12,977 research outputs found
Accurate initial conditions in mixed Dark Matter--Baryon simulations
We quantify the error in the results of mixed baryon--dark-matter
hydrodynamic simulations, stemming from outdated approximations for the
generation of initial conditions. The error at redshift 0 in contemporary large
simulations, is of the order of few to ten percent in the power spectra of
baryons and dark matter, and their combined total-matter power spectrum. After
describing how to properly assign initial displacements and peculiar velocities
to multiple species, we review several approximations: (1) {using the
total-matter power spectrum to compute displacements and peculiar velocities of
both fluids}, (2) scaling the linear redshift-zero power spectrum back to the
initial power spectrum using the Newtonian growth factor ignoring homogeneous
radiation, (3) using longitudinal-gauge velocities with synchronous-gauge
densities, and (4) ignoring the phase-difference in the Fourier modes for the
offset baryon grid, relative to the dark-matter grid. Three of these
approximations do not take into account that dark matter and baryons experience
a scale-dependent growth after photon decoupling, which results in directions
of velocity which are not the same as their direction of displacement. We
compare the outcome of hydrodynamic simulations with these four approximations
to our reference simulation, all setup with the same random seed and simulated
using Gadget-III.Comment: 10 pages, 5 figure
On the spatial distribution of neutral hydrogen in the Universe: bias and shot-noise of the HI Power Spectrum
The spatial distribution of neutral hydrogen (HI) in the Universe contains a
wealth of cosmological information. The 21 cm emission line can be used to map
the HI up to very high redshift and therefore reveal us something about the
evolution of the large scale structures in the Universe. However little is
known about the abundance and clustering properties of the HI over cosmic time.
Motivated by this, we build an analytic framework where the relevant parameters
that govern how the HI is distributed among dark matter halos can be fixed
using observations. At the same time we provide tools to study the column
density distribution function of the HI absorbers together with their
clustering properties. Our formalism is the first one able to account for all
observations at a single redshift, . The linear bias of the HI and the
mean number density of HI sources, two main ingredients in the calculation of
the signal-to-noise ratio of a cosmological survey, are then discussed in
detail, also extrapolating the results to low and high redshift. We find that
HI bias is relatively higher than the value reported in similar studies, but
the shot noise level is always sub dominant, making the HI Power Spectrum
always a high signal-to-noise measurements up to in the limit of no
instrumental noise and foreground contamination.Comment: 10 pages, 9 figure
Re-Pair Compression of Inverted Lists
Compression of inverted lists with methods that support fast intersection
operations is an active research topic. Most compression schemes rely on
encoding differences between consecutive positions with techniques that favor
small numbers. In this paper we explore a completely different alternative: We
use Re-Pair compression of those differences. While Re-Pair by itself offers
fast decompression at arbitrary positions in main and secondary memory, we
introduce variants that in addition speed up the operations required for
inverted list intersection. We compare the resulting data structures with
several recent proposals under various list intersection algorithms, to
conclude that our Re-Pair variants offer an interesting time/space tradeoff for
this problem, yet further improvements are required for it to improve upon the
state of the art
Representation of industrial products in the early stages of design: Drawing and artistic expression in industrial design
Comunicació presentada a ICERI 2018 11th annual International Conference of Education, Research and Innovation (Seville, Spain. 12-14 November, 2018)Hand drawing is a basic tool for industrial designers, as it allows them to represent and communicate concepts in an agile way during the initial design phase. Although we can find subjects related to drawing in the first years of all university degrees in industrial design, the way to implement the necessary activities is not always the most appropriate, and it may happen that, despite having practiced sketching, at the end of the course the students do not have the necessary skills to communicate their ideas effectively or adequately represent the reality that surrounds them.
This paper proposes twelve groups of activities designed to help industrial design students acquire skills related to hand drawing. The activities were implemented during the second course of the Degree in Industrial Design and Product Development Engineering at Universitat Jaume I, improving those implemented during the last course. The paper analyzes and discusses the positive results of the innovations introduced, which improved the mean grade of the course by 4.48% with respect to the grade obtained the previous year
Compact Binary Relation Representations with Rich Functionality
Binary relations are an important abstraction arising in many data
representation problems. The data structures proposed so far to represent them
support just a few basic operations required to fit one particular application.
We identify many of those operations arising in applications and generalize
them into a wide set of desirable queries for a binary relation representation.
We also identify reductions among those operations. We then introduce several
novel binary relation representations, some simple and some quite
sophisticated, that not only are space-efficient but also efficiently support a
large subset of the desired queries.Comment: 32 page
Electrically charged finite energy solutions of an and an Higgs-Chern-Simons--Yang-Mills-Higgs systems in dimensions
We study spherically symmetric finite energy solutions of two
Higgs-Chern-Simons--Yang-Mills-Higgs (HCS-YMH) models in dimensions, one
with gauge group and the other with . The Chern-Simons (CS)
densities are defined in terms of both the Yang-Mills (YM) and Higgs fields and
the choice of the two gauge groups is made so they do not vanish. The solutions
of the model carry only electric charge and zero magnetic charge, while
the solutions of the model are dyons carrying both electric and
magnetic charges like the Julia-Zee (JZ) dyon. Unlike the latter however, the
electric charge in both models receives an important contribution from the CS
dynamics. We pay special attention to the relation between the energies and
charges of these solutions. In contrast with the electrically charged JZ dyon
of the Yang-Mills-Higgs (YMH) system, whose mass is larger than that of the
electrically neutral (magnetic monopole) solutions, the masses of the
electrically charged solutions of our HCS-YMH models can be smaller than their
electrically neutral counterparts in some parts of the parameter space. To
establish this is the main task of this work, which is performed by
constructing the HCS-YMH solutions numerically. In the case of the
HCS-YMH, we have considered the question of angular momentum, and it turns out
that it vanishes.Comment: 20 pages, 10 figure
Charged Rotating Black Holes in Higher Dimensions
In recent years higher-dimensional black holes have attracted much interest
because of various developments in gravity and high energy physics. But whereas
higher-dimensional charged static (Tangherlini) and uncharged rotating
(Myers-Perry) black holes were found long ago, black hole solutions of
Einstein-Maxwell theory, are not yet known in closed form in more than 4
dimensions, when both electric charge and rotation are present. Here we
therefore study these solutions and those of Einstein-Maxwell-dilaton theory,
by using numerical and perturbative methods, and by exploiting the existence of
spacetime symmetries. The properties of these black holes reveal new
interesting features, not seen in D=4. For instance, unlike the D=4 Kerr-Newman
solution, they possess a non-constant gyromagnetic factor.Comment: 4 pages, 2 figures, to appear in Proceedings of Spanish Relativity
Meeting 2010 (ERE 2010) held in Granada, Spai
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