6,160 research outputs found
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity
We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati
braneworld model, by solving the full non-linear equations of motion for the
scalar degree of freedom in this model, the brane bending mode. While coupling
universally to matter, the brane-bending mode has self-interactions that become
important as soon as the density field becomes non-linear. These
self-interactions lead to a suppression of the field in high-density
environments, and restore gravity to General Relativity. The code uses a
multi-grid relaxation scheme to solve the non-linear field equation in the
quasi-static approximation. We perform simulations of a flat self-accelerating
DGP model without cosmological constant. The results of the DGP simulations are
compared with standard gravity simulations assuming the same expansion history,
and with DGP simulations using the linearized equation for the brane bending
mode. This allows us to isolate the effects of the non-linear self-couplings of
the field which are noticeable already on quasi-linear scales. We present
results on the matter power spectrum and the halo mass function, and discuss
the behavior of the brane bending mode within cosmological structure formation.
We find that, independently of CMB constraints, the self-accelerating DGP model
is strongly constrained by current weak lensing and cluster abundance
measurements.Comment: 21 pages; 10 figures. Revised version matching published versio
Cosmological Simulations of Normal-Branch Braneworld Gravity
We introduce a cosmological model based on the normal branch of DGP
braneworld gravity with a smooth dark energy component on the brane. The
expansion history in this model is identical to LambdaCDM, thus evading all
geometric constraints on the DGP cross-over scale r_c. This model can serve as
a first approximation to more general braneworld models whose cosmological
solutions have not been obtained yet. We study the formation of large scale
structure in this model in the linear and non-linear regime using N-body
simulations for different values of r_c. The simulations use the code presented
in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the
brane-bending mode in conjunction with the usual gravitational dynamics. The
brane-bending mode is attractive rather than repulsive in the DGP normal
branch, hence the sign of the modified gravity effects is reversed compared to
those presented in arXiv:0905.0858. We compare the simulation results with
those of ordinary LambdaCDM simulations run using the same code and initial
conditions. We find that the matter power spectrum in this model shows a
characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the
abundance of massive halos is significantly enhanced. Other results presented
here include the density profiles of dark matter halos, and signatures of the
brane-bending mode self-interactions (Vainshtein mechanism) in the simulations.
Independently of the expansion history, these results can be used to place
constraints on the DGP model and future generalizations through their effects
on the growth of cosmological structure.Comment: 17 pages, 10 figures; v2: minor changes; v3: references added; v4:
added appendix on comparison with previous results; matches published
version; v5: corrected Eqs. (2.4-2.5) and Fig. 1 following Ref. [28]; all
following results unchange
Coherence of Spin Qubits in Silicon
Given the effectiveness of semiconductor devices for classical computation
one is naturally led to consider semiconductor systems for solid state quantum
information processing. Semiconductors are particularly suitable where local
control of electric fields and charge transport are required. Conventional
semiconductor electronics is built upon these capabilities and has demonstrated
scaling to large complicated arrays of interconnected devices. However, the
requirements for a quantum computer are very different from those for classical
computation, and it is not immediately obvious how best to build one in a
semiconductor. One possible approach is to use spins as qubits: of nuclei, of
electrons, or both in combination. Long qubit coherence times are a
prerequisite for quantum computing, and in this paper we will discuss
measurements of spin coherence in silicon. The results are encouraging - both
electrons bound to donors and the donor nuclei exhibit low decoherence under
the right circumstances. Doped silicon thus appears to pass the first test on
the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200
Local versus site-level effects of algae on coral microbial communities
© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Briggs, A. A., Brown, A. L., & Osenberg, C. W. Local versus site-level effects of algae on coral microbial communities. Royal Society Open Science, 8(9), (2021): 210035, https://doi.org/10.1098/rsos.210035.Microbes influence ecological processes, including the dynamics and health of macro-organisms and their interactions with other species. In coral reefs, microbes mediate negative effects of algae on corals when corals are in contact with algae. However, it is unknown whether these effects extend to larger spatial scales, such as at sites with high algal densities. We investigated how local algal contact and site-level macroalgal cover influenced coral microbial communities in a field study at two islands in French Polynesia, Mo'orea and Mangareva. At 5 sites at each island, we sampled prokaryotic microbial communities (microbiomes) associated with corals, macroalgae, turf algae and water, with coral samples taken from individuals that were isolated from or in contact with turf or macroalgae. Algal contact and macroalgal cover had antagonistic effects on coral microbiome alpha and beta diversity. Additionally, coral microbiomes shifted and became more similar to macroalgal microbiomes at sites with high macroalgal cover and with algal contact, although the microbial taxa that changed varied by island. Our results indicate that coral microbiomes can be affected by algae outside of the coral's immediate vicinity, and local- and site-level effects of algae can obscure each other's effects when both scales are not considered.This research was supported by the University of Georgia, Odum School of Ecology's Small Grants programme and the University of Florida's John J. and Katherine C. Ewel Fellowship
BATSE Gamma-Ray Burst Line Search: V. Probability of Detecting a Line in a Burst
The physical importance of the apparent discrepancy between the detections by
pre-BATSE missions of absorption lines in gamma-ray burst spectra and the
absence of a BATSE line detection necessitates a statistical analysis of this
discrepancy. This analysis requires a calculation of the probability that a
line, if present, will be detected in a given burst. However, the connection
between the detectability of a line in a spectrum and in a burst requires a
model for the occurrence of a line within a burst. We have developed the
necessary weighting for the line detection probability for each spectrum
spanning the burst. The resulting calculations require a description of each
spectrum in the BATSE database. With these tools we identify the bursts in
which lines are most likely to be detected. Also, by assuming a small frequency
with which lines occur, we calculate the approximate number of BATSE bursts in
which lines of various types could be detected. Lines similar to the Ginga
detections can be detected in relatively few BATSE bursts; for example, in only
~20 bursts are lines similar to the GB 880205 pair of lines detectable. Ginga
reported lines at ~20 and ~40 keV whereas the low energy cutoff of the BATSE
spectra is typically above 20 keV; hence BATSE's sensitivity to lines is less
than that of Ginga below 40 keV, and greater above. Therefore the probability
that the GB 880205 lines would be detected in a Ginga burst rather than a BATSE
burst is ~0.2. Finally, we adopted a more appropriate test of the significance
of a line feature.Comment: 20 pages, AASTeX 4.0, 5 figures, Ap.J. in pres
Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts
We report evidence that the asymptotic low-energy power law slope alpha
(below the spectral break) of BATSE gamma-ray burst photon spectra evolves with
time rather than remaining constant. We find a high degree of positive
correlation exists between the time-resolved spectral break energy E_pk and
alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of
alpha was found to correlate with that of the spectral break energy E_pk at the
99.7% and 98% confidence levels respectively. We also find that in the flux
rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive
and in some bursts the maximum value of alpha is consistent with a value > +1.
BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These
findings challenge GRB spectral models in which alpha must be negative of
remain constant.Comment: 12 pages (including 6 figures), accepted to Ap
Spectral Hardness Decay with Respect to Fluence in BATSE Gamma-Ray Bursts
We have analyzed the evolution of the spectral hardness parameter Epk as a
function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with
the trend reported by Liang & Kargatis (1996) which found that Epk decays
exponentially with respect to photon fluence. We also fit these pulses with a
slight modification of this trend, where Epk decays linearly with energy
fluence. In both cases, we found the set of 41 pulses to be consistent with the
trend. For the latter trend, which we believe to be more physical, the
distribution of the decay constant is roughly log-normal, with a mean of 1.75
+/- 0.07 and a FWHM of 1.0 +/- 0.1. Regarding an earlier reported invariance in
the decay constant among different pulses in a single burst, we found
probabilities of 0.49 to 0.84 (depending on the test used) that such invariance
would occur by coincidence, most likely due to the narrow distribution of decay
constant values among pulses.Comment: 17 pages, 7 figure pages, 2 table pages, submitted to The
Astrophysical Journa
Large Scale Electronic Structure Calculations with Multigrid Acceleration
We have developed a set of techniques for performing large scale ab initio
calculations using multigrid accelerations and a real-space grid as a basis.
The multigrid methods permit efficient calculations on ill-conditioned systems
with long length scales or high energy cutoffs. The technique has been applied
to systems containing up to 100 atoms, including a highly elongated diamond
cell, an isolated C molecule, and a 32-atom cell of GaN with the Ga
d-states in valence. The method is well suited for implementation on both
vector and massively parallel architectures.Comment: 4 pages, 1 postscript figur
Two- and three-body color flux tubes in the Chromo Dielectric Model
Using the framework of the Chromo Dielectric Model we perform an analysis of
color electric flux tubes in meson-like and baryon-like quark
configurations. We discuss the Abelian color structure of the model and point
out a symmetry in color space as a remnant of the SU(3) symmetry of QCD. The
generic features of the model are discussed by varying the model parameters. We
fix these parameters by reproducing the string tension MeV/fm and
the transverse width fm of the flux tube obtained in
lattice calculations. We use a bag constant MeV, a glueball
mass MeV and a strong coupling constant . We show that the asymptotic string profile of an infinitely long flux
tube is already reached for separations fm. A connection
to the Dual Color Superconductor is made by extracting a magnetic current from
the model equations and a qualitative agreement between the two descriptions of
confinement is shown. In the study of the system we observe a
-like geometry for the color electric fields and a
\textsf{Y}-like geometry in the scalar fields both in the energy density
distribution and in the corresponding potentials. The resulting total
potential is described neither by the -picture nor by the
\textsf{Y}-picture alone.Comment: 32 pages, 35 eps-figures, revised version, some references + 1
eps-file added, to be published in Phys.Rev.
BATSE Gamma-Ray Burst Line Search: IV. Line Candidates from the Visual Search
We evaluate the significance of the line candidates identified by a visual
search of burst spectra from BATSE's Spectroscopy Detectors. None of the
candidates satisfy our detection criteria: an F-test probability less than
10^-4 for a feature in one detector and consistency among the detectors which
viewed the burst. Most of the candidates are not very significant, and are
likely to be fluctuations. Because of the expectation of finding absorption
lines, the search was biased towards absorption features. We do not have a
quantitative measure of the completeness of the search which would enable a
comparison with previous missions. Therefore a more objective computerized
search has begun.Comment: 18 pages AASTEX 4.0; 4 POSTSCRIPT figures on request from
[email protected]
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