model associated with events of the system) to asynchronous elastic systems.Peer ReviewedPostprint (author's final draft
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
A thermodynamic motivation for dark energy
It is argued that the discovery of cosmic acceleration could have been
anticipated on thermodynamic grounds, namely, the generalized second law and
the approach to equilibrium at large scale factor. Therefore, the existence of
dark energy -or equivalently, some modified gravity theory- should have been
expected. In general, cosmological models that satisfy the above criteria show
compatibility with observational data.Comment: 22 pages, 7 eps figures; Key words: dark energy, thermodynamics,
modified gravity. Comments added and arguments sharpene
Genetic effects on gene expression across human tissues
Characterization of the molecular function of the human genome and its variation across individuals is essential for identifying the cellular mechanisms that underlie human genetic traits and diseases. The Genotype-Tissue Expression (GTEx) project aims to characterize variation in gene expression levels across individuals and diverse tissues of the human body, many of which are not easily accessible. Here we describe genetic effects on gene expression levels across 44 human tissues. We find that local genetic variation affects gene expression levels for the majority of genes, and we further identify inter-chromosomal genetic effects for 93 genes and 112 loci. On the basis of the identified genetic effects, we characterize patterns of tissue specificity, compare local and distal effects, and evaluate the functional properties of the genetic effects. We also demonstrate that multi-tissue, multi-individual data can be used to identify genes and pathways affected by human disease-associated variation, enabling a mechanistic interpretation of gene regulation and the genetic basis of diseas
Genetic effects on gene expression across human tissues
Characterization of the molecular function of the human genome and its variation across individuals is essential for identifying the cellular mechanisms that underlie human genetic traits and diseases. The Genotype-Tissue Expression (GTEx) project aims to characterize variation in gene expression levels across individuals and diverse tissues of the human body, many of which are not easily accessible. Here we describe genetic effects on gene expression levels across 44 human tissues. We find that local genetic variation affects gene expression levels for the majority of genes, and we further identify inter-chromosomal genetic effects for 93 genes and 112 loci. On the basis of the identified genetic effects, we characterize patterns of tissue specificity, compare local and distal effects, and evaluate the functional properties of the genetic effects. We also demonstrate that multi-tissue, multi-individual data can be used to identify genes and pathways affected by human disease-associated variation, enabling a mechanistic interpretation of gene regulation and the genetic basis of disease
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