78,302 research outputs found
Intrinsic alignments of galaxies in the Horizon-AGN cosmological hydrodynamical simulation
The intrinsic alignments of galaxies are recognised as a contaminant to weak
gravitational lensing measurements. In this work, we study the alignment of
galaxy shapes and spins at low redshift () in Horizon-AGN, an
adaptive-mesh-refinement hydrodynamical cosmological simulation box of 100
Mpc/h a side with AGN feedback implementation. We find that spheroidal galaxies
in the simulation show a tendency to be aligned radially towards over-densities
in the dark matter density field and other spheroidals. This trend is in
agreement with observations, but the amplitude of the signal depends strongly
on how shapes are measured and how galaxies are selected in the simulation.
Disc galaxies show a tendency to be oriented tangentially around spheroidals in
three-dimensions. While this signal seems suppressed in projection, this does
not guarantee that disc alignments can be safely ignored in future weak lensing
surveys. The shape alignments of luminous galaxies in Horizon-AGN are in
agreement with observations and other simulation works, but we find less
alignment for lower luminosity populations. We also characterize the
systematics of galaxy shapes in the simulation and show that they can be safely
neglected when measuring the correlation of the density field and galaxy
ellipticities.Comment: 20 pages, 23 figure
Structure formation in Multiple Dark Matter cosmologies with long-range scalar interactions
(Abridged) An interaction between Cold Dark Matter (CDM) and a classical
scalar field playing the role of the cosmic dark energy (DE) might provide
long-range dark interactions without conflicting with solar system bounds.
Although presently available observations allow to constrain such interactions
to a few percent of the gravitational strength, some recent studies have shown
that if CDM is composed by two different particle species having opposite
couplings to the DE field, such tight constraints can be considerably relaxed,
allowing for long-range scalar forces of order gravity without significantly
affecting observations both at the background and at the linear perturbations
level. In the present work, we extend the investigation of such Multiple Dark
Matter scenarios to the nonlinear regime of structure formation, by presenting
the first N-body simulations ever performed for these cosmologies. Our results
highlight some characteristic footprints of long-range scalar forces that arise
only in the nonlinear regime for specific models that would be otherwise
practically indistinguishable from the standard LCDM scenario both in the
background and in the growth of linear density perturbations. Among these
effects, the formation of "mirror" cosmic structures in the two CDM species,
the suppression of the nonlinear matter power spectrum at k > 1 h/Mpc, and the
fragmentation of collapsed halos, represent peculiar features that might
provide a direct way to constrain this class of cosmological models.Comment: 11 pages, 4 figures. Submitted to MNRA
Occlusion resistant learning of intuitive physics from videos
To reach human performance on complex tasks, a key ability for artificial
systems is to understand physical interactions between objects, and predict
future outcomes of a situation. This ability, often referred to as intuitive
physics, has recently received attention and several methods were proposed to
learn these physical rules from video sequences. Yet, most of these methods are
restricted to the case where no, or only limited, occlusions occur. In this
work we propose a probabilistic formulation of learning intuitive physics in 3D
scenes with significant inter-object occlusions. In our formulation, object
positions are modeled as latent variables enabling the reconstruction of the
scene. We then propose a series of approximations that make this problem
tractable. Object proposals are linked across frames using a combination of a
recurrent interaction network, modeling the physics in object space, and a
compositional renderer, modeling the way in which objects project onto pixel
space. We demonstrate significant improvements over state-of-the-art in the
intuitive physics benchmark of IntPhys. We apply our method to a second dataset
with increasing levels of occlusions, showing it realistically predicts
segmentation masks up to 30 frames in the future. Finally, we also show results
on predicting motion of objects in real videos
Zero-gravity movement studies
The use of computer graphics to simulate the movement of articulated animals and mechanisms has a number of uses ranging over many fields. Human motion simulation systems can be useful in education, medicine, anatomy, physiology, and dance. In biomechanics, computer displays help to understand and analyze performance. Simulations can be used to help understand the effect of external or internal forces. Similarly, zero-gravity simulation systems should provide a means of designing and exploring the capabilities of hypothetical zero-gravity situations before actually carrying out such actions. The advantage of using a simulation of the motion is that one can experiment with variations of a maneuver before attempting to teach it to an individual. The zero-gravity motion simulation problem can be divided into two broad areas: human movement and behavior in zero-gravity, and simulation of articulated mechanisms
On the spectral density from instantons in quenched QCD
We investigate the contribution of instantons to the eigenvalue spectrum of
the Dirac operator in quenched QCD. The instanton configurations that we use
have been derived, elsewhere, from cooled SU(3) lattice gauge fields and, for
comparison, we also analyse a random `gas' of instantons. Using a set of
simplifying approximations, we find a non-zero chiral condensate. However we
also find that the spectral density diverges for small eigenvalues, so that the
chiral condensate, at zero quark mass, diverges in quenched QCD. The degree of
divergence decreases with the instanton density, so that it is negligible for
the smallest number of cooling sweeps but becomes substantial for larger number
of cools. We show that the spectral density scales, that finite volume
corrections are small and we see evidence for the screening of topological
charges. However we also find that the spectral density and chiral condensate
vary rapidly with the number of cooling sweeps -- unlike, for example, the
topological susceptibility. Whether the problem lies with the cooling or with
the identification of the topological charges is an open question. This problem
needs to be resolved before one can determine how important is the divergence
we have found for quenched QCD.Comment: 33 pages, 16 figures (RevTex), substantial revisions; to appear in
Phys.Rev.
Multiscale Phenomenology of the Cosmic Web
We analyze the structure and connectivity of the distinct morphologies that
define the Cosmic Web. With the help of our Multiscale Morphology Filter (MMF),
we dissect the matter distribution of a cosmological CDM N-body
computer simulation into cluster, filaments and walls. The MMF is ideally
suited to adress both the anisotropic morphological character of filaments and
sheets, as well as the multiscale nature of the hierarchically evolved cosmic
matter distribution. The results of our study may be summarized as follows:
i).- While all morphologies occupy a roughly well defined range in density,
this alone is not sufficient to differentiate between them given their overlap.
Environment defined only in terms of density fails to incorporate the intrinsic
dynamics of each morphology. This plays an important role in both linear and
non linear interactions between haloes. ii).- Most of the mass in the Universe
is concentrated in filaments, narrowly followed by clusters. In terms of
volume, clusters only represent a minute fraction, and filaments not more than
9%. Walls are relatively inconspicous in terms of mass and volume. iii).- On
average, massive clusters are connected to more filaments than low mass
clusters. Clusters with M h have on average
two connecting filaments, while clusters with M
h have on average five connecting filaments. iv).- Density profiles
indicate that the typical width of filaments is 2\Mpch. Walls have less well
defined boundaries with widths between 5-8 Mpc h. In their interior,
filaments have a power-law density profile with slope ,
corresponding to an isothermal density profile.Comment: 28 pages, 22 figures, accepted for publication in MNRAS. For a
high-res version see http://www.astro.rug.nl/~weygaert/webmorph_mmf.pd
A macroscopic analytical model of collaboration in distributed robotic systems
In this article, we present a macroscopic analytical model of collaboration in a group of reactive robots. The model consists of a series of coupled differential equations that describe the dynamics of group behavior. After presenting the general model, we analyze in detail a case study of collaboration, the stick-pulling experiment, studied experimentally and in simulation by Ijspeert et al. [Autonomous Robots, 11, 149-171]. The robots' task is to pull sticks out of their holes, and it can be successfully achieved only through the collaboration of two robots. There is no explicit communication or coordination between the robots. Unlike microscopic simulations (sensor-based or using a probabilistic numerical model), in which computational time scales with the robot group size, the macroscopic model is computationally efficient, because its solutions are independent of robot group size. Analysis reproduces several qualitative conclusions of Ijspeert et al.: namely, the different dynamical regimes for different values of the ratio of robots to sticks, the existence of optimal control parameters that maximize system performance as a function of group size, and the transition from superlinear to sublinear performance as the number of robots is increased
- …