31,309 research outputs found
Linear Rheological Response of a Series of Densely Branched Brush Polymers
We have examined the linear rheological responses of a series of welldefined, dense, regularly branched brush polymers. These narrow molecular weight distribution brush polymers had polynorobornene backbones with degrees of
polymerization (DP) of 200, 400, and 800 and polylactide side chains with molecular weight of 1.4 kDa, 4.4 kDa, and 8.7 kDa. The master curves for these brush polymers
were obtained by time temperature superposition (TTS) of the dynamic moduli over the range from the glassy region to the terminal flow region. Similar to other long chain branched polymers, these densely branched brush polymers show a sequence of relaxation. Subsequent to the glassy relaxation, two different relaxation processes can be observed for samples with the high molecular weight (4.4 and 8.7 kDa) side chains, corresponding to the relaxation of the side chains and the brush polymer backbone. Influenced by the large volume fraction of high molecular weight side chains, these brush polymers are unentangled. The lowest plateau observed in the dynamic response is not the rubbery entanglement plateau but is instead associated with the steady state recoverable compliance. Side chain properties affect the rheological responses of these
densely branched brush polymers and determine their glassy behaviors
Cold Dark Matter Isocurvature Perturbations: Cosmological Constraints and Applications
In this paper we present the constraints on cold dark matter (CDM)
isocurvature contributions to the cosmological perturbations. By employing
Markov Chain Monte Carlo method (MCMC), we perform a global analysis for
cosmological parameters using the latest astronomical data, such as 7-year
Wilkinson Microwave Anisotropy Probe (WMAP7) observations, matter power
spectrum from the Sloan Digital Sky Survey (SDSS) luminous red galaxies (LRG),
and "Union2" type Ia Supernovae (SNIa) sample. We find that the correlated
mixture of adiabatic and isocurvature modes are mildly better fitting to the
current data than the pure adiabatic ones, with the minimal given by
the likelihood analysis being reduced by 3.5. We also obtain a tight limit on
the fraction of the CDM isocurvature contributions, which should be less than
14.6% at 95% confidence level. With the presence of the isocurvature modes, the
adiabatic spectral index becomes slightly bigger, n_s^{\rm
adi}=0.972\pm0.014~(1\,\sigma), and the tilt for isocurvature spectrum could be
large, namely, the best fit value is n_s^{\rm iso}=3.020. Finally, we discuss
the effect on WMAP normalization priors, shift parameter R, acoustic scale l_A
and z_{*}, from the CDM isocurvaure perturbation. By fitting the mixed initial
condition to the combined data, we find the mean values of R, l_A and z_{*} can
be changed about 2.9\sigma, 2.8\sigma and 1.5\sigma respectively, comparing
with those obtained in the pure adiabatic condition.Comment: 9 pages, 5 figures, 3 tables, references adde
Cosmography beyond standard candles and rulers
We perform a cosmographic analysis using several cosmological observables
such as the luminosity distance moduli, the volume distance, the angular
diameter distance and the Hubble parameter. These quantities are determined
using different data sets: Supernovae type Ia and Gamma Ray Bursts, the
Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum
and the Hubble parameter as measured from surveys of galaxies. This data set
allows to put constraints on the cosmographic expansion with unprecedented
precision. We also present forecasts for the coefficients of the kinematic
expansion using future but realistic data sets: constraints on the coefficients
of the expansions are likely to improve by a factor ten with the upcoming large
scale structure probes. Finally, we derive the set of the cosmographic
parameters for several cosmological models (including CDM) and compare
them with our best fit set. While distance measurements are unable to
discriminate among these models, we show that the inclusion of the Hubble data
set leads to strong constraints on the lowest order coefficients and in
particular it is incompatible with CDM at 3- confidence level.
We discuss the reliability of this determination and suggest further
observations which might be of crucial importance for the viability of
cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR
Analytic Spectra of CMB Anisotropies and Polarization Generated by Relic Gravitational Waves with Modification due to Neutrino Free-Streaming
We present an analytical calculation of the spectra of CMB anisotropies and
polarizations generated by relic gravitational waves (RGWs). As a substantial
extension to the previous studies, three new ingredients are included in this
work. Firstly, the analytic and are given; especially the
latter can be useful to extract signal of RGWs from the observed data in the
zero multipole method. Secondly, a fitting formula of the decaying factor on
small scales is given, coming from the visibility function around the photon
decoupling. Thirdly, the impacts by the neutrino free-streaming (NFS) is
examined, a process that occurred in the early universe and leaves observable
imprints on CMB via RGWs.
It is found that the analytic and have profiles
agreeing with the numeric ones, except that in a range
and the trough of around have some deviations.
With the new damping factor, the analytic and match with
the numeric ones with the maximum errors only up to the first three
peaks for , improving the previous studies substantially. The
correspondence of the positions of peaks of and those of RGWs are
also demonstrated explicitly. We also find that NFS reduces the amplitudes of
by for and shifts slightly
their peaks to smaller angles. Detailed analyses show that the zero multipoles
, where crosses 0, are shifted to larger values by NFS. This
shifting effect is as important as those causedby different inflation models
and different baryon fractions.Comment: 17 pages, 7 figures. accepted by PR
On relativistic approaches to the pion self-energy in nuclear matter
We argue that, in contrast to the non-relativistic approach, a relativistic
evaluation of the nucleon--hole and delta-isobar--nucleon hole contributions to
the pion self-energy incorporates the s-wave scattering, which requires a more
accurate evaluation. Therefore relativistic approach containing only these
diagrams does not describe appropriately the pion self-energy in isospin
symmetric nuclear matter. We conclude that, a correct relativistic approach to
the pion self-energy should involve a more sophisticated calculation in order
to satisfy the known experimental results on the near-threshold behaviour of
the pion-nucleon (forward) scattering amplitude.Comment: 7 pages,1 figur
Extra Dimensions: A View from the Top
In models with compact extra dimensions, where the Standard Model fields are
confined to a 3+1 dimensional hyperplane, the production
cross-section at a hadron collider can receive significant contributions from
multiple exchange of KK modes of the graviton. These are carefully computed in
the well-known ADD and RS scenarios, taking the energy dependence of the sum
over graviton propagators into account. Using data from Run-I of the Tevatron,
95% C.L. bounds on the parameter space of both models are derived. For Run-II
of the Tevatron and LHC, discovery limits are estimated.Comment: Typos corrected, references added. 12 pages, LaTeX, 2 ps figure
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