2,848 research outputs found
The Long-Term Future of Space Travel
The fact that we apparently live in an accelerating universe places
limitations on where humans might visit. If the current energy density of the
universe is dominated by a cosmological constant, a rocket could reach a galaxy
observed today at a redshift of 1.7 on a one-way journey or merely 0.65 on a
round trip. Unfortunately these maximal trips are impractical as they require
an infinite proper time to traverse. However, calculating the rocket trajectory
in detail shows that a rocketeer could nearly reach such galaxies within a
lifetime (a long lifetime admittedly -- about 100 years). For less negative
values of the maximal redshift increases becoming infinite for .Comment: 5 pages, 3 figures, minor changes to reflect version accepted to PR
Dynamics of gas bubbles in an oscillating pressure field Quarterly report, Jul. - Sep. 1965
Mechanism of bubble formation and dynamics of bubble behavior in vibrating liquid
Figures of merit and constraints from testing General Relativity using the latest cosmological data sets including refined COSMOS 3D weak lensing
We use cosmological constraints from current data sets and a figure of merit
(FoM) approach to probe any deviations from general relativity (GR) at
cosmological scales. The FoM approach is used to study the constraining power
of various combinations of data sets on modified gravity (MG) parameters. We
use recently refined HST-COSMOS weak-lensing tomography data, ISW-galaxy cross
correlations from 2MASS and SDSS LRG surveys, matter power spectrum from
SDSS-DR7 (MPK), WMAP7 temperature and polarization spectra, BAO from 2DF and
SDSS-DR7, and Union2 compilation of supernovae, in addition to other bounds
from H_0 measurements and BBN. We use 3 parametrizations of MG parameters that
enter the perturbed field equations. In order to allow for variations with
redshift and scale, the first 2 parametrizations use recently suggested
functional forms while the third is based on binning methods. Using the first
parametrization, we find that CMB + ISW + WL provides the strongest constraints
on MG parameters followed by CMB+WL or CMB+MPK+ISW. Using the second
parametrization or binning methods, CMB+MPK+ISW consistently provides some of
the strongest constraints. This shows that the constraints are parametrization
dependent. We find that adding up current data sets does not improve
consistently uncertainties on MG parameters due to tensions between best-fit MG
parameters preferred by different data sets. Furthermore, some functional forms
imposed by the parametrizations can lead to an exacerbation of these tensions.
Next, unlike some studies that used the CFHTLS lensing data, we do not find any
deviation from GR using the refined HST-COSMOS data, confirming previous claims
in those studies that their result may have been due to some systematic effect.
Finally, we find in all cases that the values corresponding to GR are within
the 95% confidence level contours for all data set combinations. (abridged)Comment: 18 pages, 6 figures, matches version published in PR
Intelligent Data Storage and Retrieval for Design Optimisation – an Overview
This paper documents the findings of a literature review conducted by the Sir Lawrence Wackett Centre for Aerospace Design Technology at RMIT University. The review investigates aspects of a proposed system for intelligent design optimisation. Such a system would be capable of efficiently storing (and compressing if required) a range of types of design data into an intelligent database. This database would be accessed by the system during subsequent design processes, allowing for search of relevant design data for re-use in later designs, allowing it to become very efficient in reducing the time for later designs as the database grows in size. Extensive research has been performed, in both theoretical aspects of the project, and practical examples of current similar systems. This research covers the areas of database systems, database queries, representation and compression of design data, geometric representation and heuristic methods for design applications.
Layzer-Irvine equation: new perspectives and the role of interacting dark energy
We derive the Layzer-Irvine equation in the presence of a homogeneous (or
quasi-homogeneous) dark energy component with an arbitrary equation of state.
We extend the Layzer-Irvine equation to homogeneous and isotropic universes
with an arbitrary number of dimensions and obtain the corresponding virial
relation for sufficiently relaxed objects. We find analogous equations
describing the dynamics of cosmic string loops and other p-branes of arbitrary
dimensionality, discussing the corresponding relativistic and non-relativistic
limits. Finally, we generalize the Layzer-Irvine equation to account for a
non-minimal interaction between dark matter and dark energy, discussing its
practical use as a signature of such an interaction.Comment: 4 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
The Evolution of Bias - Generalized
Fry (1996) showed that galaxy bias has the tendency to evolve towards unity,
i.e. in the long run, the galaxy distribution tends to trace that of matter.
Generalizing slightly Fry's reasoning, we show that his conclusion remains
valid in theories of modified gravity (or equivalently, complex clustered dark
energy). This is not surprising: as long as both galaxies and matter are
subject to the same force, dynamics would drive them towards tracing each
other. This holds, for instance, in theories where both galaxies and matter
move on geodesics. This relaxation of bias towards unity is tempered by cosmic
acceleration, however: the bias tends towards unity but does not quite make it,
unless the formation bias were close to unity. Our argument is extended in a
straightforward manner to the case of a stochastic or nonlinear bias. An
important corollary is that dynamical evolution could imprint a scale
dependence on the large scale galaxy bias. This is especially pronounced if
non-standard gravity introduces new scales to the problem: the bias at
different scales relaxes at different rates, the larger scales generally more
slowly and retaining a longer memory of the initial bias. A consistency test of
the current (general relativity + uniform dark energy) paradigm is therefore to
look for departure from a scale independent bias on large scales. A simple way
is to measure the relative bias of different populations of galaxies which are
at different stages of bias relaxation. Lastly, we comment on the possibility
of directly testing the Poisson equation on cosmological scales, as opposed to
indirectly through the growth factor.Comment: 8 pages, 2 figures. References added. Accepted for publication in
Physical Review
CMB Constraint on Radion Evolution in the Brane World Scenario
In many versions of brane model, the modulus field of extra dimensions, the
radion, could have cosmological evolution, which induces variation of the Higgs
vacuum expectation value, , resulting in cosmological variation of the
electron mass $m_e$. The formation of Cosmic Microwave Background (CMB)
anisotropies is thus affected, causing changes both in the peaks positions and
amplitudes in the CMB power spectra. Using the three-year Wilkinson Microwave
Anisotropies Probe (WMAP) CMB data, with the Hubble parameter $H_0$ fixed to be
the Hubble Space Telescope (HST) result 72 km s$^{-1}$ Mpc$^{-1}$, we obtain a
constraint on $\rho$, the ratio of the value of at CMB recombination to
its present value, to be [0.97, 1.02].Comment: 7 pages, 6 figures, minor changes of format to conform with PRD
forma
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
Signature of the interaction between dark energy and dark matter in observations
We investigate the effect of an interaction between dark energy and dark
matter upon the dynamics of galaxy clusters. This effect is computed through
the Layser-Irvine equation, which describes how an astrophysical system reaches
virial equilibrium and was modified to include the dark interactions. Using
observational data from almost 100 purportedly relaxed galaxy clusters we put
constraints on the strength of the couplings in the dark sector. We compare our
results with those from other observations and find that a positive (in the
sense of energy flow from dark energy to dark matter) non vanishing interaction
is consistent with the data within several standard deviations.Comment: 13 pages, 3 figures; matches PRD published versio
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