On the Hagedorn Behaviour of Singular Scale-Invariant Plane Waves

As a step towards understanding the properties of string theory in time-dependent and singular spacetimes, we study the divergence of density operators for string ensembles in singular scale-invariant plane waves, i.e. those plane waves that arise as the Penrose limits of generic power-law spacetime singularities. We show that the scale invariance implies that the Hagedorn behaviour of bosonic and supersymmetric strings in these backgrounds, even with the inclusion of RR or NS fields, is the same as that of strings in flat space. This is in marked contrast to the behaviour of strings in the BFHP plane wave which exhibit quantitatively and qualitatively different thermodynamic properties.Comment: 15 pages, LaTeX2e, v2: JHEP3.cls, one reference adde

Penrose Limits and Spacetime Singularities

We give a covariant characterisation of the Penrose plane wave limit: the plane wave profile matrix $A(u)$ is the restriction of the null geodesic deviation matrix (curvature tensor) of the original spacetime metric to the null geodesic, evaluated in a comoving frame. We also consider the Penrose limits of spacetime singularities and show that for a large class of black hole, cosmological and null singularities (of Szekeres-Iyer ``power-law type''), including those of the FRW and Schwarzschild metrics, the result is a singular homogeneous plane wave with profile $A(u)\sim u^{-2}$, the scale invariance of the latter reflecting the power-law behaviour of the singularities.Comment: 9 pages, LaTeX2e; v2: additional references and cosmetic correction

ILR Impact Brief - The Sources of International Differences in Wage Inequality

Wage inequality in the U.S. exceeds that of Canada, Denmark, Finland, Italy, Netherlands, Norway, Sweden, and Switzerland. Some researchers have pointed to the higher relative rewards for higher cognitive skill and more education in the U.S. as an important cause of this difference; others emphasize the greater diversity of labor market skills within the American population. This paper uses recently collected international data on cognitive skills, earnings, age, and years of formal schooling to assess the relative importance of population heterogeneity and higher relative pay for more cognitive skill in explaining higher U.S. wage inequality

Fermi Coordinates and Penrose Limits

We propose a formulation of the Penrose plane wave limit in terms of null Fermi coordinates. This provides a physically intuitive (Fermi coordinates are direct measures of geodesic distance in space-time) and manifestly covariant description of the expansion around the plane wave metric in terms of components of the curvature tensor of the original metric, and generalises the covariant description of the lowest order Penrose limit metric itself, obtained in hep-th/0312029. We describe in some detail the construction of null Fermi coordinates and the corresponding expansion of the metric, and then study various aspects of the higher order corrections to the Penrose limit. In particular, we observe that in general the first-order corrected metric is such that it admits a light-cone gauge description in string theory. We also establish a formal analogue of the Weyl tensor peeling theorem for the Penrose limit expansion in any dimension, and we give a simple derivation of the leading (quadratic) corrections to the Penrose limit of AdS_5 x S^5.Comment: 25 page

Do Cognitive Test Scores Explain Higher US Wage Inequality?

Using microdata from the 1994-6 International Adult Literacy Survey (IALS), we examine the role of cognitive skills in explaining higher wage inequality in the US. We find that while the greater dispersion of cognitive test scores in the US plays a part in explaining higher US wage inequality, higher labor market prices (i.e., higher returns to measured human capital and cognitive performance) and greater residual inequality still play important roles for both men and women. And we find that, on average, prices are quantitatively considerably more important than differences in the distribution of test scores in explaining the relatively high level of US wage inequality. This finding holds up when we examine natives only and when we correct for sample selection.

The Feasibility and Importance of Adding Measures of Actual Experience to Cross-Sectional Data Collection

We use Michigan Panel Study of Income Dynamics data and data from a 2008 telephone survey of adults conducted by Westat for the Princeton Data Improvement Initiative (PDII) to explore the importance and feasibility of adding retrospective questions about actual work experience to cross-sectional data sets. We demonstrate that having such actual experience data is important for analyzing women's post-school human capital accumulation, residual wage inequality, and the gender pay gap. Further, our PDII survey results show that it is feasible to collect actual experience data in cross-sectional telephone surveys like the March Current Population Survey annual supplement.gender, microeconomic data collection, work experience, human capital

Changes in the Labor Supply Behavior of Married Women: 1980-2000

Using March Current Population Survey (CPS) data, we investigate married women's labor supply behavior from 1980 to 2000. We find that their labor supply function for annual hours shifted sharply to the right in the 1980s, with little shift in the 1990s. In an accounting sense, this is the major reason for the more rapid growth of female labor supply observed in the 1980s, with an additional factor being that husbands' real wages fell slightly in the 1980s but rose in the 1990s. Moreover, a major new development was that, during both decades, there was a dramatic reduction in women's own wage elasticity. And, continuing past trends, women's labor supply also became less responsive to their husbands' wages. Between 1980 and 2000, women's own wage elasticity fell by 50 to 56 percent, while their cross wage elasticity fell by 38 to 47 percent in absolute value. These patterns hold up under virtually all alternative specifications correcting for: selectivity bias in observing wage offers; selection into marriage; income taxes and the earned income tax credit; measurement error in wages and work hours; and omitted variables that affect both wage offers and the propensity to work; as well as when education groups and mothers of small children are analyzed separately.

Gender and Youth Employment Outcomes: The US and West Germany, 1984-91

This paper examines gender differences in labor market outcomes for hard-to-employ youth in the US and West Germany during the 1984-91 period. We find that young, less educated American men and especially women are far less likely to be employed than their German counterparts. Moreover, less educated young women and men in the United States have lower earnings relative to more highly educated youth in their own country, and also fare much worse than less educated German youth in absolute terms, correcting for purchasing power. The relatively high employment rates of less educated German youth combined with their relatively high wages raise the question of how they are successfully absorbed into the labor market. We present evidence that the large public sector in Germany in effect functions as an employer of last resort, absorbing some otherwise unemployable low skilled youth. Our findings also suggest that the US welfare system accounts for very little of the US-German difference in employment rates.

International Differences in Male Wage Inequality: Institutions versus Market Forces

While changes in the demand for skilled labor appear to have led to a widening of the wage structures in many countries during the 198Os,considerable differences in the level of wage inequality remain. In this paper, we examine the sources of these differences, focusing primarily on explaining the considerably higher level of wage inequality in the U.S. We find that the greater overall dispersion of the U.S. wage distribution reflects considerably more compression at the bottom of the distribution in the other countries, but relatively little difference in the degree of wage inequality at the top. While differences in the distribution of measured characteristics help to explain some aspects of the international differences, U.S. labor market prices--that is, higher rewards to labor market skills-are an important factor. Labor market institutions, chiefly the relatively decentralized wage-setting mechanisms in the U.S. compared to other countries, appear to provide the most persuasive explanation for these international differences in prices. In contrast, the pattern of cross-country differences in relative supplies of and demands for skills does not appear to be consistent with the pattern of observed differences in wage inequality.