350 research outputs found
Non-monotonic dependence of the rupture force in polymer chains on their lengths
We consider the rupture dynamics of a homopolymer chain pulled at one end at
a constant loading rate. Our model of the breakable polymer is related to the
Rouse chain, with the only difference that the interaction between the monomers
is described by the Morse potential instead of the harmonic one, and thus
allows for mechanical failure. We show that in the experimentally relevant
domain of parameters the dependence of the most probable rupture force on the
chain length may be non-monotonic, so that the medium-length chains break
easier than the short and the long ones. The qualitative theory of the effect
is presented
Thermally activated breakdown in a simple polymer model
We consider the thermally activated fragmentation of a homopolymer chain. In
our simple model the dynamics of the intact chain is a Rouse one until a bond
breaks and bond breakdown is considered as a first passage problem over a
barrier to an absorbing boundary. Using the framework of the Wilemski-Fixman
approximation we calculate activation times of individual bonds for free and
grafted chains. We show that these times crucially depend on the length of the
chain and the location of the bond yielding a minimum at the free chain ends.
Theoretical findings are qualitatively confirmed by Brownian dynamics
simulations
Lensing Corrections to Features in the Angular Two-Point Correlation Function and Power Spectrum
It is well known that magnification bias, the modulation of galaxy or quasar
source counts by gravitational lensing, can change the observed angular
correlation function. We investigate magnification-induced changes to the shape
of the observed correlation function w(\theta) and the angular power spectrum
C_{\ell}, paying special attention to the matter-radiation equality peak and
the baryon wiggles. Lensing mixes the correlation function of the source
galaxies with the matter correlation at the lower redshifts of the lenses.
Since the lenses probe structure nearer to the observer, the angular scale
dependence of the lensing terms is different from that of the sources, thus the
observed correlation function is distorted. We quantify how the lensing
corrections depend on the width of the selection function, the galaxy bias b,
and the number count slope s. The correction increases with redshift and larger
corrections are present for sources with steep number count slopes and/or broad
redshift distributions. The most drastic changes to C_{\ell} occur for
measurements at z >~1.5 and \ell <~ 100. For the source distributions we
consider, magnification bias can shift the matter-radiation equality scale by
1-6% at z ~ 1.5 and by z ~ 3.5 the shift can be as large as 30%. The baryon
bump in \theta^2w(\theta) is shifted by <~ 1% and the width is typically
increased by ~10%. Shifts of >~ 0.5% and broadening of >~ 20% occur only for
very broad selection functions and/or galaxies with (5s-2)/b>~2. However, near
the baryon bump the magnification correction is not constant but a gently
varying function which depends on the source population. Depending on how the
w(\theta) data is fitted, this correction may need to be accounted for when
using the baryon acoustic scale for precision cosmology.Comment: v2: 8 pages, 5 figures, text and figures condensed, references adde
Self-organized escape of oscillator chains in nonlinear potentials
We present the noise free escape of a chain of linearly interacting units
from a metastable state over a cubic on-site potential barrier. The underlying
dynamics is conservative and purely deterministic. The mutual interplay between
nonlinearity and harmonic interactions causes an initially uniform lattice
state to become unstable, leading to an energy redistribution with strong
localization. As a result a spontaneously emerging localized mode grows into a
critical nucleus. By surpassing this transition state, the nonlinear chain
manages a self-organized, deterministic barrier crossing. Most strikingly,
these noise-free, collective nonlinear escape events proceed generally by far
faster than transitions assisted by thermal noise when the ratio between the
average energy supplied per unit in the chain and the potential barrier energy
assumes small values
Large-Scale QSO-Galaxy Correlations and Weak Lensing
Several recent studies show that bright, intermediate and high redshift
optically and radio selected QSOs are positively correlated with nearby
galaxies on a range of angular scales up to a degree. Obscuration by unevenly
distributed Galactic dust can be ruled out as the cause, leaving weak
statistical lensing as the physical process responsible. However the amplitude
of correlations on < 1 degree scales is at least a factor of a few larger than
lensing model predictions. A possible way to reconcile the observations and
theory is to revise the weak lensing formalism. We extend the standard lensing
formulation to include the next higher order term (second order) in the
geodesic equation of motion for photons. We derive relevant equations
applicable in the weak lensing regime, and discuss qualitative properties of
the updated formulation. We then perform numerical integrations of the revised
equation and study the effect of the extra term using two different types of
cosmic mass density fluctuations. We find that nearby large-scale coherent
structures increase the amplitude of the predicted lensing-induced correlations
between QSOs and foreground galaxies by ~ 10% (not a factor of several required
by observations), while the redshift of the optimal, i.e. `most correlated'
structures is moved closer to the observer compared to what is predicted using
the standard lensing equation.Comment: extended Section 2; 20 pages, including 4 figures, accepted to Ap
Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
In Lyman-alpha forest measurements it is generally assumed that quasars are
mere background light sources which are uncorrelated with the forest.
Gravitational lensing of the quasars violates this assumption. This effect
leads to a measurement bias, but more interestingly it provides a valuable
signal. The lensing signal can be extracted by correlating quasar magnitudes
with the flux power spectrum and with the flux decrement. These correlations
will be challenging to measure but their detection provides a direct measure of
how features in the Lyman-alpha forest trace the underlying mass density field.
Observing them will test the fundamental hypothesis that fluctuations in the
forest are predominantly driven by fluctuations in mass, rather than in the
ionizing background, helium reionization or winds. We discuss ways to
disentangle the lensing signal from other sources of such correlations,
including dust, continuum and background residuals. The lensing-induced
measurement bias arises from sample selection: one preferentially collects
spectra of magnified quasars which are behind overdense regions. This
measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the
flux probability distribution. Since the effect is systematic, quantities such
as the amplitude of the flux power spectrum averaged across scales should be
interpreted with care.Comment: 22 pages, 8 figures; v2: references added, discussion expanded,
matches PRD accepted versio
Internal friction and mode relaxation in a simple chain model
We consider equilibrium relaxation properties of the end-to-end distance and
of principal components in a one-dimensional polymer chain model with nonlinear
interaction between the beads. While for the single-well potentials these
properties are similar to the ones of a Rouse chain, for the double-well
interaction potentials, modeling internal friction, they differ vastly from the
ones of the harmonic chain at intermediate times and intermediate temperatures.
This minimal description within a one-dimensional model mimics the relaxation
properties found in much more complex polymer systems. Thus, the relaxation
time of the end-to-end distance may grow by orders of magnitude at intermediate
temperatures. The principal components (whose directions are shown to coincide
with the normal modes of the harmonic chain, whatever interaction potential is
assumed) not only display larger relaxation times but also subdiffusive
scaling
Galaxy-Quasar correlations between APM galaxies and Hamburg-ESO QSOs
We detect angular galaxy-QSO cross-correlations between the APM Galaxy
Catalogue and a preliminary release (consisting of roughly half of the
anticipated final catalogue) of the Hamburg-ESO Catalogue of Bright QSOs as a
function of source QSO redshift using multiple cross-correlation estimators.
Each of the estimators yield very similar results, implying that the APM
catalogue and the Hamburg-ESO survey are both fair samples of the respective
true galaxy and QSO populations. Though the signal matches the expectations of
gravitational lensing qualitatively, the strength of the measured
cross-correlation signal is significantly greater than the CDM models of
lensing by large scale structure would suggest. This same disagreement between
models and observation has been found in several earlier studies. We estimate
our confidence in the correlation detections versus redshift by generating 1000
random realizations of the Hamburg-ESO QSO survey: We detect physical
associations between galaxies and low-redshift QSOs at 99% confidence and
detect lensing associations at roughly 95% confidence for QSOs with redshifts
between 0.6 and 1. Control cross-correlations between Galactic stars and QSOs
show no signal. Finally, the overdensities (underdensities) of galaxies near
QSO positions relative to those lying roughly 135 - 150 arcmin away are
uncorrelated with differences in Galactic extinction between the two regions,
implying that Galactic dust is not significantly affecting the QSO sample.Comment: 35 pages total, including 9 figures. Accepted by the Astrophysical
Journa
An amplicon sequencing protocol for attacker identification from DNA traces left on artificial prey
1. Clay model studies are a popular tool to identify predator–prey interactions that are challenging to observe directly in the field. But despite its wide use, the meth-od's applicability is limited by its low taxonomic resolution. Attack marks on clay models are usually identified visually, which only allows classification into higher taxonomic levels of predators. Thus, the method is often biased, lacks proof and, above all, standardization.2. Here, we tested whether precise identification of attackers can be provided by amplification and sequencing of mitochondrial DNA left in bite marks on clay models. We validated our approach in a controlled laboratory study as well as in a field experiment using clay models of a common European amphibian, the European fire salamander Salamandra salamandra. DNA-based taxonomic assign-ments were additionally compared to visual assessments of bite marks.3. We show that trace DNA of attackers can be routinely isolated and sequenced from bite marks, providing accurate species-level classification. In contrast, visual identification alone yielded a high number of unassigned predator taxa. We also highlight the sensitivity of the method and show likely sources of contamination as well as probable cases of secondary and indirect predation.4. Our standardized approach for species-level attacker identification opens up new possibilities far beyond the standard use of clay models to date, including food web studies at unprecedented detail, invasive species monitoring as well as biodi-versity inventories
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