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排序方式: 共有246条查询结果,搜索用时 15 毫秒
81.
82.
Elaine M. Sadler Roberto Ricci Ronald D. Ekers J. A. Ekers Paul J. Hancock Carole A. Jackson Michael J. Kesteven Tara Murphy Chris Phillips Robert F. Reinfrank Lister Staveley-Smith Ravi Subrahmanyan Mark A. Walker Warwick E. Wilson Gianfranco De Zotti 《Monthly notices of the Royal Astronomical Society》2006,371(2):898-914
83.
R. Casas-Miranda H. J. Mo Ravi K. Sheth G. Boerner 《Monthly notices of the Royal Astronomical Society》2002,333(4):730-738
The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function P V ( N h | δ m ) , which gives the probability of finding N h haloes in a volume V with mass density contrast δ m . We study the properties of this function using high-resolution N -body simulations, and find that P V ( N h | δ m ) is significantly non-Poisson. The ratio between the variance and the mean goes from ∼1 (Poisson) at 1+ δ m ≪1 to <1 (sub-Poisson) at 1+ δ m ∼1 to >1 (super-Poisson) at 1+ δ m ≫1 . The mean bias relation is found to be well described by halo bias models based on the Press–Schechter formalism. The sub-Poisson variance can be explained as a result of halo exclusion, while the super-Poisson variance at high δ m may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behaviour of the variance as a function of δ m . Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behaviour. We discuss the implications of the stochasticity in halo bias to the modelling of higher order moments of dark haloes and of galaxies. 相似文献
84.
Ravi K. Sheth Antonaldo Diaferio 《Monthly notices of the Royal Astronomical Society》2001,322(4):901-917
We present a simple model for the shape of the distribution function of galaxy peculiar velocities. We show how both non-linear and linear theory terms combine to produce a distribution which has an approximately Gaussian core with exponential wings. The model is easily extended to study how the statistic depends on the type of particle used to trace the velocity field (dark matter particles, dark matter haloes, galaxies), and on the density of the environment in which the test particles are located. Comparisons with simulations suggest that our model is accurate. We also show that the evolution of the peculiar velocities depends on the local, rather than the global, density. Since clusters populate denser regions on average, using cluster velocities with the linear theory scaling may lead to an overestimate of the global value of Ω0 . Conversely, using linear theory with the global value of Ω0 to scale cluster velocities from the initial to the present time results in an underestimate of their true velocities. In general, however, the directions of motions of haloes are rather well described by linear theory. Our results help to simplify models of redshift-space distortions considerably. 相似文献
85.
Ravi Subrahmanyan 《Journal of Astrophysics and Astronomy》1997,18(4):251-255
The discovery of the 3K microwave background radiation (MBR) and its interpretation as a relict of the hot big bang was probably the most important
observation that led to the elevation of the hot big bang model to the status of a ‘Standard Model’. The temperature of this
background is consistent with the primordial nucleosynthesis hypothesis. Detailed measurements of the spectrum and angular
anisotropy of this radiation background have been found — within the measurement errors - to be consistent with the expectations
of the Standard Model and with the formation of structure from the gravitational growth of primordial seed density perturbations
within this framework. 相似文献
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Incorporating carbon offsets in the design of cap-and-trade programs remains a controversial issue because of its potential unintended impacts on emissions. At the heart of this discussion is the issue of crediting of emissions reductions. Projects can be correctly, over- or under-credited for their actual emissions reductions. We develop a unified framework that considers the supply of offsets within a cap-and-trade program that allows us to compare the relative impact of over-credited offsets and under-credited emissions reductions on overall emissions under different levels of baseline stringency and carbon prices. In the context of a national carbon pricing scheme that includes offsets, we find that the emissions impacts of over-credited offsets can be fully balanced out by under-credited emissions reductions without sacrificing a significant portion of the overall supply of offsets, provided emissions baselines are stringent enough. In the presence of high predicted business-as-usual (BAU) emissions uncertainty or low carbon prices, to maintain the environmental integrity of the program, baselines need to be set at stringent levels, in some cases below 50 percent of predicted BAU emissions. As predicted BAU emissions uncertainty declines or as the carbon market achieves higher equilibrium prices, however, less stringent baselines can balance out the emissions impacts of over-credited offsets and under-credited emissions reductions. These results imply that to maintain environmental integrity of offsets programs, baseline stringency should be tailored to project characteristics and market conditions that influence the proportion of over-credited offsets to under-credited emissions reductions. 相似文献
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The ability to accurately quantify shoreline variability is essential in order to establish aggressive mitigation strategies, based on recent global climate change projections. This investigation employed a suite of coastal data (topographic maps, aerial photography, satellite imagery and lidar) to establish decadal trends of shoreline movement along Perdido Key, a sandy barrier island off Florida’s northwest coast. The technique used to detect morphologic change with time was a recently developed tool, Analyzing Moving Boundaries Using R. This innovative methodology improves our understanding of the evolution of coastal systems by modeling shoreline variance using a method that is sensitive to shoreline shape. Results show that the barrier shoreline is a highly dynamic feature with distinct zones of erosion and accretion that are pervasive over time. In general, the island is displaying a mechanism of rotational instability with the eastern half retreating, and the western portion advancing. The inflection point, around which this rotational shifting is taking place, lies at the center of a Pleistocene headland located along the island’s midpoint. The results of this study suggest that coastal evolution along the island may be meta-stable, with trends in shoreline variance corresponding mainly to discrete storm events in time. 相似文献