Domain wall formation in late-time phase transitions

We examine domain wall formulation in late time phase transitions. We find that in the invisible axion domain wall phenomenon, thermal effects alone are insufficient to drive different parts of the disconnected vacuum manifold. This suggests that domain walls do not form unless either there is some supplemental (but perhaps not unreasonable) dynamics to localize the scalar field responsible for the phase transition to the low temperature maximum (to an extraordinary precision) before the onset of the phase transition, or there is some non-thermal mechanism to produce large fluctuations in the scalar field. The fact that domain wall production is not a robust prediction of late time transitions may suggest future directions in model building

Lifting the burden: fundamental tax reform and U.S. economic growth

This paper presents a comprehensive treatment of the cost-of-capital approach for analyzing the economic impact of tax policy. This approach has provided an intellectual impetus for reforms of capital income taxation in the United States and around the world. The most dramatic example is the Tax Reform Act of 1986 in the United States. In this landmark legislation the income tax base was broadened by wholesale elimination of tax preferences for both individuals and corporations. Revenues generated by base broadening were used to finance sharp reductions in tax rates at corporate and individual levels. The cost-of-capital approach presented in this paper shows that important opportunities for tax reform still remain. This approach suggests two avenues for reform. One would retain the income tax base of the existing U.S. tax system, but would equalize tax burdens on all forms of assets as well as average and marginal rates on labor income. Elimination of differences in the tax treatment of all forms of assets would produce gains in efficiency comparable to those from the Tax Reform Act of 1986. Equalization of marginal and average tax rates on labor income would more than double these gains in efficiency. Proposals to replace income by consumption as a tax base were revived in the United States during the 1990's. The Hall-Rabushka Flat Tax proposal would produce efficiency gains comparable to those from equalizing tax burdens on all forms of assets under the income tax. However, a progressive National Retail Sales Tax, collected on personal consumption expenditures at the retail level, would generate gains in efficiency exceeding those from the Flat Tax by more than 50 percent! Equalizing marginal and average rates of taxation on consumption would double the gains from the Flat Tax.

Mathematics Course Placement Using Holistic Measures: Possibilities for Community College Students.

Background/Context: Most community colleges across the country use a placement test to determine students’ readiness for college-level coursework, yet these tests are admittedly imperfect instruments. Researchers have documented significant problems stemming from overreliance on placement testing, including placement error and misdiagnosis of remediation needs. They have also described significant consequences of misplacement, which can hinder the educational progression and attainment of community college students. Purpose/Objective/Research Question/Focus of Study: We explore possibilities for placing community college students in mathematics courses using a holistic approach that considers measures beyond placement test scores. This includes academic background measures, such as high school GPA and math courses taken, and indicators of noncognitive constructs, such as motivation, time use, and social support. Setting: The study draws upon administrative data from a large urban community college district in California that serves over 100,000 students each semester. The data enable us to link students’ placement testing results, survey data, background information, and transcript records. Research Design: We first use the supplemental survey data gathered during routine placement testing to conduct predictive exercises that identify severe placement errors under existing placement practices. We then move beyond prediction and evaluate student outcomes in two colleges where noncognitive indicators were directly factored into placement algorithms. Findings/Results: Using high school background information and noncognitive indicators to predict success reveals as many as one quarter of students may be misassigned to their math courses by status quo practices. In our subsequent analysis we find that students placed under a holistic approach that considered noncognitive indicators in addition to placement test scores performed no differently from higher scoring peers in the same course. Conclusions/Recommendations: The findings suggest a holistic approach to mathematics course placement may improve placement accuracy and provide access to higher level mathematics courses for community college students without compromising their likelihood of success

Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004

Wind-interaction models for gamma-ray burst afterglows predict that the optical emission from the reverse shock drops below that from the forward shock within 100s of seconds of the burst. The typical frequency $\nu_m$ of the synchrotron emission from the forward shock passes through the optical band typically on a timescale of minutes to hours. Before the passage of $\nu_m$, the optical flux evolves as $t^{-1/4}$ and after the passage, the decay steepens to $t^{-(3p-2)/4}$, where $p$ is the exponent for the assumed power-law energy distribution of nonthermal electrons and is typically $\sim 2$. The steepening in the slope of temporal decay should be readily identifiable in the early afterglow light curves. We propose that such a steepening was observed in the R-band light curve of GRB 021004 around day 0.1. Available data at several radio frequencies are consistent with this interpretation, as are the X-ray observations around day~1. The early evolution of GRB 021004 contrasts with that of GRB 990123, which can be described by emission from interaction with a constant density medium.Comment: 16 pages, 1 figure, submitted to ApJ

The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies

We have mapped the neutral atomic gas (HI) in the three disk-disk merger systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority of the mergers mapped in HI, in that their stellar and gaseous tidal features do not coincide. In particular, they exhibit large stellar tidal features with little if any accompanying neutral gas and large gas-rich tidal features with little if any accompanying starlight. On smaller scales, there are striking anti-correlations where the gaseous and stellar tidal features appear to cross. We explore several possible causes for these differences, including dust obscuration, ram pressure stripping, and ionization effects. No single explanation can account for all of the observed differences. The fact that each of these systems shows evidence for a starburst driven superwind expanding in the direction of the most striking anti-correlations leads us to suggest that the superwind is primarily responsible for the observed differences, either by sweeping the features clear of gas via ram pressure, or by excavating a clear sightline towards the starburst and allowing UV photons to ionize regions of the tails.Comment: 16 pages, 5 figures, uses emulateapj.sty. To appear in the March 2000 issue of AJ. Version with full resolution figures is available via http://www.cv.nrao.edu/~jhibbard/HIdisp/HIdisp.htm

The geometrically-averaged density of states as a measure of localization

Motivated by current interest in disordered systems of interacting electrons, the effectiveness of the geometrically averaged density of states, $\rho_g(\omega)$, as an order parameter for the Anderson transition is examined. In the context of finite-size systems we examine complications which arise from finite energy resolution. Furthermore we demonstrate that even in infinite systems a decline in $\rho_g(\omega)$ with increasing disorder strength is not uniquely associated with localization.Comment: 8 pages, 8 figures; revised text and figure