9,222 research outputs found
Numerical Relativity: A review
Computer simulations are enabling researchers to investigate systems which
are extremely difficult to handle analytically. In the particular case of
General Relativity, numerical models have proved extremely valuable for
investigations of strong field scenarios and been crucial to reveal unexpected
phenomena. Considerable efforts are being spent to simulate astrophysically
relevant simulations, understand different aspects of the theory and even
provide insights in the search for a quantum theory of gravity. In the present
article I review the present status of the field of Numerical Relativity,
describe the techniques most commonly used and discuss open problems and (some)
future prospects.Comment: 2 References added; 1 corrected. 67 pages. To appear in Classical and
Quantum Gravity. (uses iopart.cls
Coordinate Singularities in Harmonically-sliced Cosmologies
Harmonic slicing has in recent years become a standard way of prescribing the
lapse function in numerical simulations of general relativity. However, as was
first noticed by Alcubierre (1997), numerical solutions generated using this
slicing condition can show pathological behaviour. In this paper, analytic and
numerical methods are used to examine harmonic slicings of Kasner and Gowdy
cosmological spacetimes. It is shown that in general the slicings are prevented
from covering the whole of the spacetimes by the appearance of coordinate
singularities. As well as limiting the maximum running times of numerical
simulations, the coordinate singularities can lead to features being produced
in numerically evolved solutions which must be distinguished from genuine
physical effects.Comment: 21 pages, REVTeX, 5 figure
Assessing the impact of algorithmic trading on markets: a simulation approach
Innovative automated execution strategies like Algorithmic Trading gain significant market share on electronic market venues worldwide, although their impact on market outcome has not been investigated in depth yet. In order to assess the impact of such concepts, e.g. effects on the price formation or the volatility of prices, a simulation environment is presented that provides stylized implementations of algorithmic trading behavior and allows for modeling latency. As simulations allow for reproducing exactly the same basic situation, an assessment of the impact of algorithmic trading models can be conducted by comparing different simulation runs including and excluding a trader constituting an algorithmic trading model in its trading behavior. By this means the impact of Algorithmic Trading on different characteristics of market outcome can be assessed. The results indicate that large volumes to execute by the algorithmic trader have an increasing impact on market prices. On the other hand, lower latency appears to lower market volatility
Redshift-Space Enhancement of Line-of-Sight Baryon Acoustic Oscillations in the SDSS Main-Galaxy Sample
We show that redshift-space distortions of galaxy correlations have a strong
effect on correlation functions with distinct, localized features, like the
signature of the baryon acoustic oscillations (BAO). Near the line of sight,
the features become sharper as a result of redshift-space distortions. We
demonstrate this effect by measuring the correlation function in Gaussian
simulations and the Millennium Simulation. We also analyze the SDSS DR7
main-galaxy sample (MGS), splitting the sample into slices 2.5 degrees on the
sky in various rotations. Measuring 2D correlation functions in each slice, we
do see a sharp bump along the line of sight. Using Mexican-hat wavelets, we
localize it to (110 +/- 10) Mpc/h. Averaging only along the line of sight, we
estimate its significance at a particular wavelet scale and location at 2.2
sigma. In a flat angular weighting in the (pi,r_p) coordinate system, the noise
level is suppressed, pushing the bump's significance to 4 sigma. We estimate
that there is about a 0.2% chance of getting such a signal anywhere in the
vicinity of the BAO scale from a power spectrum lacking a BAO feature. However,
these estimates of the significances make some use of idealized Gaussian
simulations, and thus are likely a bit optimistic.Comment: 17 pages, 27 figures. Minor changes to match final version accepted
to Ap
Trumpet Initial Data for Boosted Black Holes
We describe a procedure for constructing initial data for boosted black holes
in the moving-punctures approach to numerical relativity that endows the
initial time slice from the outset with trumpet geometry within the black hole
interiors. We then demonstrate the procedure in numerical simulations using an
evolution code from the Einstein Toolkit that employs 1+log slicing. The
Lorentz boost of a single black hole can be precisely specified and multiple,
widely separated black holes can be treated approximately by superposition of
single hole data. There is room within the scheme for later improvement to
re-solve (iterate) the constraint equations in the multiple black hole case.
The approach is shown to yield an initial trumpet slice for one black hole that
is close to, and rapidly settles to, a stationary trumpet geometry. Initial
data in this new approach is shown to contain initial transient (or "junk")
radiation that is suppressed by as much as two orders of magnitude relative to
that in comparable Bowen-York initial data.Comment: 18 pages, 18 figure
An excision scheme for black holes in constrained evolution formulations: spherically symmetric case
Excision techniques are used in order to deal with black holes in numerical
simulations of Einstein equations and consist in removing a topological sphere
containing the physical singularity from the numerical domain, applying instead
appropriate boundary conditions at the excised surface. In this work we present
recent developments of this technique in the case of constrained formulations
of Einstein equations and for spherically symmetric spacetimes. We present a
new set of boundary conditions to apply to the elliptic system in the
fully-constrained formalism of Bonazzola et al. (2004), at an arbitrary
coordinate sphere inside the apparent horizon. Analytical properties of this
system of boundary conditions are studied and, under some assumptions, an
exponential convergence toward the stationary solution is exhibited for the
vacuum spacetime. This is verified in numerical examples, together with the
applicability in the case of the accretion of a scalar field onto a
Schwarzschild black hole. We also present the successful use of the excision
technique in the collapse of a neutron star to a black hole, when excision is
switched on during the simulation, after the formation of the apparent horizon.
This allows the accretion of matter remaining outside the excision surface and
for the stable long-term evolution of the newly formed black hole.Comment: 14 pages, 9 figures. New section added and changes included according
to published articl
Deep Reinforcement Learning for Resource Management in Network Slicing
Network slicing is born as an emerging business to operators, by allowing
them to sell the customized slices to various tenants at different prices. In
order to provide better-performing and cost-efficient services, network slicing
involves challenging technical issues and urgently looks forward to intelligent
innovations to make the resource management consistent with users' activities
per slice. In that regard, deep reinforcement learning (DRL), which focuses on
how to interact with the environment by trying alternative actions and
reinforcing the tendency actions producing more rewarding consequences, is
assumed to be a promising solution. In this paper, after briefly reviewing the
fundamental concepts of DRL, we investigate the application of DRL in solving
some typical resource management for network slicing scenarios, which include
radio resource slicing and priority-based core network slicing, and demonstrate
the advantage of DRL over several competing schemes through extensive
simulations. Finally, we also discuss the possible challenges to apply DRL in
network slicing from a general perspective.Comment: The manuscript has been accepted by IEEE Access in Nov. 201
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