4,930 research outputs found
Dipole anisotropies of IRAS galaxies and the contribution of a large-scale local void
Recent observations of dipole anisotropies show that the velocity of the
Local Group (\Vec v_{\rm G}) induced by the clustering of IRAS galax ies has
an amplitude and direction similar to those of the velocity of Cosmic Microwave
Background dipole anisotropy (\Vec v_{\rm CMB}), but the difference | \Vec
v_{\rm G} - \Vec v_{\rm CMB} | is still km/s, which is about 28% of
|\Vec v_{\rm CMB} |. Here we consider the possibility that the origin of this
difference comes from a hypothetical large-scale local void, with which we can
account for the accelerating behavior of type Ia supernovae due to the spatial
inhomogeneity of the Hubble constant without dark energies and derive the
constraint to the model parameters of the local void. It is found as a result
that the distance between the Local Group and the center of the void must be
Mpc, whose accurate value depends on the background model
parameters.Comment: 13 pages, 1 figure, to be published in ApJ 584, No.2 (2003
The Dyer-Roeder distance-redshift relation in inhomogeneous universes
Using Monte-Carlo methods, we determine the best-fit value of the homogeneity
parameter alpha in the Dyer-Roeder distance-redshift relation for a variety of
redshifts, inhomogeneity models and cosmological parameter values. The relation
between alpha and the fraction of compact objects, f_p, is found to be
approximately linear. This relation can be parametrized with reasonable
accuracy for all cases treated in this paper by alpha = a*f_p, where a = 0.6.Comment: 5 pages, 10 figures, submitted to Phys.Rev.
Photometric Observations of Star Formation Activity in Early Type Spirals
We observationally study the current star formation activities of early type
spiral galaxies. We construct a complete sample of 15 early type spirals having
far-infrared (FIR) to optical B band luminosity ratios, L(FIR)/L(B), larger
than the average of the type, and make their CCD imaging of the R and H-alpha
bands. The equivalent widths of H-alpha emission increase with increasing
L(FIR)/L(B), indicating that L(FIR)/L(B) can be an indicator of star formation
for such early type spirals with star formation activities higher than the
average. For all of the observed early type spirals, the extended HII regions
exist at the central regions with some asymmetric features. H-alpha emission is
more concentrated to the galactic center than the R band light, and the degree
of the concentration increases with the star formation activity. We also
analyze the relation between the star formation activities and the existence of
companion galaxies in the sample galaxies and other bright early type spirals.
No correlation is found and this suggests that the interaction is not
responsible for all of the star formation activities of early type spirals.Comment: LaTex, 23 pages (2 tables included), plus 9 Postscript figures & 1
table. To be published in AJ (November issue
Long wavelength iteration of Einstein's equations near a spacetime singularity
We clarify the links between a recently developped long wavelength iteration
scheme of Einstein's equations, the Belinski Khalatnikov Lifchitz (BKL) general
solution near a singularity and the antinewtonian scheme of Tomita's. We
determine the regimes when the long wavelength or antinewtonian scheme is
directly applicable and show how it can otherwise be implemented to yield the
BKL oscillatory approach to a spacetime singularity. When directly applicable
we obtain the generic solution of the scheme at first iteration (third order in
the gradients) for matter a perfect fluid. Specializing to spherical symmetry
for simplicity and to clarify gauge issues, we then show how the metric behaves
near a singularity when gradient effects are taken into account.Comment: 35 pages, revtex, no figure
Probability-Changing Cluster Algorithm: Study of Three-Dimensional Ising Model and Percolation Problem
We present a detailed description of the idea and procedure for the newly
proposed Monte Carlo algorithm of tuning the critical point automatically,
which is called the probability-changing cluster (PCC) algorithm [Y. Tomita and
Y. Okabe, Phys. Rev. Lett. {\bf 86} (2001) 572]. Using the PCC algorithm, we
investigate the three-dimensional Ising model and the bond percolation problem.
We employ a refined finite-size scaling analysis to make estimates of critical
point and exponents. With much less efforts, we obtain the results which are
consistent with the previous calculations. We argue several directions for the
application of the PCC algorithm.Comment: 6 pages including 8 eps figures, to appear in J. Phys. Soc. Jp
Renormalization Group Approach to Einstein Equation in Cosmology
The renormalization group method has been adapted to the analysis of the
long-time behavior of non-linear partial differential equation and has
demonstrated its power in the study of critical phenomena of gravitational
collapse. In the present work we apply the renormalization group to the
Einstein equation in cosmology and carry out detailed analysis of
renormalization group flow in the vicinity of the scale invariant fixed point
in the spherically symmetric and inhomogeneous dust filled universe model.Comment: 16 pages including 2 eps figures, RevTe
A Parallel Branch and Bound Algorithm for the Maximum Labelled Clique Problem
The maximum labelled clique problem is a variant of the maximum clique
problem where edges in the graph are given labels, and we are not allowed to
use more than a certain number of distinct labels in a solution. We introduce a
new branch-and-bound algorithm for the problem, and explain how it may be
parallelised. We evaluate an implementation on a set of benchmark instances,
and show that it is consistently faster than previously published results,
sometimes by four or five orders of magnitude.Comment: Author-final version. Accepted to Optimization Letter
Cross-Correlating Cosmic Microwave Background Radiation Fluctuations with Redshift Surveys: Detecting the Signature of Gravitational Lensing
Density inhomogeneities along the line-of-sight distort fluctuations in the
cosmic microwave background. Usually, this effect is thought of as a small
second-order effect that mildly alters the statistics of the microwave
background fluctuations. We show that there is a first-order effect that is
potentially observable if we combine microwave background maps with large
redshift surveys. We introduce a new quantity that measures this lensing
effect, , where T is the microwave
background temperature and is the lensing due to matter in the
region probed by the redshift survey. We show that the expected signal is first
order in the gravitational lensing bending angle, , and find that it should be easily detectable, (S/N) 15-35, if
we combine the Microwave Anisotropy Probe satellite and Sloan Digital Sky
Survey data. Measurements of this cross-correlation will directly probe the
``bias'' factor, the relationship between fluctuations in mass and fluctuations
in galaxy counts.Comment: 13 pages, 4 postscript figures included; Uses aaspp4.sty (AASTeX
v4.0); Accepted for publication in Astrophysical Journal, Part
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