75 research outputs found
SCML: A Structural Representation for Chinese Characters
Chinese characters are used daily by well over a billion people. They constitute the main writing system of China and Taiwan, form a major part of written Japanese, and are also used in South Korea. Anything more than a cursory glance at these characters will reveal a high degree of structure to them, but computing systems do not currently have a means to operate on this structure. Existing character databases and dictionaries treat them as numerical code points, and associate with them additional `hand-computed\u27 data, such as stroke count, stroke order, and other information to aid in specific searches. Searching by a character\u27s `shape\u27 is effectively impossible in these systems. I propose a new approach to representing these characters, through an XML-based language called SCML. This language, by encoding an abstract form of a character, allows the direct retrieval of important information such as stroke count and stroke order, and permits useful but previously impossible automated analysis of characters. In addition, the system allows the design of a view that takes abstract SCML representations as character models and outputs glyphs based on an aesthetic, facilitating the creation of `meta-fonts\u27 for Chinese characters. Finally, through the creation of a specialized database, SCML allows for efficient structural character queries to be performed against the body of inserted characters, thus allowing people to search by the most obvious of a character\u27s characteristics: its shape
Free streaming in mixed dark matter
Free streaming in a \emph{mixture} of collisionless non-relativistic dark
matter (DM) particles is studied by implementing methods from the theory of
multicomponent plasmas. The mixture includes Fermionic, condensed and non
condensed Bosonic particles decoupling in equilibrium while relativistic, heavy
non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple
\emph{out of equilibrium} when they are relativistic. The free-streaming length
is obtained from the marginal zero of the gravitational
polarization function, which separates short wavelength Landau-damped from long
wavelength Jeans-unstable \emph{collective} modes. At redshift we find ,where are the \emph{fractions} of the respective DM components of mass
that decouple when the effective number of ultrarelativistic degrees of
freedom is , and only depend on the distribution functions at
decoupling, given explicitly in all cases. If sterile neutrinos produced either
resonantly or non-resonantly that decouple near the QCD scale are the
\emph{only} DM component,we find (non-resonant), (resonant).If WIMPs with
decoupling at are present in the mixture with
, is \emph{dominated} by CDM. If a Bose Einstein condensate is a DM
component its free streaming length is consistent with CDM because of the
infrared enhancement of the distribution function.Comment: 19 pages, 2 figures. More discussions same conclusions and results.
Version to appear in Phys. Rev.
Warm dark matter at small scales: peculiar velocities and phase space density
We study the scale and redshift dependence of the power spectra for density
perturbations and peculiar velocities, and the evolution of a coarse grained
phase space density for (WDM) particles that decoupled during the radiation
dominated stage. The (WDM) corrections are obtained in a perturbative expansion
valid in the range of redshifts at which N-body simulations set up initial
conditions, and for a wide range of scales. The redshift dependence is
determined by the kurtosis of the distribution function at
decoupling. At large redshift there is an enhancement of peculiar velocities
for that contributes to free streaming and leads to further
suppression of the matter power spectrum and an enhancement of the peculiar
velocity autocorrelation function at scales smaller than the free streaming
scale. Statistical fluctuations of peculiar velocities are also suppressed on
these scales by the same effect. In the linearized approximation, the coarse
grained phase space density features redshift dependent (WDM) corrections from
gravitational perturbations determined by the power spectrum of density
perturbations and . For it \emph{grows
logarithmically} with the scale factor as a consequence of the suppression of
statistical fluctuations. Two specific models for WDM are studied in detail.
The (WDM) corrections relax the bounds on the mass.Comment: 22 pages, 9 figs, more explanations. Published versio
Controlling Chaos through Compactification in Cosmological Models with a Collapsing Phase
We consider the effect of compactification of extra dimensions on the onset
of classical chaotic "Mixmaster" behavior during cosmic contraction. Assuming a
universe that is well-approximated as a four-dimensional
Friedmann-Robertson--Walker model (with negligible Kaluza-Klein excitations)
when the contraction phase begins, we identify compactifications that allow a
smooth contraction and delay the onset of chaos until arbitrarily close the big
crunch. These compactifications are defined by the de Rham cohomology (Betti
numbers) and Killing vectors of the compactification manifold. We find
compactifications that control chaos in vacuum Einstein gravity, as well as in
string theories with N = 1 supersymmetry and M-theory. In models where chaos is
controlled in this way, the universe can remain homogeneous and flat until it
enters the quantum gravity regime. At this point, the classical equations
leading to chaotic behavior can no longer be trusted, and quantum effects may
allow a smooth approach to the big crunch and transition into a subsequent
expanding phase. Our results may be useful for constructing cosmological models
with contracting phases, such as the ekpyrotic/cyclic and pre-big bang models.Comment: 1 figure. v2/v3: minor typos correcte
Some FRW Models of Accelerating Universe with Dark Energy
The paper deals with a spatially homogeneous and isotropic FRW space-time
filled with perfect fluid and dark energy components. The two sources are
assumed to interact minimally, and therefore their energy momentum tensors are
conserved separately. A special law of variation for the Hubble parameter
proposed by Berman (1983) has been utilized to solve the field equations. The
Berman's law yields two explicit forms of the scale factor governing the FRW
space-time and constant values of deceleration parameter. The role of dark
energy with variable equation of state parameter has been studied in detail in
the evolution of FRW universe. It has been found that dark energy dominates the
universe at the present epoch, which is consistent with the observations. The
physical behavior of the universe is discussed in detail.Comment: 10 pages, 5 figure
The Structure of Structure Formation Theories
We study the general structure of models for structure formation, with
applications to the reverse engineering of the model from observations. Through
a careful accounting of the degrees of freedom in covariant gravitational
instability theory, we show that the evolution of structure is completely
specified by the stress history of the dark sector. The study of smooth,
entropic, sonic, scalar anisotropic, vector anisotropic, and tensor anisotropic
stresses reveals the origin, robustness, and uniqueness of specific model
phenomenology. We construct useful and illustrative analytic solutions that
cover cases with multiple species of differing equations of state relevant to
the current generation of models, especially those with effectively smooth
components. We present a simple case study of models with phenomenologies
similar to that of a LambdaCDM model to highlight reverse-engineering issues. A
critical-density universe dominated by a single type of dark matter with the
appropriate stress history can mimic a LambdaCDM model exactly.Comment: 31 pages, 18 figures, RevTeX, submitted to Phys. Rev.
Density pertubation of unparticle dark matter in the flat Universe
The unparticle has been suggested as a candidate of dark matter. We
investigated the growth rate of the density perturbation for the unparticle
dark matter in the flat Universe. First, we consider the model in which
unparticle is the sole dark matter and find that the growth factor can be
approximated well by , where is
the equation of state of unparticle. Our results show that the presence of
modifies the behavior of the growth factor . For the second model
where unparticle co-exists with cold dark matter, the growth factor has a new
approximation and
is a function of . Thus the growth factor of unparticle is quite
different from that of usual dark matter. These information can help us know
more about unparticle and the early evolution of the Universe.Comment: 6pages, 4 figures, accepted for publication in Eur. Phys. J.
Gravitational waves in the presence of a cosmological constant
We derive the effects of a non-zero cosmological constant on
gravitational wave propagation in the linearized approximation of general
relativity. In this approximation we consider the situation where the metric
can be written as , , where is
the background perturbation and is a modification
interpretable as a gravitational wave. For this linearization
of Einstein equations is self-consistent only in certain coordinate systems.
The cosmological Friedmann-Robertson-Walker coordinates do not belong to this
class and the derived linearized solutions have to be reinterpreted in a
coordinate system that is homogeneous and isotropic to make contact with
observations. Plane waves in the linear theory acquire modifications of order
, both in the amplitude and the phase, when considered in FRW
coordinates. In the linearization process for , we have also
included terms of order . For the background
perturbation the difference is very small but when the
term is retained the equations of motion can be
interpreted as describing massive spin-2 particles. However, the extra degrees
of freedom can be approximately gauged away, coupling to matter sources with a
strength proportional to the cosmological constant itself. Finally we discuss
the viability of detecting the modifications caused by the cosmological
constant on the amplitude and phase of gravitational waves. In some cases the
distortion with respect to gravitational waves propagating in Minkowski
space-time is considerable. The effect of could have a detectable
impact on pulsar timing arrays.Comment: 20 pages, 1 figur
Observationally Determining the Properties of Dark Matter
Determining the properties of the dark components of the universe remains one
of the outstanding challenges in cosmology. We explore how upcoming CMB
anisotropy measurements, galaxy power spectrum data, and supernova (SN)
distance measurements can observationally constrain their gravitational
properties with minimal assumptions on the theoretical side. SN observations
currently suggest the existence of dark matter with an exotic equation of state
p/rho < -1/3 that accelerates the expansion of the universe. When combined with
CMB anisotropy measurements, SN or galaxy survey data can in principle
determine the equation of state and density of this component separately,
regardless of their value, as long as the universe is spatially flat. Combining
these pairs creates a sharp consistency check. If p/rho > -1/2, then the
clustering behavior (sound speed) of the dark component can be determined so as
to test the scalar-field ``quintessence'' hypothesis. If the exotic matter
turns out instead to be simply a cosmological constant (p/rho = -1), the
combination of CMB and galaxy survey data should provide a significant
detection of the remaining dark matter, the neutrino background radiation
(NBR). The gross effect of its density or temperature on the expansion rate is
ill-constrained as it is can be mimicked by a change in the matter density.
However, anisotropies of the NBR break this degeneracy and should be detectable
by upcoming experiments.Comment: 16 pages, 10 figures, RevTeX, submitted to PR
The Clustering of Luminous Red Galaxies in the Sloan Digital Sky Survey Imaging Data
We present the 3D real space clustering power spectrum of a sample of
\~600,000 luminous red galaxies (LRGs) measured by the Sloan Digital Sky Survey
(SDSS), using photometric redshifts. This sample of galaxies ranges from
redshift z=0.2 to 0.6 over 3,528 deg^2 of the sky, probing a volume of 1.5
(Gpc/h)^3, making it the largest volume ever used for galaxy clustering
measurements. We measure the angular clustering power spectrum in eight
redshift slices and combine these into a high precision 3D real space power
spectrum from k=0.005 (h/Mpc) to k=1 (h/Mpc). We detect power on gigaparsec
scales, beyond the turnover in the matter power spectrum, on scales
significantly larger than those accessible to current spectroscopic redshift
surveys. We also find evidence for baryonic oscillations, both in the power
spectrum, as well as in fits to the baryon density, at a 2.5 sigma confidence
level. The statistical power of these data to constrain cosmology is ~1.7 times
better than previous clustering analyses. Varying the matter density and baryon
fraction, we find \Omega_M = 0.30 \pm 0.03, and \Omega_b/\Omega_M = 0.18 \pm
0.04, The detection of baryonic oscillations also allows us to measure the
comoving distance to z=0.5; we find a best fit distance of 1.73 \pm 0.12 Gpc,
corresponding to a 6.5% error on the distance. These results demonstrate the
ability to make precise clustering measurements with photometric surveys
(abridged).Comment: 23 pages, 27 figures, submitted to MNRA
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