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141.
Vilkovisky has claimed to have solved the black hole backreaction problem and finds that black holes lose only ten percent
of their mass to Hawking radiation before evaporation ceases. We examine the implications of this scenario for cold dark matter,
assuming that primordial black holes are created during the reheating period after inflation. The mass spectrum is expected
to be dominated by 10-gram black holes. Nucleosynthesis constraints and the requirement that the earth presently exist do
not come close to ruling out such black holes as dark matter candidates. They also evade the demand that the photon density
produced by evaporating primordial black holes does not exceed the present cosmic radiation background by a factor of about
one thousand. 相似文献
142.
This paper investigates the extent to which entrenched interests of stakeholder groups both maintain water use practice, and may be confronted. The focus is on the agricultural sectors of Yemen and Jordan, where water resource policymakers face resistance in their attempts to reduce water use to environmentally sustainable levels through implementation of water demand management (WDM) activities. Some farmers in both countries that have invested in irrigated production of high-value crops (such as qat and bananas) benefit from a political economy that encourages increased rather than reduced water consumption. The resultant over-exploitation of water resources affects groups in unequal measures. Stakeholder analysis demonstrates that the more ‘powerful’ groups (chiefly the large landowners and the political elites, as well as the ministries of irrigation over which they exert influence) are generally opposed to reform in water use, while the proponents of WDM (e.g. water resource managers, environmental ministries and NGOs, and the international donor community) are found to have minimal influence over water use policy and decisionmaking. Efforts and ideas attempted by this latter group to challenge the status quo are classified here as either (a) influencing or (b) challenging the power asymmetry, and the merits and limits of both approaches are discussed. The interpretation of evidence suggests current practice is likely to endure, but may be more effectively challenged if a long-term approach is taken with an awareness of opportunities generated by windows of opportunity and the participation of ‘overlap groups’. 相似文献
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The results of a laboratory investigation of the dust devil vortex are presented. Vortex velocity and temperature profiles were measured for various initial conditiions of boundary heat input and outer flow swirl. Three different categories of core structure were observed depending on the amount of axial flow force and circulation strength produced in the experiment and dust devil models are constructed based on the measured mean behavior of each core category. 相似文献
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147.
Seung-Hyun Jeong Norman W. Paton Alvaro A. A. Fernandes Tony Griffiths 《Transactions in GIS》2005,9(2):129-156
Many applications capture, or make use of, spatial data that changes over time. This requirement for effective and efficient spatio‐temporal data management has given rise to a range of research activities relating to spatio‐temporal data management. Such work has sought to understand, for example, the requirements of different categories of application, and the modelling facilities that are most effective for these applications. However, at present, there are few systems with fully integrated support for spatio‐temporal data, and thus developers must often construct custom solutions for their applications. Developers of both bespoke solutions and of generic spatio‐temporal platforms will often need to support the fusion of large spatio‐temporal data sets. Supporting such requests in a database setting involves the use of join operations with both spatial and temporal conditions – spatio‐temporal joins. However, there has been little work to date on spatio‐temporal join algorithms or their evaluation. This paper presents an evaluation of several approaches to the implementation of spatio‐temporal joins that build upon widely available indexing techniques. The evaluation explores how several algorithms perform for databases with different spatial and temporal characteristics, with a view to helping developers of generic infrastructures or custom solutions in the selection and development of appropriate spatio‐temporal join strategies. 相似文献
148.
Tony J. Chandler 《Earth》1984,20(3):246-247
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