233 research outputs found
Curvature-driven Molecular Demixing in the Budding and Breakup of Mixed Component Worm-like Miscelles
Amphiphilic block copolymers of suitable proportions can self-assemble into surprisingly long and stable worm-like micelles, but the intrinsic polydispersity of polymers as well as polymer blending efforts and the increasing use of degradable chains all raise basic questions of curvature–composition coupling and morphological stability of these high curvature assemblies. Molecular simulations here of polyethylene glycol (PEG) based systems show that a systematic increase in the hydrated PEG fraction, in both monodisperse and binary blends, induces budding and breakup into spherical and novel ‘dumbbell’ micelles—as seen in electron microscopy images of degradable worm-like micelles. Core dimension, d, in our large-scale, long-time dissipative particle dynamics (DPD) simulations is shown to scale with chain-length, N, as predicted theoretically by the strong segregation limit (d ≈ N2/3), but morphological transitions of binary mixtures are only crudely predicted by simple mixture rules. Here we show that for weakly demixing diblock copolymers, the coupling between local interfacial concentration and mean curvature can be described with a simple linear relationship. The computational methods developed here for PEG-based assemblies should be useful for many high curvature nanosystems
Magnification-Temperature Correlation: the Dark Side of ISW Measurements
Integrated Sachs-Wolfe (ISW) measurements, which involve cross-correlating
the CMB with the foreground large-scale structure (e.g. galaxies/quasars), have
proven to be an interesting probe of dark energy. We show that magnification
bias, which is the inevitable modulation of the foreground number counts by
gravitational lensing, alters both the shape and amplitude of the observed ISW
signal. This is true especially at high redshifts because (1) the intrinsic
galaxy-temperature signal diminishes greatly back in the matter dominated era,
(2) the lensing efficiency increases with redshift and (3) the number count
slope generally steepens with redshift in a magnitude limited sample. At z >~
2, the magnification-temperature correlation dominates over the intrinsic
galaxy-temperature correlation and causes the observed ISW signal to increase
with z, despite dark energy subdominance -- a result of the fact that
magnification probes structures between the observer and the sources. Ignoring
magnification bias can then lead to erroneous conclusions about dark energy.
While the lensing modulation opens up an interesting high z window for ISW
measurements, high z measurements are not expected to add much new information
to low z ones if dark energy is the cosmological constant. This is because
lensing introduces significant covariance across redshifts. The most compelling
reason to pursue high z ISW measurements is to look for a potential surprise
such as early dark energy domination or the signature of modified gravity. We
conclude with a discussion of existing measurements, the highest z of which is
at the margin of being sensitive to magnification bias. We also develop a
formalism which might be of general interest: to predict biases in estimating
parameters when certain physical effects are ignored in interpreting data.Comment: 14 pages, 12 figures, references added, minor typos corrected,
accepted for publication by PR
Local stochastic non-Gaussianity and N-body simulations
Large-scale clustering of highly biased tracers of large-scale structure has
emerged as one of the best observational probes of primordial non-Gaussianity
of the local type (i.e. f_{NL}^{local}). This type of non-Gaussianity can be
generated in multifield models of inflation such as the curvaton model.
Recently, Tseliakhovich, Hirata, and Slosar showed that the clustering
statistics depend qualitatively on the ratio of inflaton to curvaton power \xi
after reheating, a free parameter of the model. If \xi is significantly
different from zero, so that the inflaton makes a non-negligible contribution
to the primordial adiabatic curvature, then the peak-background split ansatz
predicts that the halo bias will be stochastic on large scales. In this paper,
we test this prediction in N-body simulations. We find that large-scale
stochasticity is generated, in qualitative agreement with the prediction, but
that the level of stochasticity is overpredicted by ~30%. Other predictions,
such as \xi independence of the halo bias, are confirmed by the simulations.
Surprisingly, even in the Gaussian case we do not find that halo model
predictions for stochasticity agree consistently with simulations, suggesting
that semi-analytic modeling of stochasticity is generally more difficult than
modeling halo bias.Comment: v3: minor changes matching published versio
Moment transport equations for the primordial curvature perturbation
In a recent publication, we proposed that inflationary perturbation theory
can be reformulated in terms of a probability transport equation, whose moments
determine the correlation properties of the primordial curvature perturbation.
In this paper we generalize this formulation to an arbitrary number of fields.
We deduce ordinary differential equations for the evolution of the moments of
zeta on superhorizon scales, which can be used to obtain an evolution equation
for the dimensionless bispectrum, fNL. Our equations are covariant in field
space and allow identification of the source terms responsible for evolution of
fNL. In a model with M scalar fields, the number of numerical integrations
required to obtain solutions of these equations scales like O(M^3). The
performance of the moment transport algorithm means that numerical calculations
with M >> 1 fields are straightforward. We illustrate this performance with a
numerical calculation of fNL in Nflation models containing M ~ 10^2 fields,
finding agreement with existing analytic calculations. We comment briefly on
extensions of the method beyond the slow-roll approximation, or to calculate
higher order parameters such as gNL.Comment: 23 pages, plus appendices and references; 4 figures. v2: incorrect
statements regarding numerical delta N removed from Sec. 4.3. Minor
modifications elsewher
Scale Dependence of the Halo Bias in General Local-Type Non-Gaussian Models I: Analytical Predictions and Consistency Relations
We investigate the clustering of halos in cosmological models starting with
general local-type non-Gaussian primordial fluctuations. We employ multiple
Gaussian fields and add local-type non-Gaussian corrections at arbitrary order
to cover a class of models described by frequently-discussed f_nl, g_nl and
\tau_nl parameterization. We derive a general formula for the halo power
spectrum based on the peak-background split formalism. The resultant spectrum
is characterized by only two parameters responsible for the scale-dependent
bias at large scale arising from the primordial non-Gaussianities in addition
to the Gaussian bias factor. We introduce a new inequality for testing
non-Gaussianities originating from multi fields, which is directly accessible
from the observed power spectrum. We show that this inequality is a
generalization of the Suyama-Yamaguchi inequality between f_nl and \tau_nl to
the primordial non-Gaussianities at arbitrary order. We also show that the
amplitude of the scale-dependent bias is useful to distinguish the simplest
quadratic non-Gaussianities (i.e., f_nl-type) from higher-order ones (g_nl and
higher), if one measures it from multiple species of galaxies or clusters of
galaxies. We discuss the validity and limitations of our analytic results by
comparison with numerical simulations in an accompanying paper.Comment: 25 pages, 3 figures, typo corrected, Appendix C updated, submitted to
JCA
Scale-dependent bias from the primordial non-Gaussianity with a Gaussian-squared field
We investigate the halo bias in the case where the primordial curvature
fluctuations, , are sourced from both a Gaussian random field and a
Gaussian-squared field, as , so-called "ungaussiton model". We employ the
peak-background split formula and find a new scale-dependence in the halo bias
induced from the Gaussian-squared field.Comment: 9 pages, 1 figure, comments are welcom
3D Photometric Cosmic Shear
Here we present a number of improvements to weak lensing 3D power spectrum
analysis, 3D cosmic shear, that uses the shape and redshift information of
every galaxy to constrain cosmological parameters. We show how photometric
redshift probability distributions for individual galaxies can be directly
included in this statistic with no averaging. We also include the Limber
approximation, considerably simplifying full 3D cosmic shear analysis, and we
investigate its range of applicability. Finally we show the relationship
between weak lensing tomography and the 3D cosmic shear field itself; the steps
connecting them being the Limber approximation, a harmonic-space transform and
a discretisation in wavenumber. Each method has its advantages: 3D cosmic shear
analysis allows straightforward inclusion of all relevant modes, thus ensuring
minimum error bars, and direct control of the range of physical wavenumbers
probed, to avoid the uncertain highly nonlinear regime. On the other hand,
tomography is more convenient for checking systematics through direct
investigation of the redshift dependence of the signal. Finally, for
tomography, we suggest that the angular modes probed should be
redshift-dependent, to recover some of the 3D advantages.Comment: Accepted to MNRAS. 15 pages, 7 figure
Effects and Detectability of Quasi-Single Field Inflation in the Large-Scale Structure and Cosmic Microwave Background
Quasi-single field inflation predicts a peculiar momentum dependence in the
squeezed limit of the primordial bispectrum which smoothly interpolates between
the local and equilateral models. This dependence is directly related to the
mass of the isocurvatons in the theory which is determined by the
supersymmetry. Therefore, in the event of detection of a non-zero primordial
bispectrum, additional constraints on the parameter controlling the
momentum-dependence in the squeezed limit becomes an important question. We
explore the effects of these non-Gaussian initial conditions on large-scale
structure and the cosmic microwave background, with particular attention to the
galaxy power spectrum at large scales and scale-dependence corrections to
galaxy bias. We determine the simultaneous constraints on the two parameters
describing the QSF bispectrum that we can expect from upcoming large-scale
structure and cosmic microwave background observations. We find that for
relatively large values of the non-Gaussian amplitude parameters, but still
well within current uncertainties, galaxy power spectrum measurements will be
able to distinguish the QSF scenario from the predictions of the local model. A
CMB likelihood analysis, as well as Fisher matrix analysis, shows that there is
also a range of parameter values for which Planck data may be able distinguish
between QSF models and the related local and equilateral shapes. Given the
different observational weightings of the CMB and LSS results, degeneracies can
be significantly reduced in a joint analysis.Comment: 27 pages, 14 figure
Relativistic effects and primordial non-Gaussianity in the galaxy bias
When dealing with observables, one needs to generalize the bias relation
between the observed galaxy fluctuation field to the underlying matter
distribution in a gauge-invariant way. We provide such relation at second-order
in perturbation theory adopting the local Eulerian bias model and starting from
the observationally motivated uniform-redshift gauge. Our computation includes
the presence of primordial non-Gaussianity. We show that large scale-dependent
relativistic effects in the Eulerian bias arise independently from the presence
of some primordial non-Gaussianity. Furthermore, the Eulerian bias inherits
from the primordial non-Gaussianity not only a scale-dependence, but also a
modulation with the angle of observation when sources with different biases are
correlated.Comment: 12 pages, LaTeX file; version accepted for publication in JCA
An Improved Calculation of the Non-Gaussian Halo Mass Function
The abundance of collapsed objects in the universe, or halo mass function, is
an important theoretical tool in studying the effects of primordially generated
non-Gaussianities on the large scale structure. The non-Gaussian mass function
has been calculated by several authors in different ways, typically by
exploiting the smallness of certain parameters which naturally appear in the
calculation, to set up a perturbative expansion. We improve upon the existing
results for the mass function by combining path integral methods and saddle
point techniques (which have been separately applied in previous approaches).
Additionally, we carefully account for the various scale dependent combinations
of small parameters which appear. Some of these combinations in fact become of
order unity for large mass scales and at high redshifts, and must therefore be
treated non-perturbatively. Our approach allows us to do this, and to also
account for multi-scale density correlations which appear in the calculation.
We thus derive an accurate expression for the mass function which is based on
approximations that are valid over a larger range of mass scales and redshifts
than those of other authors. By tracking the terms ignored in the analysis, we
estimate theoretical errors for our result and also for the results of others.
We also discuss the complications introduced by the choice of smoothing filter
function, which we take to be a top-hat in real space, and which leads to the
dominant errors in our expression. Finally, we present a detailed comparison
between the various expressions for the mass functions, exploring the accuracy
and range of validity of each.Comment: 28 pages, 13 figures; v2: text reorganized and some figured modified
for clarity, results unchanged, references added. Matches version published
in JCA
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