456 research outputs found
A halo bias function measured deeply into voids without stochasticity
We study the relationship between dark-matter haloes and matter in the MIP
-body simulation ensemble, which allows precision measurements of this
relationship, even deeply into voids. What enables this is a lack of
discreteness, stochasticity, and exclusion, achieved by averaging over hundreds
of possible sets of initial small-scale modes, while holding fixed large-scale
modes that give the cosmic web. We find (i) that dark-matter-halo formation is
greatly suppressed in voids; there is an exponential downturn at low densities
in the otherwise power-law matter-to-halo density bias function. Thus, the
rarity of haloes in voids is akin to the rarity of the largest clusters, and
their abundance is quite sensitive to cosmological parameters. The exponential
downturn appears both in an excursion-set model, and in a model in which
fluctuations evolve in voids as in an open universe with an effective
proportional to a large-scale density. We also find that (ii) haloes
typically populate the average halo-density field in a super-Poisson way, i.e.
with a variance exceeding the mean; and (iii) the rank-order-Gaussianized halo
and dark-matter fields are impressively similar in Fourier space. We compare
both their power spectra and cross-correlation, supporting the conclusion that
one is roughly a strictly-increasing mapping of the other. The MIP ensemble
especially reveals how halo abundance varies with `environmental' quantities
beyond the local matter density; (iv) we find a visual suggestion that at fixed
matter density, filaments are more populated by haloes than clusters.Comment: Changed to version accepted by MNRA
Evolution of magnetic component in Yang-Mills condensate dark energy models
The evolution of the electric and magnetic components in an effective
Yang-Mills condensate dark energy model is investigated. If the electric field
is dominant, the magnetic component disappears with the expansion of the
Universe. The total YM condensate tracks the radiation in the earlier Universe,
and later it becomes thus is similar to the cosmological constant.
So the cosmic coincidence problem can be avoided in this model. However, if the
magnetic field is dominant, holds for all time, suggesting that it
cannot be a candidate for the dark energy in this case.Comment: 12 pages, 4 figures, minor typos correcte
Empirical likelihood-based portmanteau tests for autoregressive moving average models with possible infinite variance innovation
It is an important task in the literature to check whether a fitted
autoregressive moving average (ARMA) model is adequate, while the currently
used tests may suffer from the size distortion problem when the underlying
autoregressive models have low persistence. To fill this gap, this paper
proposes two empirical likelihood-based portmanteau tests. The first one is
naive but can serve as a benchmark, and the second is for the case with
infinite variance innovations. The asymptotic distributions under the null
hypothesis are derived under mild moment conditions, and their usefulness is
demonstrated by simulation experiments and two real data examples.Comment: 23 pages, 2 figure
Total Angular Momentum Waves for Scalar, Vector, and Tensor Fields
Most calculations in cosmological perturbation theorydecompose those
perturbations into plane waves (Fourier modes). However, for some calculations,
particularly those involving observations performed on a spherical sky, a
decomposition into waves of fixed total angular momentum (TAM) may be more
appropriate. Here we introduce TAM waves, solutions of fixed total angular
momentum to the Helmholtz equation, for three-dimensional scalar, vector, and
tensor fields. The vector TAM waves of given total angular momentum can be
decomposed further into a set of three basis functions of fixed orbital angular
momentum (OAM), a set of fixed helicity, or a basis consisting of a
longitudinal (L) and two transverse (E and B) TAM waves. The symmetric
traceless rank-2 tensor TAM waves can be similarly decomposed into a basis of
fixed OAM or fixed helicity, or a basis that consists of a longitudinal (L),
two vector (VE and VB, of opposite parity), and two tensor (TE and TB, of
opposite parity) waves. We show how all of the vector and tensor TAM waves can
be obtained by applying derivative operators to scalar TAM waves. This operator
approach then allows one to decompose a vector field into three covariant
scalar fields for the L, E, and B components and symmetric-traceless-tensor
fields into five covariant scalar fields for the L, VE, VB, TE, and TB
components. We provide projections of the vector and tensor TAM waves onto
vector and tensor spherical harmonics. We provide calculational detail to
facilitate the assimilation of this formalism into cosmological calculations.
As an example, we calculate the power spectra of the deflection angle for
gravitational lensing by density perturbations and by gravitational waves. We
comment on an alternative approach to CMB fluctuations based on TAM waves. Our
work may have applications elsewhere in field theory and in general relativity.Comment: 32 pages, Published version in PR
Role of membrane biophysics in Alzheimer’s–related cell pathways
Cellular membrane alterations are commonly observed in many diseases, including Alzheimer's disease (AD). Membrane biophysical properties, such as membrane molecular order, membrane fluidity, organization of lipid rafts, and adhesion between membrane and cytoskeleton, play an important role in various cellular activities and functions. While membrane biophysics impacts a broad range of cellular pathways, this review addresses the role of membrane biophysics in amyloid-β peptide aggregation, Aβ-induced oxidative pathways, amyloid precursor protein processing, and cerebral endothelial functions in AD. Understanding the mechanism(s) underlying the effects of cell membrane properties on cellular processes should shed light on the development of new preventive and therapeutic strategies for this devastating disease
Amidine-Mediated Zwitterionic Ring-Opening Polymerization of N-Alkyl N-Carboxyanhydride: Mechanism, Kinetics, and Architecture Elucidation
© 2016 American Chemical Society. Zwitterionic ring-opening polymerization (ZROP) of N-butyl N-carboxyanhydrides (Bu-NCAs) has been investigated using 1,8-diazabicycloundec-7-ene (DBU), a bicyclic amidine initiator. It was found that poly(N-butylglycine)s (PNBGs) with molecular weight (Mn) in the 3.5-32.4 kg mol-1 range and polydispersity index (PDI) in the 1.02-1.12 range can be readily obtained by systematically varying the initial monomer to initiator feed ratio. The polymerization exhibits characteristics of a controlled polymerization, as evidenced by the linear increase of polymer molecular weight with conversion and the successful enchainment experiments. Kinetic studies revealed that the reaction is first-order dependent on the monomer and the DBU concentration. The rate of initiation is comparable to that of the propagation. Random copolypeptoids of poly[(N-propargylglycine)-r-(N-butylglycine)]s [P(NPgG-r-NBG)s] were also synthesized by DBU-mediated copolymerization of Bu-NCA and N-propargyl N-carboxyanhydride (Pg-NCA). Subsequent grafting with azido-terminated poly(ethylene glycol) (PEG) produces bottlebrush copolymers. Analysis of bottlebrush copolymer samples using atomic force microscopy (AFM) revealed a surface morphology of toroid-shaped nanostructures, consistent with the polypeptoid backbone having cyclic architecture. Small-angle neutron scattering (SANS) characterization of the bottlebrush polymer ensemble in solution also confirms the cyclic architecture of the polypeptoid backbones
Comparison of Surface Elevation Changes of the Greenland and Antarctic Ice Sheets from Radar and Laser Altimetry
A primary purpose of satellite altimeter measurements is determination of the mass balances of the Greenland and Antarctic ice sheets and changes with time by measurement of changes in the surface elevations. Since the early 1990's, important measurements for this purpose have been made by radar altimeters on ERS-l and 2, Envisat, and CryoSat and a laser altimeter on ICESat. One principal factor limiting direct comparisons between radar and laser measurements is the variable penetration depth of the radar signal and the corresponding location of the effective depth of the radar-measured elevation beneath the surface, in contrast to the laser-measured surface elevation. Although the radar penetration depth varies significantly both spatially and temporally, empirical corrections have been developed to account for this effect. Another limiting factor in direct comparisons is caused by differences in the size of the laser and radar footprints and their respective horizontal locations on the surface. Nevertheless, derived changes in elevation, dHldt, and time-series of elevation, H(t), have been shown to be comparable. For comparisons at different times, corrections for elevation changes caused by variations in the rate offrrn compaction have also been developed. Comparisons between the H(t) and the average dH/dt at some specific locations, such as the Vostok region of East Antarctic, show good agreement among results from ERS-l and 2, Envisat, and ICESat. However, Greenland maps of dHidt from Envisat and ICESat for the same time periods (2003-2008) show some areas of significant differences as well as areas of good agreement. Possible causes of residual differences are investigated and described
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