Selection of high-z supernovae candidates

Deep, ground based, optical wide-field supernova searches are capable of detecting a large number of supernovae over a broad redshift range up to z~1.5. While it is practically unfeasible to obtain spectroscopic redshifts of all the supernova candidates right after the discovery, we show that the magnitudes and colors of the host galaxies, as well as the supernovae, can be used to select high-z supernova candidates, for subsequent spectroscopic and photometric follow-up. Using Monte-Carlo simulations we construct criteria for selecting galaxies in well-defined redshift bands. For example, with a selection criteria using B-R and R-I colors we are able to pick out potential host galaxies for which z>0.85 with 80% confidence level and with a selection efficiency of 64-86%. The method was successfully tested using real observations from the HDF. Similarly, we show that that the magnitude and colors of the supernova discovery data can be used to constrain the redshift. With a set of cuts based on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a redshift interval sigma_z <0.15.Comment: 33 pages, 13 figures, accepted for publication in PASP (March 2002 issue

Stability of Electrically Conducting Rotating Fluid in the Presence of a Horizontal Magnetic Field

The Rayleigh instability of an incompressible, finitely conducting, inviscid fluid of variable density is investigated under the influence of an horizontal magnetic field. Due to finite conducting of the fluid, it is found that potentially stable or unstable configuration retains its character. It may further be noted that the stratification can be stabilized for a certain wave-number range in the absence of finite resistivity. The effect of finite resistivity is this to wipe out the aforesaid range of wave-number for which the system is stabilized otherwise

Supernova cosmology: legacy and future

The discovery of dark energy by the first generation of high-redshift supernova surveys has generated enormous interest beyond cosmology and has dramatic implications for fundamental physics. Distance measurements using supernova explosions are the most direct probes of the expansion history of the Universe, making them extremely useful tools to study the cosmic fabric and the properties of gravity at the largest scales. The past decade has seen the confirmation of the original results. Type Ia supernovae are among the leading techniques to obtain high-precision measurements of the dark energy equation of state parameter, and in the near future, its time dependence. The success of these efforts depends on our ability to understand a large number of effects, mostly of astrophysical nature, influencing the observed flux at Earth. The frontier now lies in understanding if the observed phenomenon is due to vacuum energy, albeit its unnatural density, or some exotic new physics. Future surveys will address the systematic effects with improved calibration procedures and provide thousands of supernovae for detailed studies.Comment: Invited review, Annual Review of Nuclear and Particle Science (submitted version

Horizontal cooling towers: riverine ecosystem services and the fate of thermoelectric heat in the contemporary Northeast US

The electricity sector is dependent on rivers to provide ecosystem services that help regulate excess heat, either through provision of water for evaporative cooling or by conveying, diluting and attenuating waste heat inputs. Reliance on these ecosystem services alters flow and temperature regimes, which impact fish habitat and other aquatic ecosystem services. We demonstrate the contemporary (2000–2010) dependence of the electricity sector on riverine ecosystem services and associated aquatic impacts in the Northeast US, a region with a high density of thermoelectric power plants. We quantify these dynamics using a spatially distributed hydrology and water temperature model (the framework for aquatic modeling in the Earth system), coupled with the thermoelectric power and thermal pollution model. We find that 28.4% of thermoelectric heat production is transferred to rivers, whereas 25.9% is directed to vertical cooling towers. Regionally, only 11.3% of heat transferred to rivers is dissipated to the atmosphere and the rest is delivered to coasts, in part due to the distribution of power plants within the river system. Impacts to the flow regime are minimal, while impacts to the thermal regime include increased river lengths of unsuitable habitats for fish with maximum thermal tolerances of 24.0, 29.0, and 34.0 ° C in segments downstream of plants by 0.6%, 9.8%, and 53.9%, respectively. Our analysis highlights the interactions among electricity production, cooling technologies, aquatic impacts, and ecosystem services, and can be used to assess the full costs and tradeoffs of electricity production at regional scales

On Classification of QCD defects via holography

We discuss classification of defects of various codimensions within a holographic model of pure Yang-Mills theories or gauge theories with fundamental matter. We focus on their role below and above the phase transition point as well as their weights in the partition function. The general result is that objects which are stable and heavy in one phase are becoming very light (tensionless) in the other phase. We argue that the $\theta$ dependence of the partition function drastically changes at the phase transition point, and therefore it correlates with stability properties of configurations. Some possible applications for study the QCD vacuum properties above and below phase transition are also discussed.Comment: 21 pages, 2 figure

Política de educação e discurso político: The American Federation for Children redes

This article presents findings from an analysis of the AFC policy network using tools from network ethnography and qualitative content analysis. Specifically, we examined tax forms and carried out extensive web searches to spatialize and map the AFC network, mined text from policy-actors in the AFC network, and analyzed the policy discourse promoted by these network actors to achieve their political goals. The task for this study was to use AFC as a heuristic device to explore the complexity of the education policy field and to understand how network policy-actors work to achieve their policy goals through advocacy and marketing. Findings from the study indicate that the AFC network demonstrates a hierarchical ordering, this hierarchical ordering is reflective of the elite planning and social engineering associated with neoliberal reforms, and that the policy-actors in the AFC network employ discursive strategies to frame an elite political project to advance school choice policies as an anti-elite movement oriented toward political empowerment and educational justice.Este artículo presenta los resultados de un análisis de la red política American Federation for Children Network (AFC) utilizando herramientas de red de la etnografía y análisis de contenido cualitativo. En concreto, examinamos formularios de impuestos y realizó extensas investigaciones en la web para espacializar y asignar la red AFC, texto extraído de políticos-actores en la red AFC, y analizados por el discurso político Promovido Estos actores de la red para alcanzar sus objetivos políticos. La tarea para este estudio fue utilizar el AFC como un dispositivo heurístico para explorar la complejidad de la política educativa y entender cómo los actores de política de red trabajan para alcanzar sus objetivos de política a través de la defensa y el marketing. Las conclusiones del estudio indican que la red AFC demuestra una ordenación jerárquica, esta ordenación jerárquica es el reflejo de la planificación de la elite y la ingeniería social asociadas a las reformas neoliberales, y que la política-actores al servicio de la red estrategias discursivas AFC para encuadrar un proyecto político élite para hacer avanzar las políticas de elección de la escuela como un movimiento político anti-elite orientado hacia la capacitación y la justicia educativa.Este artigo apresenta os resultados de uma análise da rede política American Federation for Children Network (AFC) utilizando ferramentas de rede da etnografia e análise de conteúdo qualitativa. Especificamente, nós examinamos formulários de impostos e realizou extensas pesquisas na web para espacializar e mapear a rede AFC, texto extraído de políticos-atores na rede AFC, e analisados pelo discurso político Promovido Estes atores da rede para atingir seus objetivos políticos. A tarefa para este estudo foi usar o AFC como um dispositivo heurístico para explorar a complexidade da política educacional e entender como os atores de política de rede trabalham para alcançar seus objetivos de política por meio de advocacy e marketing. Conclusões do estudo indicam que a rede AFC Demonstra uma ordenação hierárquica, esta ordenação hierárquica é o reflexo do planejamento elite e engenharia social associados às reformas neoliberais, e que a política-atores a serviço da rede estratégias discursivas AFC para enquadrar um projeto político elite para fazer avançar as políticas de escolha da escola como um movimento político anti-elite voltada para a capacitação e justiça educacional

Casimir forces in Bose-Einstein condensates: finite size effects in three-dimensional rectangular cavities

The Casimir force due to {\it thermal} fluctuations (or pseudo-Casimir force) was previously calculated for the perfect Bose gas in the slab geometry for various boundary conditions. The Casimir pressure due to {\it quantum} fluctuations in a weakly-interacting dilute Bose-Einstein condensate (BEC) confined to a parallel plate geometry was recently calculated for Dirichlet boundary conditions. In this paper we calculate the Casimir energy and pressure due to quantum fluctuations in a zero-temperature homogeneous weakly-interacting dilute BEC confined to a parallel plate geometry with periodic boundary conditions and include higher-order corrections which we refer to as Bogoliubov corrections. The leading order term is identified as the Casimir energy of a massless scalar field moving with wave velocity equal to the speed of sound in the BEC. We then obtain the leading order Casimir pressure in a general three-dimensional rectangular cavity of arbitrary lengths and obtain the finite-size correction to the parallel plate scenario.Comment: 12 pages; no figures; v.2: version accepted for publication in JSTAT v.3: references adde

Correlation Between Student Collaboration Network Centrality and Academic Performance

We compute nodal centrality measures on the collaboration networks of students enrolled in three upper-division physics courses, usually taken sequentially, at the Colorado School of Mines. These are complex networks in which links between students indicate assistance with homework. The courses included in the study are intermediate Classical Mechanics, introductory Quantum Mechanics, and intermediate Electromagnetism. By correlating these nodal centrality measures with students' scores on homework and exams, we find four centrality measures that correlate significantly with students' homework scores in all three courses: in-strength, out-strength, closeness centrality, and harmonic centrality. These correlations suggest that students who not only collaborate often, but also collaborate significantly with many different people tend to achieve higher grades. Centrality measures between simultaneous collaboration networks (analytical vs. numerical homework collaboration) composed of the same students also correlate with each other, suggesting that students' collaboration strategies remain relatively stable when presented with homework assignments targeting different skills. Additionally, we correlate centrality measures between collaboration networks from different courses and find that the four centrality measures with the strongest relationship to students' homework scores are also the most stable measures across networks involving different courses. Correlations of centrality measures with exam scores were generally smaller than the correlations with homework scores, though this finding varied across courses.Comment: 10 pages, 4 figures, submitted to Phys. Rev. PE

The localization transition at finite temperatures: electric and thermal transport

The Anderson localization transition is considered at finite temperatures. This includes the electrical conductivity as well as the electronic thermal conductivity and the thermoelectric coefficients. An interesting critical behavior of the latter is found. A method for characterizing the conductivity critical exponent, an important signature of the transition, using the conductivity and thermopower measurements, is outlined.Comment: Article for the book: "50 Years of Anderson Localization", edited by E. Abrahams (World Scientific, Singapore, 2010

Ulta-slow relaxation in discontinuous-film based electron glasses

We present field effect measurements on discontinuous 2D thin films which are composed of a sub monolayer of nano-grains of Au, Ni, Ag or Al. Like other electron glasses these systems exhibit slow conductance relaxation and memory effects. However, unlike other systems, the discontinuous films exhibit a dramatic slowing down of the dynamics below a characteristic temperature $T^*$. $T^*$ is typically between 10-50K and is sample dependent. For $T<T^*$ the sample exhibits a few other peculiar features such as repeatable conductance fluctuations in millimeter size samples. We suggest that the enhanced system sluggishness is related to the current carrying network becoming very dilute in discontinuous films so that the system contains many parts which are electrically very weakly connected and the transport is dominated by very few weak links. This enables studying the glassy properties of the sample as it transitions from a macroscopic sample to a mesocopic sample, hence, the results provide new insight on the underlying physics of electron glasses.Comment: 4 pages, 4 figure