Stability of marginally outer trapped surfaces and existence of marginally outer trapped tubes

The present work extends our short communication Phys. Rev. Lett. 95, 111102 (2005). For smooth marginally outer trapped surfaces (MOTS) in a smooth spacetime we define stability with respect to variations along arbitrary vectors v normal to the MOTS. After giving some introductory material about linear non self-adjoint elliptic operators, we introduce the stability operator L_v and we characterize stable MOTS in terms of sign conditions on the principal eigenvalue of L_v. The main result shows that given a strictly stable MOTS S contained in one leaf of a given reference foliation in a spacetime, there is an open marginally outer trapped tube (MOTT), adapted to the reference foliation, which contains S. We give conditions under which the MOTT can be completed. Finally, we show that under standard energy conditions on the spacetime, the MOTT must be either locally achronal, spacelike or null.Comment: 33 pages, no figures, typos corrected, minor changes in presentatio

Do as the Neighbors Do: The Impact of Social Networks on Immigrant Employment

Substantial immigrant segregation in the United States, combined with the increase in the share of the U.S. foreign-born population, have led to great interest in the causes and consequences of immigrant concentration, including those related to the functioning of labor markets. This paper provides robust evidence that both the size and the quality of an immigrant enclave affects the labor market outcomes of new immigrants. We develop new measures of the quality, or information value, of immigrant networks by exploiting data based on worker earnings records matched to firm and Census information. We demonstrate the importance of immigrant employment links: network members are much more likely than other immigrants to be employed in the same firm as their geographic neighbors. Immigrants living with large numbers of employed neighbors are more likely to have jobs than immigrants in areas with fewer employed neighbors. The effects are quantitatively important and robust under alternative specifications. For example, in a high value network – one with an average employment rate in the 90th percentile – a one standard deviation increase in the log of the number of contacts in the network is associated with almost a 5% increase in the employment rate. Earnings, conditional on employment, increase by about 0.7%.social networks, immigrant enclaves, labor market intermediaries

The Parenthood “Happiness Penalty”: The Effects of Social Policies in 22 Countries

A large body of research has established that parents are less happy than nonparents. But is it always true that parents are less happy than nonparents? This research brief, by PRC faculty research associate Jennifer Glass and colleagues, shows that the “happiness penalty” is entirely explained by the presence or absence of social policies that allow parents to better combine paid work with family obligations.Population Research Cente

Cities, Matching and the Productivity Gains of Agglomeration

The striking geographical concentration of economic activities suggests that there are substantial benefits toagglomeration. However, the nature of those benefits remains unclear. In this paper we take advantage of a newdataset to quantify the role of one of the main contenders - the matching of workers and jobs. Using individuallevel data for two large US states we show that thicker urban labour markets are associated with moreassortative matching between workers and firms. Another critical condition is required for this to generatehigher productivity: complementarity of worker and firm quality in the production function. Usingestablishment level productivity regressions, we show that such complementarity is found in our data. Puttingtogether the production and matching relationships, we show that production complementarity and assortativematching is an important source of the urban productivity premium.Urban Productivity, Matching, Agglomeration

Boumediene v. Bush: Another Chapter in the Court’s Jurisprudence on Civil Liberties at Guantanamo Bay

A recent surge in the usage of instant messaging (IM) applications on mobile devices has brought the energy efficiency of these applications into focus of attention. Although IM applications are changing the message communication landscape, this work illustrates that the current versions of IM applications differ vastly in energy consumption when using the third generation (3G) cellular communication. This paper shows the interdependency between energy consumption and IM data patterns in this context. We analyse the user interaction pattern using a IM dataset, consisting of 1043370 messages collected from 51 mobile users. Based on the usage characteristics, we propose a message bundling technique that aggregates consecutive messages over time, reducing the energy consumption with a trade-off against latency. The results show that message bundling can save up to 43% in energy consumption while still maintaining the conversation function. Finally, the energy cost of a common functionality used in IM applications that informs that the user is currently typing a response, so called typing notification, is evaluated showing an energy increase ranging from 40-104%

The time evolution of marginally trapped surfaces

In previous work we have shown the existence of a dynamical horizon or marginally trapped tube (MOTT) containing a given strictly stable marginally outer trapped surface (MOTS). In this paper we show some results on the global behavior of MOTTs assuming the null energy condition. In particular we show that MOTSs persist in the sense that every Cauchy surface in the future of a given Cauchy surface containing a MOTS also must contain a MOTS. We describe a situation where the evolving outermost MOTS must jump during the coalescence of two seperate MOTSs. We furthermore characterize the behavior of MOTSs in the case that the principal eigenvalue vanishes under a genericity assumption. This leads to a regularity result for the tube of outermost MOTSs under the genericity assumption. This tube is then smooth up to finitely many jump times. Finally we discuss the relation of MOTSs to singularities of a space-time.Comment: 21 pages. This revision corrects some typos and contains more detailed proofs than the original versio

Changing ground layer : ground lichen after clear-cutting

Skogsbruk och renskötsel är två olika näringar som i dagsläget verkar på samma marker. Renskötseln använder marken för bete samtidigt som skogsbruket producerar förnybara råvaror i form av träprodukter från skogen. Studier pekar på att vinterbetestillgången för renskötseln i form av marklavar minskar till följd av olika skogsbruksåtgärder men att det finns potential att lindra denna trend. I denna rapport sökte vi att bidra till förståelsen för hur marklavar utvecklas efter en slutavverkning. Data som följer permanenta ytor från Riksskogstaxeringens inventeringar mellan åren 1983–2017 användes och en generalized estimating equation model gjordes för att modellera utvecklingen. Resultaten visade att den lavtäckningsgrad som marken tillhörde innan en avverkning hade signifikant påverkan på vilken täckningsgrad som återfanns i senare inventeringar. Dock visades att tid sen avverkning inte var signifikant. Detta tyder alltså på att ingen förändring över tid för marklavarna sågs efter en slutavverkning och att vidare forskning behövs för att definiera sambandet bättre.Forestry and reindeer husbandry are two competing forms of land use on mutually shared land. Reindeer husbandry utilizes the land for grazing whilst forestry uses it for producing timber and pulpwood. Studies have shown that winter grazing opportunities for reindeer are declining, as a result of various forestry measures, as well as the potential for alleviating this trend. In this study we tried to contribute with further understanding of how clearcutting effects ground lichen, and how they develop succeedingly. Data tracking current states of permanent plots from the Swedish national forest inventory during the years 1983-2017 were used and fitted a generalized estimation equation model to track change. The results show that the degree of lichen cover before clear cutting can be used to model what state would be found after. However, a significant relation over time could not be found. This suggests that change in lichen cover over time is not determined solely on its state before clearcutting and that further research is required to determine the influencing factors

Targeting for energy efficiency and improved energy collaboration between different companies using total site analysis (TSA)

Rising fuel prices, increasing costs associated with emissions of green house gases and the threat of global warming make efficient use of energy more and more important. Industrial clusters have the potential to significantly increase energy efficiency by energy collaboration. In this paper Sweden’s largest chemical cluster is analysed using the total site analysis (TSA) method. TSA delivers targets for the amount of utility consumed and generated through excess energy recovery by the different processes. The method enables investigation of opportunities to deliver waste heat from one process to another using a common utility system. The cluster consists of 5 chemical companies producing a variety of products, including polyethylene (PE), polyvinyl chloride (PVC), amines, ethylene, oxygen/nitrogen and plasticisers. The companies already work together by exchanging material streams. In this study the potential for energy collaboration is analysed in order to reach an industrial symbiosis. The overall heating and cooling demands of the site are around 442 MW and 953 MW, respectively. 122 MW of heat is produced in boilers and delivered to the processes. TSA is used to stepwise design a site-wide utility system which improves energy efficiency. It is shown that heat recovery in the cluster can be increased by 129 MW, i.e. the current utility demand could be completely eliminated and further 7 MW excess steam can be made available. The proposed retrofitted utility system involves the introduction of a site-wide hot water circuit, increased recovery of low pressure steam and shifting of heating steam pressure to lower levels in a number heat exchangers when possible. Qualitative evaluation of the suggested measures shows that 60 MW of the savings potential could to be achieved with moderate changes to the process utility system corresponding to 50% of the heat produced from purchased fuel in the boilers of the cluster. Further analysis showed that after implementation of the suggested energy efficiency measures there is still a large excess of heat at temperatures of up to 137 °C

Total Site Analysis (TSA) Stenungsund

This project was carried out in cooperation between the Division of Heat and Power Technology at Chalmers University of Technology, CIT Industriell Energianalys AB, AGA Gas AB, Akzo Nobel Sverige AB, Borealis AB, INEOS Sverige AB and Perstorp Oxo AB. A Total Site Analysis (TSA) was performed in this study which can be used as a basis for future implementations of energy system integration at the chemical cluster in Stenungsund. At first stream data (Tstart, Ttarget, Q) and data on overall utility consumption of all the processes in the cluster was collected. The analysis is based on data collected on process streams heated or cooled with utility exceeding a heat load of 300 kW. Additionally steam from by-product incineration which cannot be utilised in another way is considered as process heat. With this data the current energy system was analysed by determining steam excess and deficit at each steam level and company. After that, the data was represented in curves, the so called total site profiles (TSP) and the total site composite curves (TSC). The curves were used to determine the site pinch (the limiting factor for further integration) and to identify measures to increase heat recovery. The measures found by TSA were assessed qualitatively with respect to feasibility to determine the most attractive measures. Finally the site wide potential for cogeneration and measures for reduction of external cooling demand below ambient temperature was analysed. Main findings are presented in the following: From the stream data collected is can be seen that the total demand of hot and cold utility of the cluster is 442 MW and 953 MW respectively. By-products, which have to be incinerated on-site provide 40 MW of steam. To cover the external heat demand additional 122 MW of heat is supplied by steam/hot oil from boilers or directly by flue gas from added fuels purchased or available on site. The TSP and TSC curves show a site pinch at the 2 bar(g) steam system (132 °C). The site pinch limits the potential for heat integration. To increase energy savings by heat integration it is necessary to change the position of the site pinch. It was shown that theoretically by introducing a site-wide hot water circuit, increased recovery of 2 bar(g) steam and adjustment steam levels in several heat exchangers the pinch point can be moved so that hot utility savings of 122 MW plus excess of 7 MW steam at 85 bar(g) can be realised. Only introducing a hot water circuit can save 51 MW of steam from added fuels, which corresponds to estimated savings of 122 MSEK/year. It is possible to replace more steam by hot water, but the demand for 2 bar(g) steam is limited. Therefore a demand for low pressure steam must be created by adjusting steam levels in order to utilise more waste heat in a hot water circuit. The present delivery of heat to the district heating system is not affected by a site wide hot water circuit. There is potential for increased recovery of 33 MW of 2 bar(g) steam from process heat. This would replace the production of the same amount of steam in the boilers, worth 79 MSEK/year. A qualitative assessment on the implementation of a hot water circuit shows estimated steam savings of 55.2 MW (132 MSEK/year) with moderate changes (83.5 MW including more complex changes, 200 MSEK/year). Technically the introduction of a hot water circuit includes hot water pipes between several plants, as most of the consumers of heat are situated at the cracker site and at Perstorp but the sources are spread out across the cluster. Also piping is necessary to transfer the 2 bar(g) steam replaced by hot water to other plants with steam deficit. The practical potential for increased 2 bar(g) steam recovery is estimated to 4.2 MW (10 MSEK/year) with moderate changes and 26.6 MW including more complex changes (64 MSEK/year). Increased 2 bar(g) recovery implies the construction of steam pipes from Borealis to Perstorp and INEOS, as most of the potential steam sources are located at Borealis but Perstorp and INEOS have a demand for 2 bar(g) steam The theoretical cogeneration potential in the cluster is 19 MWel in addition to the 10 MWel generated today (additional revenue is 40 MSEK/year) assuming that steam demand at all pressure levels remains the same but the steam systems are connected with each other. A practical option to increase cogeneration with the existing equipment is to supply steam below 8.8 bar(g) produced at Borealis to INEOS, Akzo and Perstorp. This would result in additional 8.6 MWel by cogeneration in Borealis turbo-alternator (estimated revenue: 18 MSEK/year). Some process streams below ambient temperature are heated with steam. It has been shown that 6.5 MW steam is used for heating stream well below ambient temperature. This steam can be saved and the cooling energy can be recovered. This decreases the energy usage in the cooling system and also saves heating steam. Savings up to 48 MSEK/year were estimated. It has been shown that by site wide collaboration it is possible to increase heat recovery, cogeneration and utilisation of waste heat. The results from this study are the bases to identify concrete projects which contribute to cost and CO2 emissions savings. The study also shows the advantages of TSA in order to find solutions for process integration by the utility system on a site wide level

Asymptotically simple solutions of the vacuum Einstein equations in even dimensions

We show that a set of conformally invariant equations derived from the Fefferman-Graham tensor can be used to construct global solutions of the vacuum Einstein equations, in all even dimensions. This gives, in particular, a new, simple proof of Friedrich's result on the future hyperboloidal stability of Minkowski space-time, and extends its validity to even dimensions.Comment: 25p