785 research outputs found
Towards Mixed Gr{\"o}bner Basis Algorithms: the Multihomogeneous and Sparse Case
One of the biggest open problems in computational algebra is the design of
efficient algorithms for Gr{\"o}bner basis computations that take into account
the sparsity of the input polynomials. We can perform such computations in the
case of unmixed polynomial systems, that is systems with polynomials having the
same support, using the approach of Faug{\`e}re, Spaenlehauer, and Svartz
[ISSAC'14]. We present two algorithms for sparse Gr{\"o}bner bases computations
for mixed systems. The first one computes with mixed sparse systems and
exploits the supports of the polynomials. Under regularity assumptions, it
performs no reductions to zero. For mixed, square, and 0-dimensional
multihomogeneous polynomial systems, we present a dedicated, and potentially
more efficient, algorithm that exploits different algebraic properties that
performs no reduction to zero. We give an explicit bound for the maximal degree
appearing in the computations
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
Constraining the second half of reionization with the Ly ÎČ forest
We present an analysis of the evolution of the Lyman-series forest into the
epoch of reionization using cosmological radiative transfer simulations in a
scenario where reionization ends late. We explore models with different
midpoints of reionization and gas temperatures. We find that once the
simulations have been calibrated to match the mean flux of the observed
Lyman- forest at , they also naturally reproduce the
distribution of effective optical depths of the Lyman- forest in this
redshift range. We note that the tail of the largest optical depths that is
most challenging to match corresponds to the long absorption trough of ULAS
J0148+0600, which we have previously shown to be rare in our simulations. We
consider the evolution of the Lyman-series forest out to higher redshifts, and
show that future observations of the Lyman- forest at will
discriminate between different reionization histories. The evolution of the
Lyman- and Lyman- forests are less promising as a tool for
pushing studies of reionization to higher redshifts due to the stronger
saturation and foreground contamination, respectively
Le traumatisme de guerre dans le roman europĂ©en (1920 - 1940) : entre âhystĂ©rie masculineâ et âmythe de la guerreâ, problĂšmes dâune histoire culturelle
Actes du XXXVe CongrÚs de la SFLGC, Université de Bourgogne, 2008</p
V.I. Vernadsky and the noosphere concept: Russian understandings of society-nature interaction
Recent Russian legislative and policy documentation concerning national progress towards sustainable development has suggested that the attainment of such a state would represent the first stage in the development of the noosphere as outlined by the Russian scientist Vladimir Ivanovich Vernadsky (1863â1945). This paper explores Vernadskyâs model of evolutionary change through a focus on his work on the biosphere and noosphere in an attempt to further understanding of the way in which Russia is approaching the concept of sustainable development in the contemporary period. It is argued that the official Russian interpretation of the noosphere idea tends to obscure the evolutionary and materialist foundations of Vernadskyâs biosphereânoosphere conceptualisation. At the same time, the concluding section of the paper suggests that the scope of Vernadskyâs work can be used to stimulate the search for a more coherent approach to work in areas of sustainable development and sustainability across the span of the social and physical sciences
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Large Ly α opacity fluctuations and low CMB Ï in models of late reionization with large islands of neutral hydrogen extending to z < 5.5
High-redshift QSO spectra show large spatial fluctuations in the Ly-alpha
opacity of the intergalactic medium on surprisingly large scales at z>~5.5. We
present a radiative transfer simulation of cosmic reionization driven by
galaxies that reproduces this large scatter and the rapid evolution of the
Ly-alpha opacity distribution at 5<z<6. The simulation also reproduces the low
Thomson scattering optical depth reported by the latest CMB measurement and is
consistent with the observed short near-zones and strong red damping wings in
the highest-redshift QSOs. It also matches the rapid disappearance of observed
Ly-alpha emission by galaxies at z>~6. Reionization is complete at z=5.3 in our
model, and 50% of the volume of the Universe is ionized at z=7. Agreement with
the Ly-alpha forest data in such a late reionization model requires a rapid
evolution of the ionizing emissivity of galaxies that peaks at z~6.8. The late
end of reionization results in a large scatter in the photoionisation rate and
the neutral hydrogen fraction at redshifts as low as z<~5.5 with large residual
neutral 'islands' that can produce very long Gunn-Peterson troughs resembling
those seen in the data
CPT symmetry and antimatter gravity in general relativity
The gravitational behavior of antimatter is still unknown. While we may be
confident that antimatter is self-attractive, the interaction between matter
and antimatter might be either attractive or repulsive. We investigate this
issue on theoretical grounds. Starting from the CPT invariance of physical
laws, we transform matter into antimatter in the equations of both
electrodynamics and gravitation. In the former case, the result is the
well-known change of sign of the electric charge. In the latter, we find that
the gravitational interaction between matter and antimatter is a mutual
repulsion, i.e. antigravity appears as a prediction of general relativity when
CPT is applied. This result supports cosmological models attempting to explain
the Universe accelerated expansion in terms of a matter-antimatter repulsive
interaction.Comment: 6 pages, to be published in EPL (http://epljournal.edpsciences.org/
"Dark energy" in the Local Void
The unexpected discovery of the accelerated cosmic expansion in 1998 has
filled the Universe with the embarrassing presence of an unidentified "dark
energy", or cosmological constant, devoid of any physical meaning. While this
standard cosmology seems to work well at the global level, improved knowledge
of the kinematics and other properties of our extragalactic neighborhood
indicates the need for a better theory. We investigate whether the recently
suggested repulsive-gravity scenario can account for some of the features that
are unexplained by the standard model. Through simple dynamical considerations,
we find that the Local Void could host an amount of antimatter
() roughly equivalent to the mass of a typical
supercluster, thus restoring the matter-antimatter symmetry. The antigravity
field produced by this "dark repulsor" can explain the anomalous motion of the
Local Sheet away from the Local Void, as well as several other properties of
nearby galaxies that seem to require void evacuation and structure formation
much faster than expected from the standard model. At the global cosmological
level, gravitational repulsion from antimatter hidden in voids can provide more
than enough potential energy to drive both the cosmic expansion and its
acceleration, with no need for an initial "explosion" and dark energy.
Moreover, the discrete distribution of these dark repulsors, in contrast to the
uniformly permeating dark energy, can also explain dark flows and other
recently observed excessive inhomogeneities and anisotropies of the Universe.Comment: 6 pages, accepted as a Letter to the Editor by Astrophysics and Space
Scienc
The meaning of life in a developing universe
The evolution of life on Earth has produced an organism that is beginning to model and understand its own evolution and the possible future evolution of life in the universe. These models and associated evidence show that evolution on Earth has a trajectory. The scale over which living processes are organized cooperatively has increased progressively, as has its evolvability. Recent theoretical advances raise the possibility that this trajectory is itself part of a wider developmental process. According to these theories, the developmental process has been shaped by a larger evolutionary process that involves the reproduction of universes. This evolutionary process has tuned the key parameters of the universe to increase the likelihood that life will emerge and develop to produce outcomes that are successful in the larger process (e.g. a key outcome may be to produce life and intelligence that intentionally reproduces the universe and tunes the parameters of âoffspringâ universes). Theory suggests that when life emerges on a planet, it moves along this trajectory of its own accord. However, at a particular point evolution will continue to advance only if organisms emerge that decide to advance the evolutionary process intentionally. The organisms must be prepared to make this commitment even though the ultimate nature and destination of the process is uncertain, and may forever remain unknown. Organisms that complete this transition to intentional evolution will drive the further development of life and intelligence in the universe. Humanityâs increasing understanding of the evolution of life in the universe is rapidly bringing it to the threshold of this major evolutionary transition
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