3,376 research outputs found
Probing gravitational non-minimal coupling with dark energy surveys
We investigate observational constraints on a specific one-parameter
extension to the minimal quintessence model, where the quintessence field
acquires a quadratic coupling to the scalar curvature through a coupling
constant . The value of is highly suppressed in typical tracker
models if the late-time cosmic acceleration is driven at some field values near
the Planck scale. We test in a second class of models in which the field
value today becomes a free model parameter. We use the combined data from
type-Ia supernovae, cosmic microwave background, baryon acoustic oscillations
and matter power spectrum, to weak lensing measurements and find a best-fit
value where is excluded outside the 95 per cent
confidence region. The effective gravitational constant subject
to the hint of a non-zero is constrained to at the same confidence level on cosmological scales, and can be narrowed
down to when combining with Solar
System tests.Comment: Context extended, figures and references added, title changed to
match with accepted version for publicatio
PARENTAL SUPPORT AND ENGLISH ACHIEVEMENT: THE MEDIATING ROLE OF ACADEMIC SELF-CONCEPT
The contribution of parental support to academic achievement has been adequately explored. However, the mediating mechanisms between parental support and academic achievement should be more studied, especially in teaching English as a foreign language (EFL). Consequently, this study was designed to examine whether academic self-concept a mediating between parental support and academic achievement among Chinese EFL learners. Participants were 499 Chinese secondary EFL learners selected through convenience sampling. The relationship between parental support, academic self-concept, and English achievement was explored by structural equation modelling analysis (SEM) and mediation analysis while controlling for gender and age. Results showed that parental support could affect English achievement directly and indirectly through academic self-concept. Additionally, academic self-concept fully mediated the relationship between parental support and English achievement. Theoretical and practical implications, limitations and directions for future research are discussed. Article visualizations
"Teleparallel" Dark Energy
Using the "teleparallel" equivalent of General Relativity as the
gravitational sector, which is based on torsion instead of curvature, we add a
canonical scalar field, allowing for a nonminimal coupling with gravity.
Although the minimal case is completely equivalent to standard quintessence,
the nonminimal scenario has a richer structure, exhibiting quintessence-like or
phantom-like behavior, or experiencing the phantom-divide crossing. The richer
structure is manifested in the absence of a conformal transformation to an
equivalent minimally-coupled model.Comment: 5 pages, 1 figure, Version published in PLB704 (2011) 384-38
Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures
Hydrogen-rich compounds are important for understanding the dissociation of
dense molecular hydrogen, as well as searching for room temperature
Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure
experiment reported the successful synthesis of novel insulating lithium
polyhydrides when above 130 GPa. However, the results are in sharp contrast to
previous theoretical prediction by PBE functional that around this pressure
range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. In order to
address this discrepancy, we perform unbiased structure search with first
principles calculation by including the van der Waals interaction that was
ignored in previous prediction to predict the high pressure stable structures
of LiHn (n = 2-11, 13) up to 200 GPa. We reproduce the previously predicted
structures, and further find novel compositions that adopt more stable
structures. The van der Waals functional (vdW-DF) significantly alters the
relative stability of lithium polyhydrides, and predicts that the stable
stoichiometries for the ground-state should be LiH2 and LiH9 at 130-170 GPa,
and LiH2, LiH8 and LiH10 at 180-200 GPa. Accurate electronic structure
calculation with GW approximation indicates that LiH, LiH2, LiH7, and LiH9 are
insulative up to at least 208 GPa, and all other lithium polyhydrides are
metallic. The calculated vibron frequencies of these insulating phases are also
in accordance with the experimental infrared (IR) data. This reconciliation
with the experimental observation suggests that LiH2, LiH7, and LiH9 are the
possible candidates for lithium polyhydrides synthesized in that experiment.
Our results reinstate the credibility of density functional theory in
description H-rich compounds, and demonstrate the importance of considering van
der Waals interaction in this class of materials.Comment: 34 pages, 15 figure
Constraints on primordial curvature power spectrum with pulsar timing arrays
The stochastic signal detected by NANOGrav, PPTA, EPTA, and CPTA can be
explained by the scalar-induced gravitational waves. In order to determine the
scalar-induced gravitational waves model that best fits the stochastic signal,
we employ both single- and double-peak parameterizations for the power spectrum
of the primordial curvature perturbations, where the single-peak scenarios
include the -function, box, lognormal, and broken power law model, and
the double-peak scenario is described by the double lognormal form. Using
Bayesian inference, we find that there is no significant evidence for or
against the single-peak scenario over the double-peak model, with (Bayes
factors) among these models . Therefore, we are not able
to distinguish the different shapes of the power spectrum of the primordial
curvature perturbation with the current sensitivity of pulsar timing arrays.Comment: 19 pages, 1 table, 7 figure
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