285 research outputs found
Parity-violation in bouncing cosmology
We investigate the possibility of the enhancement of parity-violation signal
in bouncing cosmology. Specifically, we are interested in deciding which phase
should generate the most significant parity-violation signals. We find that the
dominant contribution comes from the bouncing phase, while the contraction
phase has a smaller contribution. Therefore, bouncing cosmology can enhance the
parity-violation signals during the bouncing phase. Moreover, since the
bouncing phase has the highest energy scale in bouncing cosmology, we can also
probe new physics at this scale by studying the parity-violation effect.Comment: 28 pages, 22 figure
Pulsar timing array observations as possible hints for nonsingular cosmology
Recent pulsar timing array (PTA) experiments have reported strong evidence of
the stochastic gravitational wave background (SGWB). If interpreted as
primordial gravitational waves (GWs), the signal favors a strongly blue-tilted
spectrum. Consequently, the nonsingular cosmology, which is able to predict a
strongly blue-tilted GW spectrum with on certain scales, offers
a potential explanation for the observed SGWB signal. In this paper, we present
a Genesis-inflation model capable of explaining the SGWB signal observed by the
PTA collaborations while also overcoming the initial singularity problem
associated with the inflationary cosmology. Furthermore, our model predicts
distinctive features in the SGWB spectrum, which might be examined by
forthcoming space-based gravitational wave experiments.Comment: 20 pages, 8 figures; references added, published in EPJ
Higher order correction to weak-field lensing of Ellis-Bronnikov wormhole
Gravitational lensing effect at higher order under weak field approximation
is believed to be important to distinguish black holes and other compact
objects like wormholes. The deflection angle of a generic wormhole is difficult
to solve analytically, thus approximation methods are implemented. In this
paper, we investigate the weak-field deflection angle of a specific wormhole,
the Ellis-Bronnikov wormhole (EBWH), up to the 1/b^4 order. We use different
approximation formalism, study their precision at 1/b^4 order by a comparison
to a purely numerical result, and finally rank these formalism by their
accuracy. Moreover, we find that certain formalism are sensitive to the choice
of coordinate system, when the corresponding deflection angle approaches 0 in
the negative-mass branch of universe.Comment: Comments are welcome
Prediction of Yield Surface of Single Crystal Copper from Discrete Dislocation Dynamics and Geometric Learning
A yield surface of a material is a set of critical stress conditions beyond
which macroscopic plastic deformation begins. For crystalline solids, plastic
deformation occurs by the motion of dislocations, which can be captured by
discrete dislocation dynamics (DDD) simulations. In this paper, we predict the
yield surfaces and strain-hardening behaviors using DDD simulations and a
geometric manifold learning approach. The yield surfaces in the
three-dimensional space of plane stress are constructed for single-crystal
copper subjected to uniaxial loading along the and directions,
respectively. With increasing plastic deformation under loading, the
yield surface expands nearly uniformly in all directions, corresponding to
isotropic hardening. In contrast, under loading, latent hardening is
observed, where the yield surface remains nearly unchanged in the orientations
in the vicinity of the loading direction itself, but expands in other
directions, resulting in an asymmetric shape. This difference in hardening
behaviors is attributed to the different dislocation multiplication behaviors
on various slip systems under the two loading conditions
Microlensing effect of charged spherically symmetric wormhole
We systematically investigate the microlensing effect of charged spherically
symmetric wormhole, where the light source is remote from the throat.
Remarkably, there will be at most three images by considering the charge part.
We study all situations including three images, two images, and one image,
respectively. The numerical result shows that the range of total magnification
is from to depending on various metrics. In the case of three
images, there will be two maximal values of magnification (a peak, and a gentle
peak) when the contribution via mass is much less than that of charge. However,
we cannot distinguish the case that forms three images or only one image as the
total magnification is of order . Finally, our theoretical investigation
could shed new light on exploring the wormhole with the microlensing effect.Comment: 10 pages, 9 figure
Enhance Primordial Black Hole Abundance through the Non-linear Processes around Bounce Point
The non-singular bouncing cosmology is an alternative paradigm to inflation,
wherein the background energy density vanishes at the bounce point, in the
context of Einstein gravity. Therefore, the non-linear effects in the evolution
of density fluctuations () may be strong in the bounce phase,
which potentially provides a mechanism to enhance the abundance of primordial
black holes (PBHs). This article presents a comprehensive illustration for PBH
enhancement due to the bounce phase. To calculate the non-linear evolution of
, the Raychaudhuri equation is numerically solved here. Since the
non-linear processes may lead to a non-Gaussian probability distribution
function for after the bounce point, the PBH abundance is
calculated in a modified Press-Schechter formalism. In this case, the criterion
of PBH formation is complicated, due to complicated non-linear evolutionary
behavior of during the bounce phase. Our results indicate that
the bounce phase indeed has potential to enhance the PBH abundance
sufficiently. Furthermore, the PBH abundance is applied to constrain the
parameters of bounce phase, providing a complementary to the surveys of cosmic
microwave background and large scale structure.Comment: 17 pages, 6 figure
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