9,683 research outputs found
The Physical State of the Intergalactic Medium or Can We Measure Y?
We present an argument for a {\it lower limit} to the Compton- parameter
describing spectral distortions of the cosmic microwave background (CMB). The
absence of a detectable Gunn-Peterson signal in the spectra of high redshift
quasars demands a high ionization state of the intergalactic medium (IGM).
Given an ionizing flux at the lower end of the range indicated by the proximity
effect, an IGM representing a significant fraction of the
nucleosynthesis-predicted baryon density must be heated by sources other than
the photon flux to a temperature \go {\rm few} \times 10^5\, K. Such a gas at
the redshift of the highest observed quasars, , will produce a y\go
10^{-6}. This lower limit on rises if the Universe is open, if there is a
cosmological constant, or if one adopts an IGM with a density larger than the
prediction of standard Big Bang nucleosynthesis.Comment: Proceedings of `Unveiling the Cosmic Infrared Background', April
23-25, 1995, Maryland. Self-unpacking uuencoded, compressed tar file with two
figures include
Kinematics of a Spacetime with an Infinite Cosmological Constant
A solution of the sourceless Einstein's equation with an infinite value for
the cosmological constant \Lambda is discussed by using Inonu-Wigner
contractions of the de Sitter groups and spaces. When \Lambda --> infinity,
spacetime becomes a four-dimensional cone, dual to Minkowski space by a
spacetime inversion. This inversion relates the four-cone vertex to the
infinity of Minkowski space, and the four-cone infinity to the Minkowski
light-cone. The non-relativistic limit c --> infinity is further considered,
the kinematical group in this case being a modified Galilei group in which the
space and time translations are replaced by the non-relativistic limits of the
corresponding proper conformal transformations. This group presents the same
abstract Lie algebra as the Galilei group and can be named the conformal
Galilei group. The results may be of interest to the early Universe Cosmology.Comment: RevTex, 7 pages, no figures. Presentation changes, including a new
Title. Version to appear in Found. Phys. Let
Time-Reversal Symmetry Breaking and Decoherence in Chaotic Dirac Billiards
In this work, we perform a statistical study on Dirac Billiards in the
extreme quantum limit (a single open channel on the leads). Our numerical
analysis uses a large ensemble of random matrices and demonstrates the
preponderant role of dephasing mechanisms in such chaotic billiards. Physical
implementations of these billiards range from quantum dots of graphene to
topological insulators structures. We show, in particular, that the role of
finite crossover fields between the universal symmetries quickly leaves the
conductance to the asymptotic limit of unitary ensembles. Furthermore, we show
that the dephasing mechanisms strikingly lead Dirac billiards from the extreme
quantum regime to the semiclassical Gaussian regime
Two novel evolutionary formulations of the graph coloring problem
We introduce two novel evolutionary formulations of the problem of coloring
the nodes of a graph. The first formulation is based on the relationship that
exists between a graph's chromatic number and its acyclic orientations. It
views such orientations as individuals and evolves them with the aid of
evolutionary operators that are very heavily based on the structure of the
graph and its acyclic orientations. The second formulation, unlike the first
one, does not tackle one graph at a time, but rather aims at evolving a
`program' to color all graphs belonging to a class whose members all have the
same number of nodes and other common attributes. The heuristics that result
from these formulations have been tested on some of the Second DIMACS
Implementation Challenge benchmark graphs, and have been found to be
competitive when compared to the several other heuristics that have also been
tested on those graphs.Comment: To appear in Journal of Combinatorial Optimizatio
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