Comparing Fully General Relativistic and Newtonian Calculations of Structure Formation

In the standard approach to studying cosmological structure formation, the overall expansion of the Universe is assumed to be homogeneous, with the gravitational effect of inhomogeneities encoded entirely in a Newtonian potential. A topic of ongoing debate is to what degree this fully captures the dynamics dictated by general relativity, especially in the era of precision cosmology. To quantitatively assess this, we directly compare standard N-body Newtonian calculations to full numerical solutions of the Einstein equations, for cold matter with various magnitude initial inhomogeneities on scales comparable to the Hubble horizon. We analyze the differences in the evolution of density, luminosity distance, and other quantities defined with respect to fiducial observers. This is carried out by reconstructing the effective spacetime and matter fields dictated by the Newtonian quantities, and by taking care to distinguish effects of numerical resolution. We find that the fully general relativistic and Newtonian calculations show excellent agreement, even well into the nonlinear regime. They only notably differ in regions where the weak gravity assumption breaks down, which arise when considering extreme cases with perturbations exceeding standard values.Comment: 17 pages, 14 figures; revised to match PRD versio

Validating modelling assumptions of alpha particles in electrostatic turbulence

To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium distribution must be used. In the work, the response of high-energy alpha particles to electrostatic turbulence has been analyzed for several different tokamak parameters. Our results are consistent with known scalings and experimental evidence that alpha particles are generally well-confined: on the order of several seconds. It is also confirmed that the effect of alphas on the turbulence is negligible at realistically low concentrations, consistent with linear theory. It is demonstrated that the usual practice of using a high-temperature Maxwellian gives incorrect estimates for the radial alpha particle flux, and a method of correcting it is provided. Furthermore, we see that the timescales associated with collisions and transport compete at moderate energies, calling into question the assumption that alpha particles remain confined to a flux surface that is used in the derivation of the slowing-down distribution.Comment: 23 pages, 13 figures, submitted to the Journal of Plasma Physic

Composting the City: Urban Farming in the Ethnographic Moment

This thesis outlines the social texture of an urban agriculture project: City Leaf Farm in Washington, DC. Drawing on field notes and interviews recorded through ethnographic research, this study outlines the everyday patterns of work and play that constitute the urban agriculture project, as well as the informant and researcher\u27s shared ethnographic moment. By focusing on the positive existence of City Leaf Farm as a space, this paper is juxtaposed with an evaluative approach to urban agriculture research, which conceptualizes cultivars and their social existence in terms of their solution to utilitarian scarcities and public issues. The acknowledgement of this positive existence is shown to have implications for scientific understandings of urban governance: far from managing scarcity, as the term food desert might connote, governance at City Leaf Farm consists of managing surpluses of social energy, volunteer labor, and grant funds. Contrary to a theory of social entrepreneurship, management, and the labor that it necessitates, do not go away

Design of a sequential detector

The communication engineer has in the past treated the detection problem from a limited point of view. Detectors have been designed to reach a decision based on a specified number of samples. This approach has proved valuable, but it offers no provisions for an early decision if a conclusion becomes obvious early in the sampling. Neither is there an opportunity for additional sampling if necessary to reach a definite conclusion, because of these limitations it is desirable to find more general detectors. This study looks at a specific statistics (the sign test) and designs a sequential detector to utilize this statistic. The sequential detector is a type of detector where the decision to terminate depends only on the previous samples. This detector is also of the nonparametric class in that a complete knowledge of the signal and noise is not necessary for its operation --Abstract, page i

EXTERNAL DEBT SERVICING AND CAPITAL FORMATION IN GHANA: AN AUTOREGRESSIVE DISTRIBUTED LAG ANALYSIS

This study empirically examines the effects of external debt servicing on capital formation in Ghana. Using data from 1980 to 2019, the study estimates the Autoregressive Distributed Lag (ARDL) model and finds that the effect of external debt servicing is negative both in the long and the short run due to the tax disincentive effect. This suggests that as a result of the potentially high debt servicing due to the high debt stock, any future investment may attract high marginal tax rates and would tend to reduce investment in the economy. The result further shows that external debt servicing affects private capital formation more than public capital formation. However, the effect of the external debt stock on private investment is negative in the long run but positive in the short run confirming the direct effect of the debt hypothesis’ existence in Ghana suggesting that external debt discourages a long-term investment which is critical for economic growth. Additionally, there exists complementarity between public and private investments indicating that some public investments attract private ones into the country. Therefore, external debt service payment crowds out private investment through excessive interest charges, so government should determine a threshold of borrowing in order to minimize the high debt servicing. JEL: E22, E31, E62, F31, G31, H63, P24  Article visualizations

Shapes and Probabilities of Galaxy Clusters II: Comparisons with observations

We identify low redshift clusters and groups in the Sloan Digital Sky Survey (SDSS) and estimate their kinetic and correlation potential energies. We compare the distribution of these energies to the predictions by Yang and Saslaw (2012) and in the process estimate a measure of an average 3-dimensional velocity and spatial anisotropy of a sample of clusters. We find that the inferred velocity anisotropy is correlated with the inferred spatial anisotropy. We also find that the general shape of the energy distribution agrees with theory over a wide range of scales from small groups to superclusters once the uncertainties and fluctuations in the estimated energies are included.Comment: 18 Pages, 8 figures. Accepted by ApJ. Latest version incorporates the referee's suggestions and comment

Calakmul, Campeche, México: la estructura II-H, sus entierros y otras funciones ceremoniales y habitacionales.

Sin resume

Microstability of β∼1\beta \sim 1 tokamak equilibria

High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high $\beta$ value (the ratio of thermal to magnetic pressure), i.e., $\beta \sim 1$. However, a $\beta \sim 1$ state may be unstable to various pressure- and current-driven instabilities or have unfavorable microstability properties. To explore these possibilities, we generate $\beta \sim 1$ equilibria and investigate their stability. Initially, we study an analytical technique that was used in the past to generate $\beta \sim 1$ equilibria and outline its limitations. Hence, we demonstrate the generation of high-$\beta$ equilibria with the computer code $\texttt{VMEC}$. We then analyze these equilibria to determine their stability against the infinite-$n$ ideal ballooning mode. We follow that by engaging in a detailed microstability study, beginning with assessments of electrostatic ITG and TEM instabilities. We observe interesting behavior for the high-$\beta$ equilibria -- stabilization of these modes through two distinct mechanisms. Finally, we perform electromagnetic gyrokinetic simulations and again observe stabilizing trends in the equilibria at high $\beta$. These trends are different from their lower $\beta$ counterparts and offer an alternative, potentially favorable regime of tokamak operation.Comment: 39 pages, 23 figure