2,791 research outputs found
Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations
We test the ability of the numerical action method (NAM) to recover the
individual orbit histories of mass tracers in an expanding universe in a region
of radius 26Mpc/h, given the masses and redshift-space coordinates at the
present epoch. The mass tracers are represented by dark matter haloes
identified in a high resolution N-body simulation of the standard LCDM
cosmology. Since previous tests of NAM at this scale have traced the underlying
distribution of dark matter particles rather than extended haloes, our study
offers an assessment of the accuracy of NAM in a scenario which more closely
approximates the complex dynamics of actual galaxy haloes. We show that NAM can
recover present-day halo distances with typical errors of less than 3 per cent,
compared to 5 per cent errors assuming Hubble flow distances. The total halo
mass and the linear bias were both found to be constained at the 50 per cent
level. The accuracy of individual orbit reconstructions was limited by the
inability of NAM, in some instances, to correctly model the positions of haloes
at early times solely on the basis of the redshifts, angular positions, and
masses of the haloes at the present epoch. Improvements in the quality of NAM
reconstructions may be possible using the present-day three-dimensional halo
velocities and distances to further constrain the dynamics. This velocity data
is expected to become available for nearby galaxies in the coming generations
of observations by SIM and GAIA.Comment: 12 pages, 9 figures. submitted to MNRA
Delayed Recombination
Under the standard model for recombination of the primeval plasma, and the
cold dark matter model for structure formation, recent measurements of the
first peak in the angular power spectrum of the cosmic microwave background
temperature indicate the spatial geometry of the universe is nearly flat. If
sources of Lya resonance radiation, such as stars or active galactic nuclei,
were present at z ~ 1000 they would delay recombination, shifting the first
peak to larger angular scales, and producing a positive bias in this measure of
space curvature. It can be distinguished from space curvature by its
suppression of the secondary peaks in the spectrum.Comment: submitted to ApJ
Active Integrity Constraints and Revision Programming
We study active integrity constraints and revision programming, two
formalisms designed to describe integrity constraints on databases and to
specify policies on preferred ways to enforce them. Unlike other more commonly
accepted approaches, these two formalisms attempt to provide a declarative
solution to the problem. However, the original semantics of founded repairs for
active integrity constraints and justified revisions for revision programs
differ. Our main goal is to establish a comprehensive framework of semantics
for active integrity constraints, to find a parallel framework for revision
programs, and to relate the two. By doing so, we demonstrate that the two
formalisms proposed independently of each other and based on different
intuitions when viewed within a broader semantic framework turn out to be
notational variants of each other. That lends support to the adequacy of the
semantics we develop for each of the formalisms as the foundation for a
declarative approach to the problem of database update and repair. In the paper
we also study computational properties of the semantics we consider and
establish results concerned with the concept of the minimality of change and
the invariance under the shifting transformation.Comment: 48 pages, 3 figure
Limits on the integration constant of the dark radiation term in Brane Cosmology
We consider the constraints from primordial Helium abundances on the constant
of integration of the dark radiation term of the brane-world generalized
Friedmann equation derived from the Randall-Sundrum Single brane model. We
found that -- using simple, approximate and semianalytical Method -- that the
constant of integration is limited to be between -8.9 and 2.2 which limits the
possible contribution from dark radiation term to be approximately between -27%
to 7% of the background photon energy density.Comment: 8 page
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
Beyond single-level accounts: the role of cognitive architectures in cognitive scientific explanation
We consider approaches to explanation within the cognitive sciences that begin with Marr’s computational level (e.g., purely Bayesian accounts of cognitive phenomena) or Marr’s implementational level (e.g., reductionist accounts of cognitive phenomena based only on neural level evidence) and argue that each is subject to fundamental limitations which impair their ability to provide adequate explanations of cognitive phenomena. For this reason, it is argued, explanation cannot proceed at either level without tight coupling to the algorithmic and representation level. Even at this level, however, we argue that additional constraints relating to the decomposition of the cognitive system into a set of interacting subfunctions (i.e., a cognitive architecture) are required. Integrated cognitive architectures that permit abstract specification of the functions of components and that make contact with the neural level provide a powerful bridge for linking the algorithmic and representational level to both the computational level and the implementational level
Adiabatic instability in coupled dark energy-dark matter models
We consider theories in which there exists a nontrivial coupling between the
dark matter sector and the sector responsible for the acceleration of the
universe. Such theories can possess an adiabatic regime in which the
quintessence field always sits at the minimum of its effective potential, which
is set by the local dark matter density. We show that if the coupling strength
is much larger than gravitational, then the adiabatic regime is always subject
to an instability. The instability, which can also be thought of as a type of
Jeans instability, is characterized by a negative sound speed squared of an
effective coupled dark matter/dark energy fluid, and results in the exponential
growth of small scale modes. We discuss the role of the instability in specific
coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and
clarify for these theories the regimes in which the instability can be evaded
due to non-adiabaticity or weak coupling.Comment: 20 pages, 2 figures; final published versio
Particle linear theory on a self-gravitating perturbed cubic Bravais lattice
Discreteness effects are a source of uncontrolled systematic errors of N-body
simulations, which are used to compute the evolution of a self-gravitating
fluid. We have already developed the so-called "Particle Linear Theory" (PLT),
which describes the evolution of the position of self-gravitating particles
located on a perturbed simple cubic lattice. It is the discrete analogue of the
well-known (Lagrangian) linear theory of a self-gravitating fluid. Comparing
both theories permits to quantify precisely discreteness effects in the linear
regime. It is useful to develop the PLT also for other perturbed lattices
because they represent different discretizations of the same continuous system.
In this paper we detail how to implement the PLT for perturbed cubic Bravais
lattices (simple, body and face-centered) in a cubic simulation box. As an
application, we will study the discreteness effects -- in the linear regime --
of N-body simulations for which initial conditions have been set-up using these
different lattices.Comment: 9 pages, 4 figures and 4 tables. Minor corrections to match published
versio
The Measure of Cosmological Parameters
New, large, ground and space telescopes are contributing to an exciting and
rapid period of growth in observational cosmology. The subject is now far from
its earlier days of being data-starved and unconstrained, and new data are
fueling a healthy interplay between observations and experiment and theory. I
briefly review here the status of measurements of a number of quantities of
interest in cosmology: the Hubble constant, the total mass-energy density, the
matter density, the cosmological constant or dark energy component, and the
total optical background light.Comment: 12 pages, 4 figures, to be published in "2001: A Spacetime Odyssey:
Proceedings of the Inaugural Conference of the Michigan Center for
Theoretical Physics", Michael J. Duff & James T. Liu, eds., (World
Scientific, Singapore), in pres
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