28 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
Shape enhancement for rapid prototyping
Many applications, for instance in the reverse engineering and cultural heritage field, require to build a physical replica of 3D digital models. Recent 3D printers can easily perform this task in a relatively short time and using color to reproduce object textures. However, the finite resolution of printers and, most of all, some peculiar optical and physical properties of the used materials reduce their perceptual quality. The contribution of this paper is a shape enhancing technique, which allows users to increase readability of the tiniest details in physical replicas, without requiring manual post-reproduction interventions.831-840Pubblicat
Simulating Populations in Massive Urban Environments
This short paper reviews some of the results obtained withing the European Project CRIMSON.The United Nations recently reported that the global proportion of urban population reached 49% in 2005 and that 60% of the global population is expected to live in cities by 2030. Urbanised areas are extremely vulnerable to all sorts of threats. Indeed, the combination of heavy population concentrations, critical infrastructures and built environments make it possible for environmental, industrial or man-made incidents to rapidly escalate into major disorders. Recent events have forcefully demonstrated that
authorities at all levels of government turn out to be inadequately prepared for the intricacies and dilemmas of disasters in large urban environments. Therefore, innovative tools are needed to assist them in the studies, planning and inter-organizational preparation efforts, enabling to understand vulnerabilities and security issues, define and assess crisis management procedures, and train personnel. The CRIMSON research project has been funded by the European Commission in the field of Security Research to address this challenging need by researching, implementing and validating an innovative framework combining the latest virtual reality and simulation technologies. For that purpose, several technological challenges have been tackled by an international team of researchers, industrials and users, and important advances have
been made in the following fields
Planet-Sized Batched Dynamic Adaptive Meshes (P-BDAM)
This paper describes an efficient technique for out-of-core management and interactive rendering of planet sized textured terrain surfaces. The technique, called planet-sized batched dynamic adaptive meshes (P-BDAM), extends the BDAM approach by using as basic primitive a general triangulation of points on a displaced triangle. The proposed framework introduces several advances with respect to the state of the art: thanks to a batched host-to-graphics communication model, we outperform current adaptive tessellation solutions in terms of rendering speed; we guarantee overall geometric continuity, exploiting programmable graphics hardware to cope with the accuracy issues introduced by single precision floating points; we exploit a compressed out of core representation and speculative prefetching for hiding disk latency during rendering of out-of-core data; we efficiently construct high quality simplified representations with a novel distributed out of core simplification algorithm working on a standard PC network.147-15
Interactive Out-of-core Visualization of Very Large Landscapes on Commodity Graphics Platforms
We recently introduced an efficient technique for out-of-core rendering and management of large textured landscapes. The technique, called Batched Dynamic Adaptive Meshes (BDAM), is based on a paired tree structure: a tiled quadtree for texture data and a pair of bintrees of small triangular patches for the geometry. These small patches are TINs that are constructed and optimized off-line with high quality simplification and tristripping algorithms. Hierarchical view frustum culling and view-dependendent texture/geometry refinement is performed at each frame with a stateless traversal algorithm that renders a continuous adaptive terrain surface by assembling out of core data. Thanks to the batched CPU/GPU communication model, the proposed technique is not processor intensive and fully harnesses the power of current graphics hardware. This paper summarizes the method and discusses the results obtained in a virtual flythrough over a textured digital landscape derived from aerial imaging.21-2