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961.
Lian-Zhong Lü 《中国天文和天体物理学报》2007,7(5):649-656
Recent applications of type Ia supernovae(SNe Ia)in cosmology have successfully revealed the accelerating expansion of the universe.However,as distance indicators used in measuring the expansion history of the universe and probing the nature of dark energy,these objects must pass more strict tests.We propose a K-S test to investigate if there exists any systematic bias when deriving the luminosity distances under the standard candle assumption. Two samples,one comprising 71 high-redshift SNe Ia and the other,44 nearby ones,are used in our investigation.We find that it is likely there exists a bias in the adopted samples,which is probably caused by a systematic error,e.g.in the color parameter used in the luminosity calibration and a bias may be caused by the SN evolution or by varying properties of the dust surrounding the SNe Ia. 相似文献
962.
Long-Term Earthquake Clustering 总被引:22,自引:0,他引:22
963.
Nikhil Padmanabhan David J. Schlegel Uro Seljak Alexey Makarov Neta A. Bahcall Michael R. Blanton Jonathan Brinkmann Daniel J. Eisenstein Douglas P. Finkbeiner James E. Gunn David W. Hogg eljko Ivezi Gillian R. Knapp Jon Loveday Robert H. Lupton Robert C. Nichol Donald P. Schneider Michael A. Strauss Max Tegmark Donald G. York 《Monthly notices of the Royal Astronomical Society》2007,378(3):852-872
964.
M. Sereno 《Monthly notices of the Royal Astronomical Society》2007,380(3):1207-1218
Knowledge of the intrinsic shape of galaxy clusters is very important in investigating cosmic structure formation and astrophysical processes. The reconstruction of the 3D structure usually relies on deprojecting 2D X-ray, Sunyaev–Zeldovich (SZ) and/or gravitational lensing observations. As known, a joint analysis of these data sets can provide the elongation of the cluster along the line of sight together with its length and width in the plane of the sky. An unbiased measurement of the Hubble constant can be also inferred. Due to some intrinsic degeneracies, the observational constraints obtained from such projected data sets are not enough to allow an unique inversion. In general, the projected maps can be at the same time compatible with prolate, oblate and with many triaxial configurations. Even a prolate cluster might be interpreted as an oblate system and vice versa. Assuming that the cluster is axially symmetric is likely to overestimate the intrinsic ellipticity, whereas the system always looks rounder performing the inversion under the hypothesis of a triaxial cluster aligned with the line of sight. In general, analysing triaxial clusters under the prolate or oblate assumption may introduce strong biases even when the clusters are actually near to axial symmetry whereas the systematics introduced assuming the cluster to be aligned with the line of sight are more under control. 相似文献
965.
J. S. Bagla 《Journal of Astrophysics and Astronomy》2002,23(3-4):185-196
We describe the TreePM method for carrying out large N-Body simulations to study formation and evolution of the large scale
structure in the Universe. This method is a combination of Barnes and Hut tree code and Particle-Mesh code. It combines the
automatic inclusion of periodic boundary conditions of PM simulations with the high resolution of tree codes. This is done
by splitting the gravitational force into a short range and a long range component. We describe the splitting of force between
these two parts. We outline the key differences between TreePM and some other N-Body methods. 相似文献
966.
General circulation models (GCMs), or stand‐alone models that are forced by the output from GCMs, are increasingly being used to simulate the interactions between snow cover, snowmelt, climate and water resources. The variation in snowpack extent, and hence albedo, through time in a cell is likely to be substantial, especially in mid‐latitude mountainous regions. As a consequence, the energy budget simulation by a GCM relies on a realistic representation of snowpack extent. Similarly, from a water resource perspective, the spatial extent of the pack is key in predicting meltwater discharges into rivers. In this paper a simple computationally efficient regional snow model has been developed, which is based on a degree‐day approach and simulates the fraction of the model domain covered by snow, the spatially averaged melt rate and the mean snowpack depth. Computational efficiency is achieved through a novel spatial averaging procedure, which relies on the assumptions that precipitation and temperature scale linearly with elevation and that the distribution of elevations in the domain can be modelled by a continuous function. The resulting spatially averaged model is compared with both observations of the duration of snow cover throughout Austria and with results from a distributed model based on the same underlying assumptions but applied at a fine spatial resolution. The new spatially averaged model successfully simulated the seasonal snow duration observations and reproduced the daily dynamics of snow cover extent, the spatially averaged melt rate and mean pack depth simulated by the distributed model. It, therefore, offers a computationally efficient and easily applied alternative to the current crop of regional snow models. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
967.
M.A. Hendry S. Rauzy S.P. Goodwin J. Gribbin 《Monthly notices of the Royal Astronomical Society》2001,324(3):717-724
We obtain a robust, non-parametric, estimate of the Hubble constant from the linear diameters and rotation velocities of galaxies in the recent KLUN sample, calibrated using Cepheid distances to Hubble Space Telescope Key Project galaxies. There are two key features that make our analysis considerably more robust than previous work. First, the method is independent of the spatial distribution of galaxies and is insensitive to Malmquist bias. It may, therefore, be applied to more distant samples than so-called 'plateau' methods – making it much less vulnerable to the impact of peculiar motions in the Local Supercluster. Secondly, we include information on the galaxy rotation velocities in a fully non-parametric manner: unlike the conventional Tully–Fisher relation we reconstruct a robust estimate of the cumulative distribution function of galaxy diameter at given rotation velocity, without requiring the assumption of, for example, a linear Tully–Fisher relation with symmetric Gaussian residuals.
Using this robust method we find H0 =65±6 km s−1 Mpc−1 from our analysis – in excellent agreement with many recent determinations of the Hubble parameter, although somewhat larger than previous results using galaxy diameters. 相似文献
Using this robust method we find H
968.
969.
SOCIAL CONSTRUCTION OF HYDROPOLITICS: THE GEOGRAPHICAL SCALES OF WATER AND SECURITY IN THE INDUS BASIN* 总被引:1,自引:0,他引:1
Daanish Mustafa 《Geographical review》2007,97(4):484-501
ABSTRACT. The article identifies important themes and future research directions for analyzing water and conflict dynamics at the subnational scale in the Indus Basin. A historical overview of water development in the Indus Basin suggests that the water‐security nexus was always a salient theme in the minds of water developers, even in the nineteenth century. Conflicts over contemporary large‐scale water‐development projects in the Indian and Pakistani parts of the Indus Basin are reviewed. Engineers' single‐minded focus on megaprojects, to the neglect of the wider set of values that societies attach to water resources in the eastern and western Indus Basin are largely to blame for continuing low‐grade conflict in the basin. A review of local‐level conflicts over water supply and sanitation in Karachi and the distribution of irrigation water in Pakistani Punjab illustrates the critical role of governance and differential social power relations in accentuating conflict. The article argues against neo‐Malthusian assumptions about the inevitability of conflict over water because of its future absolute scarcity. Instead, the article seeks to demonstrate that, despite evidence suggesting that international armed conflict over water does not exist, the potential for political instability over domestic water distribution and development issues is real. The question of whether conflict at the subnational scale will culminate in violence will depend on how water‐resources institutions in the basin behave. 相似文献
970.