12,325 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
Fundamental Plane of Sunyaev-Zeldovich clusters
Sunyaev-Zel'dovich (SZ) cluster surveys are considered among the most
promising methods for probing dark energy up to large redshifts. However, their
premise is hinged upon an accurate mass-observable relationship, which could be
affected by the (rather poorly understood) physics of the intracluster gas. In
this letter, using a semi-analytic model of the intracluster gas that
accommodates various theoretical uncertainties, I develop a Fundamental Plane
relationship between the observed size, thermal energy, and mass of galaxy
clusters. In particular, I find that M ~ (Y_{SZ}/R_{SZ,2})^{3/4}, where M is
the mass, Y_{SZ} is the total SZ flux or thermal energy, and R_{SZ,2} is the SZ
half-light radius of the cluster. I first show that, within this model, using
the Fundamental Plane relationship reduces the (systematic+random) errors in
mass estimates to 14%, from 22% for a simple mass-flux relationship. Since
measurement of the cluster sizes is an inevitable part of observing the SZ
clusters, the Fundamental Plane relationship can be used to reduce the error of
the cluster mass estimates by ~ 34%, improving the accuracy of the resulting
cosmological constraints without any extra cost. I then argue why our
Fundamental Plane is distinctly different from the virial relationship that one
may naively expect between the cluster parameters. Finally, I argue that while
including more details of the observed SZ profile cannot significantly improve
the accuracy of mass estimates, a better understanding of the impact of
non-gravitational heating/cooling processes on the outskirts of the
intracluster medium (apart from external calibrations) might be the best way to
reduce these errors.Comment: 5 pages, 1 figure, added an analytic derivation of the Fundametal
Plane relation (which is distinctly different from the virial relation),
submitted to Ap
Topics in Born-Infeld Electrodynamics
Classical version of Born-Infeld electrodynamics is recalled and its most
important properties discussed. Then we analyze possible abelian and
non-abelian generalizations of this theory, and show how certain soliton-like
configurations can be obtained. The relationship with the Standard Model of
electroweak interactions is also mentioned.Comment: (One new reference added). 15 pages, LaTeX. To be published in the
Proceedings of XXXVII Karpacz Winter School edited in the Proceedings Series
of American Mathematical Society, editors J. Lukierski and J. Rembielinsk
A Statistical Strategy for the Sunyaev-Zel'dovich Effect's Cluster Data
We present a statistical strategy for the efficient determination of the
cluster luminosity function from the Sunyaev-Zel'dovich (SZ) effects survey. To
determine the cluster luminosity function from the noise contaminated SZ map,
we first define the zeroth-order cluster luminosity function as a discrepancy
between the measured peak number density of the SZ map and the mean number
density of noise. Then we demonstrate that the noise contamination effects can
be removed by the stabilized deconvolution of the zeroth-order cluster
luminosity function with the one-dimensional Gaussian distribution. We test
this analysis technique against Monte-Carlo simulations, and find that it works
quite well especially in the medium amplitude range where the conventional
cluster identification method based on the threshold cut-off usually fails.Comment: final version, accepted by ApJ Letter
Cosmic microwave background constraints on the epoch of reionization
We use a compilation of cosmic microwave anisotropy data to constrain the
epoch of reionization in the Universe, as a function of cosmological
parameters. We consider spatially-flat cosmologies, varying the matter density
(the flatness being restored by a cosmological constant), the Hubble
parameter and the spectral index of the primordial power spectrum. Our
results are quoted both in terms of the maximum permitted optical depth to the
last-scattering surface, and in terms of the highest allowed reionization
redshift assuming instantaneous reionization. For critical-density models,
significantly-tilted power spectra are excluded as they cannot fit the current
data for any amount of reionization, and even scale-invariant models must have
an optical depth to last scattering of below 0.3. For the currently-favoured
low-density model with and a cosmological constant, the
earliest reionization permitted to occur is at around redshift 35, which
roughly coincides with the highest estimate in the literature. We provide
general fitting functions for the maximum permitted optical depth, as a
function of cosmological parameters. We do not consider the inclusion of tensor
perturbations, but if present they would strengthen the upper limits we quote.Comment: 9 pages LaTeX file with ten figures incorporated (uses mn.sty and
epsf). Corrects some equation typos, superseding published versio
Fast and secure key distribution using mesoscopic coherent states of light
This work shows how two parties A and B can securely share sequences of
random bits at optical speeds. A and B possess true-random physical sources and
exchange random bits by using a random sequence received to cipher the
following one to be sent. A starting shared secret key is used and the method
can be described as an unlimited one-time-pad extender. It is demonstrated that
the minimum probability of error in signal determination by the eavesdropper
can be set arbitrarily close to the pure guessing level. Being based on the
-ry encryption protocol this method also allows for optical amplification
without security degradation, offering practical advantages over the BB84
protocol for key distribution.Comment: 11 pages and 4 figures. This version updates the one published in PRA
68, 052307 (2003). Minor changes were made in the text and one section on
Mutual Information was adde
Network conduciveness with application to the graph-coloring and independent-set optimization transitions
We introduce the notion of a network's conduciveness, a probabilistically
interpretable measure of how the network's structure allows it to be conducive
to roaming agents, in certain conditions, from one portion of the network to
another. We exemplify its use through an application to the two problems in
combinatorial optimization that, given an undirected graph, ask that its
so-called chromatic and independence numbers be found. Though NP-hard, when
solved on sequences of expanding random graphs there appear marked transitions
at which optimal solutions can be obtained substantially more easily than right
before them. We demonstrate that these phenomena can be understood by resorting
to the network that represents the solution space of the problems for each
graph and examining its conduciveness between the non-optimal solutions and the
optimal ones. At the said transitions, this network becomes strikingly more
conducive in the direction of the optimal solutions than it was just before
them, while at the same time becoming less conducive in the opposite direction.
We believe that, besides becoming useful also in other areas in which network
theory has a role to play, network conduciveness may become instrumental in
helping clarify further issues related to NP-hardness that remain poorly
understood
Expectations For an Interferometric Sunyaev-Zel'dovich Effect Survey for Galaxy Clusters
Non-targeted surveys for galaxy clusters using the Sunyaev-Zel'dovich effect
(SZE) will yield valuable information on both cosmology and evolution of the
intra-cluster medium (ICM). The redshift distribution of detected clusters will
constrain cosmology, while the properties of the discovered clusters will be
important for studies of the ICM and galaxy formation. Estimating survey yields
requires a detailed model for both cluster properties and the survey strategy.
We address this by making mock observations of galaxy clusters in cosmological
hydrodynamical simulations. The mock observatory consists of an interferometric
array of ten 2.5 m diameter telescopes, operating at a central frequency of 30
GHz with a bandwidth of 8 GHz. We find that clusters with a mass above will be detected at any redshift, with the
exact limit showing a very modest redshift dependence. Using a Press-Schechter
prescription for evolving the number densities of clusters with redshift, we
determine that such a survey should find hundreds of galaxy clusters per year,
many at high redshifts and relatively low mass -- an important regime uniquely
accessible to SZE surveys. Currently favored cosmological models predict
roughly 25 clusters per square degree.Comment: revised to match published versio
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