51,068 research outputs found
Primordial Perturbations During a Slow Expansion
Recently, it has been showed that a slow expansion, which is asymptotically a
static state in infinite past and may be described as an evolution with
\epsilon \ll -1, of early universe may lead to the generation of primordial
perturbation responsible for the structure formation of observable universe.
However, its feasibility depends on whether the growing mode of Bardeen
potential before phase transition can be inherited by the constant mode of
curvature perturbation after phase transition. In this note, we phenomenally
regard this slow expansion as that driven by multi NEC violating scalar fields.
We calculate the curvature perturbation induced by the entropy perturbation
before phase transition, and find that the spectrum is naturally scale
invariant with a slight red tilt. The result has an interesting similarity to
that of slow roll inflation.Comment: 5 pages, 1 eps figure, minor changes and refs. added, to publish in
PR
Entropy of Microwave Background Radiation in Observable Universe
We show that the cosmological constant at late time places a bound on the
entropy of microwave background radiation deposited in the future event horizon
of a given observer, . This bound is independent of
the energy scale of reheating and the FRW evolution after reheating. We also
discuss why the entropy of microwave background in our observable universe has
its present value.Comment: 3 pages, no figures, revised version to publish in PRD brief repor
Seeding of Primordial Perturbations During a Decelerated Expansion
A scalar field with a modified dispersion relation may seed, under certain
conditions, the primordial perturbations during a decelerated expansion. In
this note we examine whether and how these perturbations can be responsible for
the structure formation of observable universe. We discuss relevant
difficulties and possible solutions.Comment: 5 pages, 3 figures, more details and clarifications added, refs.
added, new version with 6 pages has been splitted into various sections, to
be published in PRD, v3, matched to published versio
Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States
Traveltime tomographic models of the LA RISTRA transect produce excellent waveform fits if we amplify the damped images. We observe systematic waveform distortions across the western edge of the Great Plains from South American events, starting about 300 km east of the centre of the Rio Grande Rift. The amplitude decreases by more than 50 per cent within array stations spanning less than 200 km while the pulse width increases by more than a factor of 2. This feature is not observed for the data arriving from the northwest. While the S-wave tomographic image shows a fast slab-like feature dipping to the southeast beneath the western edge of the Great Plains, synthetics generated from this model do not reproduce the waveform characteristics. However, once we modify the tomographic image by amplifying the velocity contrast between the slab and adjoining mantle by a factor of 2–3, the synthetics produce observed amplitude decay and pulse broadening. In addition to the traveltime delay, amplitude variation due to wave phenomena such as slab diffraction, focusing and defocusing provide much tighter constraints on the geometry of the fast anomaly and its amplitude and sharpness as demonstrated by a forward sensitivity test and snapshots of the seismic wavefield. Our preferred model locates the slab 200 km east of the Rio Grande Rift dipping 70°–75° to the southeast, extending to a depth near 600 km with a thickness of 120 km and a velocity of about 4 per cent fast. In short, adding waveform and amplitude components to regional tomographic studies can help validate and establish structural geometry, sharpness and velocity contrast
Energetics and kinetics of Li intercalation in irradiated graphene scaffolds
In the present study we investigate the irradiation-defects hybridized
graphene scaffold as one potential building material for the anode of Li-ion
batteries. Designating the Wigner V22 defect as a representative, we illustrate
the interplay of Li atoms with the irradiation-defects in graphene scaffolds.
We examine the adsorption energetics and diffusion kinetics of Li in the
vicinity of a Wigner V22 defect using density functional theory calculations.
The equilibrium Li adsorption sites at the defect are identified and shown to
be energetically preferable to the adsorption sites on pristine (bilayer)
graphene. Meanwhile the minimum energy paths and corresponding energy barriers
for Li migration at the defect are determined and computed. We find that while
the defect is shown to exhibit certain trapping effects on Li motions on the
graphene surface, it appears to facilitate the interlayer Li diffusion and
enhance the charge capacity within its vicinity because of the reduced
interlayer spacing and characteristic symmetry associated with the defect. Our
results provide critical assessment for the application of irradiated graphene
scaffolds in Li-ion batteries.Comment: 23 pages, 5 figure
Possible Explanation to Low CMB Quadrupole
The universe might experience many cycles with different vacua. The slow-roll
inflation may be preceded by kinetic-dominated contraction occurring in
"adjacent" vacua during some cycles. In this report we briefly show this
phenomenon may lead to a cutoff of primordial power spectrum. Thus in some
sense the CMB at large angular scale might encode the information of other
vacua.Comment: 10 pages, 3 eps figures, accepted for publication in PRD, v2 revised
with published versio
"Phantom" Inflation in Warped Compactification
In this paper, in a class of warped compactifications with the brane/flux
annihilation, we find that the inflation may be driven by a flat direction
identified as that along the number p of anti D3-branes located at the tip of
the Klebanov-Strassler throat. The spectrum of adiabatic perturbation generated
during inflation is nearly scale invariant, which may be obtained by using the
results shown in the phantom inflation, since in a four-dimension effective
description the evolution of energy density along the direction is slowly
increasing.Comment: 5 pages, 2 figures, some arguments on the reliability of our resulits
added, ref. added, to publish in PRD, matched to published versio
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