625 research outputs found
Nonlinear Effects in the Cosmic Microwave Background
Major advances in the observation and theory of cosmic microwave background
anisotropies have opened up a new era in cosmology. This has encouraged the
hope that the fundamental parameters of cosmology will be determined to high
accuracy in the near future. However, this optimism should not obscure the
ongoing need for theoretical developments that go beyond the highly successful
but simplified standard model. Such developments include improvements in
observational modelling (e.g. foregrounds, non-Gaussian features), extensions
and alternatives to the simplest inflationary paradigm (e.g. non-adiabatic
effects, defects), and investigation of nonlinear effects. In addition to well
known nonlinear effects such as the Rees-Sciama and Ostriker-Vishniac effects,
further nonlinear effects have recently been identified. These include a
Rees-Sciama-type tensor effect, time-delay effects of scalar and tensor
lensing, nonlinear Thomson scattering effects and a nonlinear shear effect.
Some of the nonlinear effects and their potential implications are discussed.Comment: Invited contribution to Relativistic Cosmology Symposium (celebrating
the 60th year of GFR Ellis); to appear Gen. Rel. Gra
A computational framework to emulate the human perspective in flow cytometric data analysis
Background: In recent years, intense research efforts have focused on developing methods for automated flow cytometric data analysis. However, while designing such applications, little or no attention has been paid to the human perspective that is absolutely central to the manual gating process of identifying and characterizing cell populations. In particular, the assumption of many common techniques that cell populations could be modeled reliably with pre-specified distributions may not hold true in real-life samples, which can have populations of arbitrary shapes and considerable inter-sample variation.
<p/>Results: To address this, we developed a new framework flowScape for emulating certain key aspects of the human perspective in analyzing flow data, which we implemented in multiple steps. First, flowScape begins with creating a mathematically rigorous map of the high-dimensional flow data landscape based on dense and sparse regions defined by relative concentrations of events around modes. In the second step, these modal clusters are connected with a global hierarchical structure. This representation allows flowScape to perform ridgeline analysis for both traversing the landscape and isolating cell populations at different levels of resolution. Finally, we extended manual gating with a new capacity for constructing templates that can identify target populations in terms of their relative parameters, as opposed to the more commonly used absolute or physical parameters. This allows flowScape to apply such templates in batch mode for detecting the corresponding populations in a flexible, sample-specific manner. We also demonstrated different applications of our framework to flow data analysis and show its superiority over other analytical methods.
<p/>Conclusions: The human perspective, built on top of intuition and experience, is a very important component of flow cytometric data analysis. By emulating some of its approaches and extending these with automation and rigor, flowScape provides a flexible and robust framework for computational cytomics
A Texture Bestiary
Textures are topologically nontrivial field configurations which can exist in
a field theory in which a global symmetry group is broken to a subgroup
, if the third homotopy group \p3 of is nontrivial. We compute this
group for a variety of choices of and , revealing what symmetry breaking
patterns can lead to texture. We also comment on the construction of texture
configurations in the different models.Comment: 34 pages, plain Tex. (Minor corrections to an old paper.
Skewness in the Cosmic Microwave Background Anisotropy from Inflationary Gravity Wave Background
In the context of inflationary scenarios, the observed large angle anisotropy
of the Cosmic Microwave Background (CMB) temperature is believed to probe the
primordial metric perturbations from inflation. Although the perturbations from
inflation are expected to be gaussian random fields, there remains the
possibility that nonlinear processes at later epochs induce ``secondary''
non-gaussian features in the corresponding CMB anisotropy maps. The
non-gaussianity induced by nonlinear gravitational instability of scalar
(density) perturbations has been investigated in existing literature. In this
paper, we highlight another source of non-gaussianity arising out of higher
order scattering of CMB photons off the metric perturbations. We provide a
simple and elegant formalism for deriving the CMB temperature fluctuations
arising due to the Sachs-Wolfe effect beyond the linear order. In particular,
we derive the expression for the second order CMB temperature fluctuations. The
multiple scattering effect pointed out in this paper leads to the possibility
that tensor metric perturbation, i.e., gravity waves (GW) which do not exhibit
gravitational instability can still contribute to the skewness in the CMB
anisotropy maps. We find that in a flat universe, the skewness in
CMB contributed by gravity waves via multiple scattering effect is comparable
to that from the gravitational instability of scalar perturbations for equal
contribution of the gravity waves and scalar perturbations to the total rms CMB
anisotropy. The secondary skewness is found to be smaller than the cosmic
variance leading to the conclusion that inflationary scenarios do predict that
the observed CMB anisotropy should be statistically consistent with a gaussian
random distribution.Comment: 10 pages, Latex (uses revtex), 1 postscript figure included. Accepted
for publication in Physical Review
Cosmic optical activity in the spacetime of a scalar-tensor screwed cosmic string
Measurements of radio emission from distant galaxies and quasars verify that
the polarization vectors of these radiations are not randomly oriented as
naturally expected. This peculiar phenomenon suggests that the spacetime
intervening between the source and observer may be exhibiting some sort of
optical activity, the origin of which is not known. In the present paper we
provide a plausible explanation to this phenomenon by investigating the r\^ole
played by a Chern-Simons-like term in the background of an ordinary or
superconducting screwed cosmic string in a scalar-tensor gravity. We discuss
the possibility that the excess in polarization of the light from
radio-galaxies and quasars can be understood as if the electromagnetic waves
emitted by these cosmic objects interact with a scalar-tensor screwed cosmic
string through a Chern-Simons coupling. We use current astronomical data to
constrain possible values for the coupling constant of this theory, and show
that it turns out to be: eV, which is two orders of
magnitude larger than in string-inspired theories.Comment: Revised version, to appear in Phys. Rev.
Late Cenozoic Climate History of the Ross Embayment from the AND-1B Drill Hole: Culmination of Three Decades of Antarctic Margin Drilling
Because of the paucity of exposed rock, the direct physical
record of Antarctic Cenozoic glacial history has become
known only recently and then largely from offshore shelf
basins through seismic surveys and drilling. The number
of holes on the continental shelf has been small and largely
confined to three areas (McMurdo Sound, Prydz Bay, and
Antarctic Peninsula), but even in McMurdo Sound, where
Oligocene and early Miocene strata are well cored, the late
Cenozoic is poorly known and dated. The latest Antarctic
geological drilling program, ANDRILL, successfully cored
a 1285-m-long record of climate history spanning the last 13
m.y. from subsea-floor sediment beneath the McMurdo Ice
Shelf (MIS), using drilling systems specially developed for
operating through ice shelves. The cores provide the most
complete Antarctic record to date of ice-sheet and climate
fluctuations for this period of Earth’s history. The >60 cycles
of advance and retreat of the grounded ice margin preserved
in the AND-1B record the evolution of the Antarctic ice sheet
since a profound global cooling step in deep-sea oxygen
isotope records ~14 m.y.a. A feature of particular interest is a
~90-m-thick interval of diatomite deposited during the warm
Pliocene and representing an extended period (~200,000
years) of locally open water, high phytoplankton productivity,
and retreat of the glaciers on land
The Angular Trispectra of CMB Temperature and Polarization
We develop the formalism necessary to study four-point functions of the
cosmic microwave background (CMB) temperature and polarization fields. We
determine the general form of CMB trispectra, with the constraints imposed by
the assumption of statistical isotropy of the CMB fields, and derive
expressions for their estimators, as well as their Gaussian noise properties.
We apply these techniques to initial non-Gaussianity of a form motivated by
inflationary models. Due to the large number of four-point configurations, the
sensitivity of the trispectra to initial non-Gaussianity approaches that of the
temperature bispectrum at high multipole moment. These trispectra techniques
will also be useful in the study of secondary anisotropies induced for example
by the gravitational lensing of the CMB by the large scale structure of the
universe.Comment: 16 pages, 4 figures; typographical errors correcte
Astrometry and geodesy with radio interferometry: experiments, models, results
Summarizes current status of radio interferometry at radio frequencies
between Earth-based receivers, for astrometric and geodetic applications.
Emphasizes theoretical models of VLBI observables that are required to extract
results at the present accuracy levels of 1 cm and 1 nanoradian. Highlights the
achievements of VLBI during the past two decades in reference frames, Earth
orientation, atmospheric effects on microwave propagation, and relativity.Comment: 83 pages, 19 Postscript figures. To be published in Rev. Mod. Phys.,
Vol. 70, Oct. 199
Acoustic Signatures in the Primary Microwave Background Bispectrum
If the primordial fluctuations are non-Gaussian, then this non-Gaussianity
will be apparent in the cosmic microwave background (CMB) sky. With their
sensitive all-sky observation, MAP and Planck satellites should be able to
detect weak non-Gaussianity in the CMB sky. On large angular scale, there is a
simple relationship between the CMB temperature and the primordial curvature
perturbation. On smaller scales; however, the radiation transfer function
becomes more complex. In this paper, we present the angular bispectrum of the
primary CMB anisotropy that uses the full transfer function. We find that the
bispectrum has a series of acoustic peaks that change a sign, and a period of
acoustic oscillations is twice as long as that of the angular power spectrum.
Using a single non-linear coupling parameter to characterize the amplitude of
the bispectrum, we estimate the expected signal-to-noise ratio for COBE, MAP,
and Planck experiments. We find that the detection of the primary bispectrum by
any kind of experiments should be problematic for the simple slow-roll
inflationary scenarios. We compare the sensitivity of the primary bispectrum to
the primary skewness and conclude that when we can compute the predicted form
of the bispectrum, it becomes a ``matched filter'' for detecting the
non-Gaussianity in the data, and much more powerful tool than the skewness. We
also show that MAP and Planck can separate the primary bispectrum from various
secondary bispectra on the basis of the shape difference. The primary CMB
bispectrum is a test of the inflationary scenario, and also a probe of the
non-linear physics in the very early universe.Comment: Submitted to Physical Review D. (v1) letter version [4 pages, 3
figures]. (v2) full paper version including the primary skewness, secondary
bispectra, and the foreground separation [17 pages, 5 figures
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