25,722 research outputs found
Dark energy, MOND and sub-millimeter tests of gravity
We consider modifications of General Relativity obtained by adding the
logarithm of some curvature invariants to the Einstein-Hilbert action. These
non-linear actions can explain the late-time acceleration of the universe
giving an expansion history that differs from that of a pure cosmological
constant. We show that they also modify the Newtonian potential below a fixed
acceleration scale given by the late-time Hubble constant times the speed of
light. This is exactly what is required in MOND, a phenomenological
modification of the Newtonian potential that is capable of explaining galactic
rotation curves without the need to introduce dark matter. We show that this
kind of modification also predicts short distance deviations of Newton's law at
the sub-mm scale and an anomalous shift in the precession of the Moon's orbit
around the Earth, both effects of a size that is less than an order of
magnitude below current bounds.Comment: 6 pages, to appear in proceedings of the XLIrst Rencontres de Morion
Long distance modifications of gravity in four dimensions
We discuss some general characteristics of modifications of the 4D
Einstein-Hilbert action that become important for low space-time curvatures. In
particular we focus on the chameleon-like behaviour of the massive
gravitational degrees of freedom. Generically there is at least one extra
scalar that is light on cosmic scales, but for certain models it becomes heavy
close to any mass source.Comment: 4 pages, contribution to the proceedings of the Rencontres de
Moriond: Contents and Structures of the Universe, March 18-25, 2006, La
Thuil
From Theory to Practice: Plug and Play with Succinct Data Structures
Engineering efficient implementations of compact and succinct structures is a
time-consuming and challenging task, since there is no standard library of
easy-to- use, highly optimized, and composable components. One consequence is
that measuring the practical impact of new theoretical proposals is a difficult
task, since older base- line implementations may not rely on the same basic
components, and reimplementing from scratch can be very time-consuming. In this
paper we present a framework for experimentation with succinct data structures,
providing a large set of configurable components, together with tests,
benchmarks, and tools to analyze resource requirements. We demonstrate the
functionality of the framework by recomposing succinct solutions for document
retrieval.Comment: 10 pages, 4 figures, 3 table
The Legacy Effect of Squatter Settlements on Urban Redevelopment
The paper presents a theoretical model that seeks to answer the question of why former squatter settlements tend to upgrade/redevelop at a slower pace than otherwise similar settlements originating in the formal sector. We argue that squatter settlers’ initial strategy to access urban land creates a ‘legacy effect’ that curtails settlement upgrading possibilities even after the settlements are granted property titles. We test our model using the case of Cochabamba, Bolivia and obtain results consistent with our theoretical model prediction. Our results suggest that the commonly used ‘benign neglect while keeping the threat of eviction’ policy has profound impacts on how land is developed in the informal sector and this poses costly consequences for local governments after legalization.squatters, informal settlements, urban development, neighbourhood upgrading, urban redevelopment
Gas-Rich Companions of Isolated Galaxies
We have used the VLA to search for gaseous remnants of the galaxy formation
process around six extremely isolated galaxies. We found two distinct HI clouds
around each of two galaxies in our sample (UGC 9762 & UGC 11124). These clouds
are rotating and appear to have optical counterparts, strongly implying that
they are typical dwarf galaxies. The companions are currently weakly
interacting with the primary galaxy, but have short dynamical friction
timescales (~1 Gyr) suggesting that these triple galaxy systems will shortly
collapse into one massive galaxy. Given that the companions are consistent with
being in circular rotation about the primary galaxy, and that they have small
relative masses, the resulting merger will be a minor one. The companions do,
however, contain enough gas that the merger will represent a significant
infusion of fuel to drive future star formation, bar formation, or central
activity, while building up the mass of the disk thus making these systems
important pieces of the galaxy formation and evolution process.Comment: Corrected dynamical friction calculation error. Revised discussion &
conclusions. 7 pages, 4 tables, 6 figures, to appear in May 1999 Astronomical
Journa
Document Retrieval on Repetitive Collections
Document retrieval aims at finding the most important documents where a
pattern appears in a collection of strings. Traditional pattern-matching
techniques yield brute-force document retrieval solutions, which has motivated
the research on tailored indexes that offer near-optimal performance. However,
an experimental study establishing which alternatives are actually better than
brute force, and which perform best depending on the collection
characteristics, has not been carried out. In this paper we address this
shortcoming by exploring the relationship between the nature of the underlying
collection and the performance of current methods. Via extensive experiments we
show that established solutions are often beaten in practice by brute-force
alternatives. We also design new methods that offer superior time/space
trade-offs, particularly on repetitive collections.Comment: Accepted to ESA 2014. Implementation and experiments at
http://www.cs.helsinki.fi/group/suds/rlcsa
Hollowgraphy Driven Holography: Black Hole with Vanishing Volume Interior
Hawking-Bekenstein entropy formula seems to tell us that no quantum degrees
of freedom can reside in the interior of a black hole. We suggest that this is
a consequence of the fact that the volume of any interior sphere of finite
surface area simply vanishes. Obviously, this is not the case in general
relativity. However, we show that such a phenomenon does occur in various
gravitational theories which admit a spontaneously induced general relativity.
In such theories, due to a phase transition (one parameter family degenerates)
which takes place precisely at the would have been horizon, the recovered
exterior Schwarzschild solution connects, by means of a self-similar transition
profile, with a novel 'hollow' interior exhibiting a vanishing spatial volume
and a locally varying Newton constant. This constitutes the so-called
'hollowgraphy' driven holography.Comment: Honorable Mention Essay - Gravity Research Foundation (2010
D=5 Einstein-Maxwell-Chern-Simons Black Holes
5-dimensional Einstein-Maxwell-Chern-Simons theory with Chern-Simons
coefficient has supersymmetric black holes with vanishing horizon
angular velocity, but finite angular momentum. Here supersymmetry is associated
with a borderline between stability and instability, since for a
rotational instability arises, where counterrotating black holes appear, whose
horizon rotates in the opposite sense to the angular momentum. For
black holes are no longer uniquely characterized by their global charges, and
rotating black holes with vanishing angular momentum appear.Comment: 4 pages, 4 figures, RevTeX styl
Observation of small scale structure using sextupole lensing
Weak gravitational lensing seeks to determine shear by measuring induced
quadrupole (elliptical) shapes in background galaxy images. Small impact
parameter (a few kpc) gravitational lensing by foreground core masses between 2
10^{9} and 2 10^{12} M_\odot will additionally induce a sextupole shape with
the quadrupole and sextupole minima aligned. This correlation in relative
orientation of the quadrupole and sextupole provides a sensitive method to
identify images which have been slightly curved by lensing events. A general
theoretical framework for sextupole lensing is developed which includes several
low order coefficients in a general lensing map. Tools to impute map
coefficients from the galaxy images are described and applied to the north
Hubble deep field. Instrumental PSFs, camera charge diffusion, and image
composition methods are modelled in the coefficient determination process.
Estimates of Poisson counting noise for each galaxy are used to cut galaxies
with signals too small to reliably establish curvature. Curved galaxies are
found to be spatially clumped, as would be expected if the curving were due to
small impact parameter lensing by localized ensembles of dark matter haloes.
Simulations provide an estimate of the total required lensing mass and the
acceptable mass range of the constituent haloes. The overdensities and
underdensities of visible galaxies and their locations in the Hubble foreground
is found to be consistent with our observations and their interpretation as
lensing events.Comment: 40 pages, 44 figure
Canonical Equivalence of a Generic 2D Dilaton Gravity Model and a Bosonic String Theory
We show that a canonical tranformation converts, up to a boundary term, a
generic 2d dilaton gravity model into a bosonic string theory with a
Minkowskian target space.Comment: LaTeX file, 9 pages, no figure
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