2,579 research outputs found
On the Three Primordial Numbers
Cosmological observations have provided us with the measurement of just three
numbers that characterize the very early universe: , and
. Although each of the three numbers individually carries
limited information about the physics of inflation, one may hope to extract
non-trivial information from relations among them. Invoking minimality, namely
the absence of ad hoc large numbers, we find two viable and mutually exclusive
inflationary scenarios. The first is the well-known inverse relation between
and . The second implies a new relation between
and , which might provide us with a handle on the beginning of
inflation and predicts the intriguing bound on the
tensor-to-scalar ratio ( CL).Comment: 5 pages, 3 figure
Large-scale anomalies from primordial dissipation
We analyze an inflationary model in which part of the power in density
perturbations arises due to particle production. The amount of particle
production is modulated by an auxiliary field. Given an initial gradient for
the auxiliary field, this model produces a hemispherical power asymmetry and a
suppression of power at low multipoles similar to those observed by WMAP and
Planck in the CMB temperature. It also predicts an additive contribution to
with support only at very small that is aligned with the
direction of the power asymmetry and has a definite sign, as well as small
oscillations in the power spectrum at all .Comment: 1+15 pages, 7 figure
Inflation from Flux Cascades
When electric-type flux threads compact extra dimensions, a quantum
nucleation event can break a flux line and initiate a cascade that unwinds many
units of flux. Here, we present a novel mechanism for inflation based on this
phenomenon. From the 4D point of view, the cascade begins with the formation of
a bubble containing an open Friedmann-Robertson-Walker cosmology, but the
vacuum energy inside the bubble is initially only slightly reduced, and
subsequently decreases gradually throughout the cascade. If the initial flux
number Q_0 ~ O(100), during the cascade the universe can undergo N ~ 60 efolds
of inflationary expansion with gradually decreasing Hubble constant, producing
a nearly scale-invariant spectrum of adiabatic density perturbations with
amplitude and tilt consistent with observation, and a potentially observable
level of non-Gaussianity and tensor modes. The power spectrum has a small
oscillatory component that does not decay away during inflation, with a period
set approximately by the light-crossing time of the compact dimension(s). Since
the ingredients are fluxes threading compact dimensions, this mechanism fits
naturally into the string landscape, but does not appear to suffer from the eta
problem or require fine-tuning (beyond the usual anthropic requirement of small
vacuum energy after reheating).Comment: 5 pages, 1 figur
D-brane scattering and annihilation
We study the dynamics of parallel brane-brane and brane-antibrane scattering
in string theory in flat spacetime, focusing on the pair production of open
strings that stretch between the branes. We are particularly interested in the
case of scattering at small impact parameter , where there is a
tachyon in the spectrum when a brane and an antibrane approach within a string
length. Our conclusion is that despite the tachyon, branes and antibranes can
pass through each other with only a very small probability of annihilating, so
long as is small and the relative velocity is neither too small nor
too close to 1. Our analysis is relevant also to the case of charged open
string production in world-volume electric fields, and we make use of this
T-dual scenario in our analysis. We briefly discuss the application of our
results to a stringy model of inflation involving moving branes.Comment: 25+7 pages, 5 figure
Unwinding Inflation
Higher-form flux that extends in all 3+1 dimensions of spacetime is a source
of positive vacuum energy that can drive meta-stable eternal inflation. If the
flux also threads compact extra dimensions, the spontaneous nucleation of a
bubble of brane charged under the flux can trigger a classical cascade that
steadily unwinds many units of flux, gradually decreasing the vacuum energy
while inflating the bubble, until the cascade ends in the self-annihilation of
the brane into radiation. With an initial number of flux quanta Q_{0} \simgeq
N, this can result in N efolds of inflationary expansion while producing a
scale-invariant spectrum of adiabatic density perturbations with amplitude and
tilt consistent with observation. The power spectrum has an oscillatory
component that does not decay away during inflation, relatively large tensor
power, and interesting non-Gaussianities. Unwinding inflation fits naturally
into the string landscape, and our preliminary conclusion is that consistency
with observation can be attained without fine-tuning the string parameters. The
initial conditions necessary for the unwinding phase are produced automatically
by bubble formation, so long as the critical radius of the bubble is smaller
than at least one of the compact dimensions threaded by flux.Comment: 29+15 pages, 10 figures, published versio
Interactive rendering of massive geometric models
Booklet2005-02Conference held in Pisa, ItalyTutorial notes, Eurographics Italy. Conference held in Pisa, Italy, February 17--18, CDROM Proceedings, February 200
A Novel Framework for Highlight Reflectance Transformation Imaging
We propose a novel pipeline and related software tools for processing the multi-light image collections (MLICs) acquired in different application contexts to obtain shape and appearance information of captured surfaces, as well as to derive compact relightable representations of them. Our pipeline extends the popular Highlight Reflectance Transformation Imaging (H-RTI) framework, which is widely used in the Cultural Heritage domain. We support, in particular, perspective camera modeling, per-pixel interpolated light direction estimation, as well as light normalization correcting vignetting and uneven non-directional illumination. Furthermore, we propose two novel easy-to-use software tools to simplify all processing steps. The tools, in addition to support easy processing and encoding of pixel data, implement a variety of visualizations, as well as multiple reflectance-model-fitting options. Experimental tests on synthetic and real-world MLICs demonstrate the usefulness of the novel algorithmic framework and the potential benefits of the proposed tools for end-user applications.Terms: "European Union (EU)" & "Horizon 2020" / Action: H2020-EU.3.6.3. - Reflective societies - cultural heritage and European identity / Acronym: Scan4Reco / Grant number: 665091DSURF project (PRIN 2015) funded by the Italian Ministry of University and ResearchSardinian Regional Authorities under projects VIGEC and Vis&VideoLa
Time-critical multiresolution rendering of large complex models
Very large and geometrically complex scenes, exceeding millions of polygons and hundreds of objects, arise naturally in many areas of interactive computer graphics. Time-critical rendering of such scenes requires the ability to trade visual quality with speed. Previous work has shown that this can be done by representing individual scene components as multiresolution triangle meshes, and performing at each frame a convex constrained optimization to choose the mesh resolutions that maximize image quality while meeting timing constraints. In this paper we demonstrate that the nonlinear optimization problem with linear constraints associated to a large class of quality estimation heuristics is efficiently solved using an active-set strategy. By exploiting the problem structure, Lagrange multipliers estimates and equality constrained problem solutions are computed in linear time. Results show that our algorithms and data structures provide low memory overhead, smooth level-of-detail control, and guarantee, within acceptable limits, a uniform, bounded frame rate even for widely changing viewing conditions. Implementation details are presented along with the results of tests for memory needs, algorithm timing, and efficacy.785-803Pubblicat
WebVideo: simple web tools for video viewing and browsing
Poster SessionsIn this paper, we describe WebVideo, a library of simple tools for video browsing and viewing. The library offers a solid basis for developing web applications to view and browse video sequences and slide sets, as well as for distributing videos and images from real-time video sources. Since video transmission is based on a de-facto standard technology, the server-push method, remote users do not need any specific hardware or software in addition to a web browser. The tools offer an adequate support for applications where image quality and simplicity of use are more important than high frame rates and perfect audio synchronization. WebVideo is available in the public domain and has been used for over one year in applications such as distance learning, slides presentations, and Internet TV broadcasting.1997-04Santa Clara, CA, USASixth International World Wide Web Conferenc
Exploring virtual prototypes using time-critical rendering techniques
Scientists at CRS4, the Center for Advanced Studies, Research and Development in Cagliari, Sardinia, Italy, have developed a time-critical rendering algorithm that relies upon a scene description in which objects are represented as multiresolution meshes. In collaboration with other European partners, this technique has been applied to the visual and collaborative exploration of large digital mock-ups.Pubblicat
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