7,220 research outputs found
Cosmology of Antisymmetric Tensor Field in D-brane Universe
We analyze homogeneous, anisotropic cosmology driven by a self-interacting
``massive'' antisymmetric tensor field which is present in string
theories with D-branes. Time-dependent magnetic field existing in the early
universe can lead to the Bianchi type I universe. Evolutions of such a tensor
field are solved exactly or numerically in the universe dominated by vacuum
energy, radiation, and field itself. The matter-like behavior of the
field (dubbed as ``-matter'') ensures that the anisotropy disappears at late
time and thus becomes unobservable in a reasonable cosmological scenario. Such
a feature should be contrasted to the cosmology of the conventional massless
antisymmetric tensor field.Comment: 13 page
Cosmology of Light Moduli
In string/M-theory with a large compactification radius, some axion-like
moduli can be much lighter than the gravitino. Generic moduli in gauge-mediated
supersymmetry breaking models also have a mass far below the weak scale.
Motivated by these, we examine the cosmological implications of light moduli
for the mass range from the weak scale to an extremely small scale of order
10^{-26} eV, and obtain an upper bound on the initial moduli misalignment for
both cases with and without a late entropy production.Comment: 23 pages including 3 pictex-figures, Late
String Cosmology of the D-brane Universe
We analyze homogeneous anisotropic cosmology driven by the dilaton and the
self-interacting ``massive'' antisymmetric tensor field which are indispensable
bosonic degrees with the graviton in the NS-NS sector of string theories with
D-branes. We found the attractor solutions for this system, which show the
overall features of general solutions, and confirmed it through numerical
analysis. The dilaton possesses the potential due to the presence of the
D-brane and the curvature of extra dimensions. In the presence of the
non-vanishing antisymmetric tensor field, the homogeneous universe expands
anisotropically while the D-brane term dominates. The isotropy is recovered as
the dilaton rolls down and the curvature term dominates. With the stabilizing
potential for the dilaton, the isotropy can also be recovered.Comment: 23 pages, 8 figures. Final version, to appear in Phys. Rev.
Taste symmetry breaking with HYP-smeared staggered fermions
We study the impact of hypercubic (HYP) smearing on the size of taste
breaking for staggered fermions, comparing to unimproved and to asqtad-improved
staggered fermions. As in previous studies, we find a substantial reduction in
taste-breaking compared to unimproved staggered fermions (by a factor of 4-7 on
lattices with spacing fm). In addition, we observe that
discretization effects of next-to-leading order in the chiral expansion () are markedly reduced by HYP smearing. Compared to asqtad valence
fermions, we find that taste-breaking in the pion spectrum is reduced by a
factor of 2.5-3, down to a level comparable to the expected size of generic
effects. Our results suggest that, once one reaches a lattice
spacing of fm, taste-breaking will be small enough after HYP
smearing that one can use a modified power counting in which , simplify fitting to phenomenologically interesting quantities.Comment: 14 pages, 13 figures, references updated, minor change
A Practical Tessellation-Based Approach for Optimizing Cell-Specific Bias Values in LTE-A Heterogeneous Cellular Networks
In order to implement an optimized solution for cell range expansion (CRE) and enhanced intercell interference coordination (eICIC) schemes in long-term evolution-advanced (LTE-A) heterogeneous cellular networks (HCNs) and to realize good load-balancing performance in existing LTE-A systems, a practical tessellation-based algorithm is proposed. In this algorithm, a globalized cell-specific bias optimization and a localized almost blank subframe (ABS) ratio update are proposed. The proposed scheme does not require major changes to existing protocols. Thus, it can be implemented in existing LTE-A systems with any legacy user equipment (UE) with only a partial update to the BSs and core networks. From simulation results, it is shown that the tessellation formed by the proposed approach is quite consistent with the optimal one for various realistic scenarios. Thus, the proposed scheme can provide a much better load-balancing capability compared with the conventional common bias scheme. Owing to the improved load-balancing capability, the user rate distribution of the proposed scheme is much better than that obtained from the conventional scheme and is even indistinguishable from that of the ideal joint user association scheme
Formation of Warped Disks by Galactic Fly-by Encounters. I. Stellar Disks
Warped disks are almost ubiquitous among spiral galaxies. Here we revisit and
test the `fly-by scenario' of warp formation, in which impulsive encounters
between galaxies are responsible for warped disks. Based on N-body simulations,
we investigate the morphological and kinematical evolution of the stellar
component of disks when galaxies undergo fly-by interactions with adjacent dark
matter halos. We find that the so-called `S'-shaped warps can be excited by
fly-bys and sustained for even up to a few billion years, and that this
scenario provides a cohesive explanation for several key observations. We show
that disk warp properties are governed primarily by the following three
parameters; (1) the impact parameter, i.e., the minimum distance between two
halos, (2) the mass ratio between two halos, and (3) the incident angle of the
fly-by perturber. The warp angle is tied up with all three parameters, yet the
warp lifetime is particularly sensitive to the incident angle of the perturber.
Interestingly, the modeled S-shaped warps are often non-symmetric depending on
the incident angle. We speculate that the puzzling U- and L-shaped warps are
geometrically superimposed S-types produced by successive fly-bys with
different incident angles, including multiple interactions with a satellite on
a highly elongated orbit.Comment: 16 pages, 13 figures, 3 tables. Accepted for publication in Ap
- …