104,721 research outputs found
Constraint on the early Universe by relic gravitational waves: From pulsar timing observations
Recent pulsar timing observations by the Parkers Pulsar Timing Array and
European Pulsar Timing Array teams obtained the constraint on the relic
gravitational waves at the frequency , which provides the
opportunity to constrain , the Hubble parameter when these waves crossed
the horizon during inflation. In this paper, we investigate this constraint by
considering the general scenario for the early Universe: we assume that the
effective (average) equation-of-state before the big bang nucleosynthesis
stage is a free parameter. In the standard hot big-bang scenario with ,
we find that the current PPTA result follows a bound H_*\leq
1.15\times10^{-1}\mpl, and the EPTA result follows H_*\leq
6.92\times10^{-2}\mpl. We also find that these bounds become much tighter in
the nonstandard scenarios with . When , the bounds become
H_*\leq5.89\times10^{-3}\mpl for the current PPTA and
H_*\leq3.39\times10^{-3}\mpl for the current EPTA. In contrast, in the
nonstandard scenario with , the bound becomes H_*\leq7.76\mpl for the
current PPTA.Comment: 8 pages, 3 figures, 1 table, PRD in pres
Relic gravitational waves: latest revisions and preparations for new data
The forthcoming release of data from the Planck mission, and possibly from
the next round of Wilkinson Microwave Anisotropy Probe (WMAP) observations,
make it necessary to revise the evaluations of relic gravitational waves in the
existing data and, at the same time, to refine the assumptions and data
analysis techniques in preparation for the arrival of new data. We reconsider
with the help of the commonly used CosmoMC numerical package the previously
found indications of relic gravitational waves in the 7-year (WMAP7) data. The
CosmoMC approach reduces the confidence of these indications from approximately
2 level to approximately 1 level, but the indications do not
disappear altogether. We critically analyze the assumptions that are currently
used in the Cosmic Microwave Background (CMB) data analyzes and outline the
strategy that should help avoid the oversight of relic gravitational waves in
the future CMB data. In particular, it is important to keep away from the
unwarranted assumptions about density perturbations. The prospects of confident
detection of relic gravitational waves by the Planck satellite have worsened,
but they are still good. It appears that more effort will be required in order
to mitigate the foreground contamination.Comment: 11 pages, 4 figures, 3 tables; v.3: improvements, published versio
Modelling of Electroluminescence in Polymers Using a Bipolar Charge Transport Model
Electroluminescence (EL) in polymeric materials is thought to occur due to the energy dissipation process from the recombination of opposite polarity charge carriers. It is considered as an indication of storage and transport of charge carriers in cable insulation subject to electrical stresses and may indicate the change in charge movement due to aging or degradation processes. Under ac electric fields, the interaction of opposite polarity charge carriers at the interface of polymer/conductor is enhanced compared with dc conditions, and seems to contribute a lot to the electroluminescence rather than the charge behaviours in the bulk of polymers. The dynamics of charge carriers both at the interface of polymer/conductor and in the bulk of polymers is investigated through a simulation work using a bipolar charge transport model. Figure 1 compares experimental electroluminescence results with simulated data from the recombination of injected charge carriers. The paper will give more details on EL model and comparison under various waveforms and frequencies
Magnetic rotations in 198Pb and 199Pb within covariant density functional theory
Well-known examples of shears bands in the nuclei 198Pb and 199Pb are
investigated within tilted axis cranking relativistic mean-field theory. Energy
spectra, the relation between spin and rotational frequency, deformation
parameters and reduced and transition probabilities are calculated.
The results are in good agreement with available data and with calculations
based on the phenomenological pairing plus-quadrupole-quadrupole tilted-axis
cranking model. It is shown that covariant density functional theory provides a
successful microscopic and fully self-consistent description of magnetic
rotation in the Pb region showing the characteristic properties as the shears
mechanism and relatively large B(M1) transitions decreasing with increasing
spin.Comment: 22 pages, 8 figure
Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood
Two distinct halo populations were found in the solar neighborhood by a
series of works. They can be clearly separated by [alpha\Fe] and several other
elemental abundance ratios including [Cu/Fe]. Very recently, a non-local
thermodynamic equilibrium (non-LTE) study revealed that relatively large
departures exist between LTE and non-LTE results in copper abundance analysis.
We aim to derive the copper abundances for the stars from the sample of Nissen
et al (2010) with both LTE and non-LTE calculations. Based on our results, we
study the non-LTE effects of copper and investigate whether the high-alpha
population can still be distinguished from the low-alpha population in the
non-LTE [Cu/Fe] results. Our differential abundance ratios are derived from the
high-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.
Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesis
method, we derive the non-LTE copper abundances based on the new atomic model
with current atomic data obtained from both laboratory and theoretical
calculations. The copper abundances determined from non-LTE calculations are
increased by 0.01 to 0.2 dex depending on the stellar parameters compared with
the LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one in
the metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects into
consideration, the high- and low-alpha stars still show distinguishable copper
abundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus
[Fe/H]. The non-LTE effects are strong for copper, especially in metal-poor
stars. Our results confirmed that there are two distinct halo populations in
the solar neighborhood. The dichotomy in copper abundance is a peculiar feature
of each population, suggesting that they formed in different environments and
evolved obeying diverse scenarios.Comment: 9 pages, 7 figures, 2 table
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