10,116 research outputs found
Generalized Chaplygin gas with and the cosmological model
The generalized Chaplygin gas model is characterized by the equation of state
. It is generally stated that the case is equivalent to a model with cosmological constant and dust (). In this work we show that, if this is true for the background equations,
this is not true for the perturbation equations. Hence, the mass spectrum
predicted for both models may differ.Comment: Latex file, 4 pages, 2 figures in eps forma
Interoperability and Standards: The Way for Innovative Design in Networked Working Environments
Organised by: Cranfield UniversityIn todayâs networked economy, strategic business partnerships and outsourcing has become the dominant
paradigm where companies focus on core competencies and skills, as creative design, manufacturing, or
selling. However, achieving seamless interoperability is an ongoing challenge these networks are facing,
due to their distributed and heterogeneous nature. Part of the solution relies on adoption of standards for
design and product data representation, but for sectors predominantly characterized by SMEs, such as the
furniture sector, implementations need to be tailored to reduce costs. This paper recommends a set of best
practices for the fast adoption of the ISO funStep standard modules and presents a framework that enables
the usage of visualization data as a way to reduce costs in manufacturing and electronic catalogue design.Mori Seiki â The Machine Tool Compan
Different faces of the phantom
The SNe type Ia data admit that the Universe today may be dominated by some
exotic matter with negative pressure violating all energy conditions. Such
exotic matter is called {\it phantom matter} due to the anomalies connected
with violation of the energy conditions. If a phantom matter dominates the
matter content of the universe, it can develop a singularity in a finite future
proper time. Here we show that, under certain conditions, the evolution of
perturbations of this matter may lead to avoidance of this future singularity
(the Big Rip). At the same time, we show that local concentrations of a phantom
field may form, among other regular configurations, black holes with
asymptotically flat static regions, separated by an event horizon from an
expanding, singularity-free, asymptotically de Sitter universe.Comment: 6 pages, presented at IRGAC 2006, Barcelona, 11-15 July 200
No-horizon theorem for spacetimes with spacelike G1 isometry groups
We consider four-dimensional spacetimes which obey the
Einstein equations , and admit a global spacelike
isometry group. By means of dimensional reduction and local
analyis on the reduced (2+1) spacetime, we obtain a sufficient condition on
which guarantees that cannot contain apparent
horizons. Given any (3+1) spacetime with spacelike translational isometry, the
no-horizon condition can be readily tested without the need for dimensional
reduction. This provides thus a useful and encompassing apparent horizon test
for -symmetric spacetimes. We argue that this adds further evidence
towards the validity of the hoop conjecture, and signals possible violations of
strong cosmic censorship.Comment: 8 pages, LaTeX, uses IOP package; published in Class. Quantum Gra
Cosmic homogeneity: a spectroscopic and model-independent measurement
Cosmology relies on the Cosmological Principle, i.e., the hypothesis that the
Universe is homogeneous and isotropic on large scales. This implies in
particular that the counts of galaxies should approach a homogeneous scaling
with volume at sufficiently large scales. Testing homogeneity is crucial to
obtain a correct interpretation of the physical assumptions underlying the
current cosmic acceleration and structure formation of the Universe. In this
Letter, we use the Baryon Oscillation Spectroscopic Survey to make the first
spectroscopic and model-independent measurements of the angular homogeneity
scale . Applying four statistical estimators, we show that the
angular distribution of galaxies in the range 0.46 < z < 0.62 is consistent
with homogeneity at large scales, and that varies with
redshift, indicating a smoother Universe in the past. These results are in
agreement with the foundations of the standard cosmological paradigm.Comment: 5 pages, 2 figures, Version accepted by MNRA
Perturbative analysis of generalized Einstein's theories
The hypothesis that the energy-momentum tensor of ordinary matter is not
conserved separately, leads to a non-adiabatic expansion and, in many cases, to
an Universe older than usual. This may provide a solution for the entropy and
age problems of the Standard Cosmological Model. We consider two different
theories of this type, and we perform a perturbative analysis, leading to
analytical expressions for the evolution of gravitational waves, rotational
modes and density perturbations. One of these theories exhibits satisfactory
properties at this level, while the other one should be discarded.Comment: 14 pages, Latex fil
Scaling limits of a tagged particle in the exclusion process with variable diffusion coefficient
We prove a law of large numbers and a central limit theorem for a tagged
particle in a symmetric simple exclusion process in the one-dimensional lattice
with variable diffusion coefficient. The scaling limits are obtained from a
similar result for the current through -1/2 for a zero-range process with bond
disorder. For the CLT, we prove convergence to a fractional Brownian motion of
Hurst exponent 1/4.Comment: 9 page
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