Traversable wormholes: minimum violation of null energy condition revisited

It was argued in literature that traversable wormholes can exist with arbitrarily small violation of null energy conditions. We show that if the amount of exotic material near the wormhole throat tends to zero, either this leads to a horn instead of a wormhole or the throat approaches the horizon in such a way that infnitely large stresses develop on the throat.Comment: 12 pages. To appear in PR

A cluster expansion approach to renormalization group transformations

The renormalization group (RG) approach is largely responsible for the considerable success which has been achieved in developing a quantitative theory of phase transitions. This work treats the rigorous definition of the RG map for classical Ising-type lattice systems in the infinite volume limit at high temperature. A cluster expansion is used to justify the existence of the partial derivatives of the renormalized interaction with respect to the original interaction. This expansion is derived from the formal expressions, but it is itself well-defined and convergent. Suppose in addition that the original interaction is finite-range and translation-invariant. We will show that the matrix of partial derivatives in this case displays an approximate band property. This in turn gives an upper bound for the RG linearization.Comment: 13 page

HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies

It has been recently argued that the HCN J=1--0 line emission may not be an unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous Infrared Galaxies (LIRGs: $\rm L_{FIR}> 10^{11} L_{\odot}$) and HCO$^+$ J=1--0 may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting doubt into earlier claims supporting the former as a good tracer of such gas (Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J=1--0 transition from either species a poor proxy of its mass. Moreover the well-known sensitivity of the HCO$^+$ abundance on the ionization degree of the molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO$^+$ as dense gas tracers) may severely reduce the HCO$^+$ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result to the decreasing HCO$^+$/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and casts doubts on the HCO$^+$ rather than the HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer, its relation to ongoing star formation, and constrain what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical Journa

Inverse Square Law of Gravitation in (2+1)-Dimensional Space-Time as a Consequence of Casimir Energy

The gravitational effect of vacuum polarization in space exterior to a particle in (2+1)-dimensional Einstein theory is investigated. In the weak field limit this gravitational field corresponds to an inverse square law of gravitational attraction, even though the gravitational mass of the quantum vacuum is negative. The paradox is resolved by considering a particle of finite extension and taking into account the vacuum polarization in its interior.Comment: 10 pages, LaTeX, Report: UPR-0540-T, To appear in Physica Script

Charged Rotating Black Holes on a 3-Brane

We study exact stationary and axisymmetric solutions describing charged rotating black holes localized on a 3-brane in the Randall-Sundrum braneworld. The charges of the black holes are considered to be of two types, the first being an induced tidal charge that appears as an imprint of nonlocal gravitational effects from the bulk space and the second is a usual electric charge arising due to a Maxwell field trapped on the brane. We assume a special ansatz for the metric on the brane taking it to be of the Kerr-Schild form and show that the Kerr-Newman solution of ordinary general relativity in which the electric charge is superceded by a tidal charge satisfies a closed system of the effective gravitational field equations on the brane. It turns out that the negative tidal charge may provide a mechanism for spinning up the black hole so that its rotation parameter exceeds its mass. This is not allowed in the framework of general relativity. We also find a new solution that represents a rotating black hole on the brane carrying both charges. We show that for a rapid enough rotation the combined influence of the rotational dynamics and the local bulk effects of the "squared" energy momentum tensor on the brane distort the horizon structure of the black hole in such a way that it can be thought of as composed of non-uniformly rotating null circles with growing radii from the equatorial plane to the poles. We finally study the geodesic motion of test particles in the equatorial plane of a rotating black hole with tidal charge. We show that the effects of negative tidal charge tend to increase the horizon radius, as well as the radii of the limiting photon orbit, the innermost bound and the innermost stable circular orbits for both direct and retrograde motions of the particles.Comment: RevTeX 4, 33 pages, 4 figures, new references adde

The Impacts (Positive and Negative) of ICT on Education in Nigeria

During the last decades, considerable resources have been invested in hardware, software, connections, training and support actions under the scope of improving the quality of teaching and learning. A major tenet of the policies that supported the introduction of information and communication technologies (ICT) in education was that they can become catalysts for change. Undoubtedly, some countries have made considerable progress in bringing networked ICT into education and made it possible for teachers and learners to use them on a daily basis. In many other cases, however, implementation policies have not been a consequence of systematic analysis and reflection. As a consequence, we still know little about the impact and effectiveness of ICT in education. To close this gap, this paper will be examining the positive and negative impact of ICT on education. Keywords: Information and Communication Technology (ICT), education, performance

The Social Construction of Black Fatherhood in Responsible Fatherhood Policies

Responsible fatherhood legislation bridges the gap between two explicit family policies in order to serve fathers: The Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA) of 1996 and the Child Support Enforcement Act of 1975. Historically, these two polices have been racialized and discourse surrounding them may contribute to negative cultural formula stories about the primary targets of responsible fatherhood programs: low-income Black fathers. The first article addresses the question of whether and how congressional discourse disrupts or legitimizes negative cultural formula stories about Black fatherhood. This study examines congressional discourse during hearings on fatherhood legislation. Members of congress legitimized cultural formula stories by constructing welfare fathers as deadbeats. Primary themes included serial illegitimacy and parental abandonment. Members of congress also disrupted cultural formula stories by constructing welfare fathers as dead broke. The second article addresses the same question, but it is asked of the first Black President of the United States, Barack Obama. Presidential statements surrounding the unconcerned Black father served to reproduce negative cultural formula stories of Black fatherhood by depicting this kind of father as lazy, prone to fathering children with multiple women (serial illegitimacy), and disinterested in fatherhood overall. The institutionalized Black father embodies negative characteristics but his absence and instability stems from historical oppression and socio-economic disadvantages. In addition, it changes the single story often found in negative cultural formula stories of Black fatherhood. Lastly, the self-sacrificing Black father embodies Obama’s ideal characteristics of Black fatherhood by working tirelessly and sacrificing his own well-being for the betterment of his family. The third article compares congressional and presidential discourse to better understand interpretive conflicts in the meanings of responsible fatherhood. Both members of congress and President Obama presented some negative cultural codes in their discourse on fathers. For members of congress, a large-scale demonstration project conducted with low-income, non-residential, non-custodial fathers helped to shift their cultural formula stories. On the other hand, President Obama’s in-group status allowed him to construct a more diverse spectrum of Black fatherhood

Cosmic balloons

Cosmic balloons, consisting of relativistic particles trapped inside a spherical domain wall, may be created in the early universe. We calculate the balloon mass $M$ as a function of the radius $R$ and the energy density profile, $\rho (r)$, including the effects of gravity. At the maximum balloon mass $2GM/R\approx 0.52$ for any value of the mass density of the wall.Comment: 9 pages, LaTeX, 2 figures in separate file, UPTP-93-1