5,469 research outputs found
The inflationary origin of the Cold Spot anomaly
Single-field inflation, arguably the simplest and most compelling paradigm
for the origin of our Universe, is strongly supported by the recent results of
the Planck satellite and the BICEP2 experiment. The results from Planck,
however, also confirm the presence of a number of anomalies in the Cosmic
Microwave Background (CMB), whose origin becomes problematic in single-field
inflation. Among the most prominent and well-tested of these anomalies is the
Cold Spot, which constitutes the only significant deviation from gaussianity in
the CMB. Planck's non-detection of primordial non-gaussianity on smaller scales
thus suggests the existence of a physical mechanism whereby significant
non-gaussianity is generated on large angular scales only. In this letter, we
address this question by developing a localized version of the inhomogeneous
reheating scenario, which postulates the existence of a scalar field able to
modify the decay of the inflaton on localized spatial regions only. We
demonstrate that if the Cold Spot is due to an overdensity in the last
scattering surface, the localization mechanism offers a feasible explanation
for it, thus providing a physical mechanism for the generation of localized
non-gaussianity in the CMB. If, on the contrary, the Cold Spot is caused by a
newly discovered supervoid (as recently claimed), we argue that the
localization mechanism, while managing to enhance underdensities, may well shed
light on the rarity of the discovered supervoid.Comment: 12 pages, 4 figures. v3 Comments and references added. It matches
published versio
On the coupling of vector fields to the Gauss-Bonnet invariant
Inflationary models including vector fields have attracted a great deal of
attention over the past decade. Such an interest owes to the fact that they
might contribute to, or even be fully responsible for, the curvature
perturbation imprinted in the CMB. However, the necessary breaking of the
vector field's conformal invariance during inflation is not without problems.
In recent years it has been realized that a number of instabilities endangering
the consistency of the theory arise when the conformal invariance is broken by
means of a non-minimal coupling to gravity. In this paper we consider a massive
vector field non-minimally coupled to gravity through the Gauss-Bonnet
invariant, and investigate whether the vector can obtain a nearly
scale-invariant perturbation spectrum while evading the emergence of
perturbative instabilities. We find that the strength of the coupling must be
extremely small if the vector field is to have a chance to contribute to the
total curvature perturbation.Comment: 8 pages, 1 figur
Topological Quintessence
A global monopole (or other topological defect) formed during a recent phase
transition with core size comparable to the present Hubble scale, could induce
the observed accelerating expansion of the universe. In such a model,
topological considerations trap the scalar field close to a local maximum of
its potential in a cosmologically large region of space. We perform detailed
numerical simulations of such an inhomogeneous dark energy system (topological
quintessence) minimally coupled to gravity, in a flat background of initially
homogeneous matter. We find that when the energy density of the field in the
monopole core starts dominating the background density, the spacetime in the
core starts to accelerate its expansion in accordance to a \Lambda CDM model
with an effective inhomogeneous spherical dark energy density parameter
\Omega_\Lambda(r). The matter density profile is found to respond to the global
monopole profile via an anti-correlation (matter underdensity in the monopole
core). Away from the monopole core, the spacetime is effectively
Einstein-deSitter (\Omega_\Lambda(r_{out}) -> 0) while at the center
\Omega_\Lambda(r ~ 0) is maximum. We fit the numerically obtained expansion
rate at the monopole core to the Union2 data and show that the quality of fit
is almost identical to that of \Lambda CDM. Finally, we discuss potential
observational signatures of this class of inhomogeneous dark energy models.Comment: Accepted in Phys. Rev. D (to appear). Added observational bounds on
parameters. 10 pages (two column revtex), 6 figures. The Mathematica files
used to produce the figures of this study may be downloaded from
http://leandros.physics.uoi.gr/topquin
DBI Galileon inflation in the light of Planck 2015
In this work we consider a DBI Galileon (DBIG) inflationary model and
constrain its parameter space with the Planck 2015 and BICEP2/Keck array and
Planck (BKP) joint analysis data by means of a potential independent analysis.
We focus our attention on inflationary solutions characterized by a constant or
varying sound speed as well as warp factor. We impose bounds on stringy aspects
of the model, such as the warp factor and the induced gravity
parameter . We study the parameter space of the model
and find that the tensor-to-scalar ratio can be as low as
and inflation happens to be at GUT scale. In addition,
we obtain the tilt of the tensor power spectrum and test the standard
inflationary consistency relation against the latest
bounds from the combined results of BKP+Laser Interferometer
Gravitational-Waves Observatory (LIGO), and find that DBIG inflation predicts a
red spectral index for the tensor power spectrum.Comment: Version accepted in JCAP. 25 pages, 10 figures, new refs adde
Concurrent Geometric Multicasting
We present MCFR, a multicasting concurrent face routing algorithm that uses
geometric routing to deliver a message from source to multiple targets. We
describe the algorithm's operation, prove it correct, estimate its performance
bounds and evaluate its performance using simulation. Our estimate shows that
MCFR is the first geometric multicast routing algorithm whose message delivery
latency is independent of network size and only proportional to the distance
between the source and the targets. Our simulation indicates that MCFR has
significantly better reliability than existing algorithms
Micropulse Transscleral Cyclophotocoagulation: A Hypothesis for the Ideal Parameters
MicroPulse transscleral cyclophotocoagulation (IRIDEX Corp., Mountain View, CA) is a novel technique that uses repetitive micropulses of active diode laser (On cycles) interspersed with resting intervals (Off cycles). It has been proposed that the OFF cycles allow thermal dissipation and, therefore, reduce collateral damage. The literature suggests that Micropulse has a better safety profile compared to traditional continuous-wave cyclophotocoagulation. However, because it is a relatively new technique, there are no clear guidelines stating the ideal laser parameters that would allow the best balance between high and sustained effectiveness with minimal side effects. This research reviewed the literature to approximate ideal parameters for single-session treatment. To simplify the comparison between studies, this study used Joules (J) as a way to standardize the energy levels employed. The reviewed clinical publications allowed reduction of these parameters to a range between 112 and 150 J of total energy, which allows a moderate IOP lowering effect of around 30% with few/no complications. An additional narrowing of the parameters was achieved after analyzing recently published experimental data. These data suggest a different mechanism of action for the Micropulse, similar to that of the pilocarpine. This effect was maximum at 150 J. Since clinical studies show few or no complications, even at those energy levels, it could be hypothesized that the ideal parameters can be located at a point closer to 150 J. This data also leads to the concept of dosimetry; the capacity to dose mTSCPC treatment based on desired IOP lowering effect and risk exposure. Further prospective studies are needed to test the proposed evidence-based hypothesis
Bitplane image coding with parallel coefficient processing
Image coding systems have been traditionally tailored for multiple instruction, multiple data (MIMD) computing. In general, they partition the (transformed) image in codeblocks that can be coded in the cores of MIMD-based processors. Each core executes a sequential flow of instructions to process the coefficients in the codeblock, independently and asynchronously from the others cores. Bitplane coding is a common strategy to code such data. Most of its mechanisms require sequential processing of the coefficients. The last years have seen the upraising of processing accelerators with enhanced computational performance and power efficiency whose architecture is mainly based on the single instruction, multiple data (SIMD) principle. SIMD computing refers to the execution of the same instruction to multiple data in a lockstep synchronous way. Unfortunately, current bitplane coding strategies cannot fully profit from such processors due to inherently sequential coding task. This paper presents bitplane image coding with parallel coefficient (BPC-PaCo) processing, a coding method that can process many coefficients within a codeblock in parallel and synchronously. To this end, the scanning order, the context formation, the probability model, and the arithmetic coder of the coding engine have been re-formulated. The experimental results suggest that the penalization in coding performance of BPC-PaCo with respect to the traditional strategies is almost negligible
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