Breaking the cosmological background degeneracy by two-fluid perturbations in f(R) gravity

One of the exact solutions of f(R) theories of gravity in the presence of different forms of matter exactly mimics the LCDM solution of general relativity at the background level. In this work we study the evolution of scalar cosmological perturbations in the covariant and gauge-invariant formalism and show that although the background in such a model is indistinguishable from the standard LCDM cosmology, this degeneracy is broken at the level of first-order perturbations. This is done by predicting different rates of structure formation in LCDM and the f(R) model both in the complete and quasi\hs static regimes.Comment: 17 pages, 19 figure

Getting connected : competition and diffusion in African mobile telecommunications markets

The author studies the determinants of the diffusion of mobile telecommunications in Africa in a fixed effects model. He uses data from 1987-2000 on 41 African countries that have adopted cellular telecommunications technologies. He finds that competition is the driving force behind the mobile telecommunications explosion in Africa. Duopoly and triopoly markets grow significantly faster than monopoly markets, although growth does not appear to differ between the first two markets. Evidence of preemptive behavior is found in competitive sequential entries into the market, but the major effect of competition on diffusion occurs after the actual year of entry. The introduction of digital technology has a positive and significant effect on the diffusion of mobile phones. The presence of an incumbent-owned cellular operator has a negative effect on the diffusion of mobiles, suggesting an abuse of a dominant position by the incumbent fixed-line operator. However, privatization of the incumbent fixed-line cellular operator accelerates mobile growth and mitigates that negative effect.Trade Finance and Investment,Economic Theory&Research,Environmental Economics&Policies,ICT Policy and Strategies,Health Economics&Finance,Economic Theory&Research,ICT Policy and Strategies,Environmental Economics&Policies,Governance Indicators,Health Economics&Finance

Inflationary f(R) cosmologies

This paper discusses a simple procedure to reconstruct f (R)-gravity models from exact cosmological solutions of the Einstein field equations with a noninteracting classical scalar field-and-radiation background. From the kind of inflationary scenario we want, we show how the potential functions can be obtained. We then show how an f (R) gravitational Lagrangian density that mimics the same cosmological expansion as the scalar field-driven inflation of General Relativity can be reconstructed. As a demonstration, we calculate the slow-roll parameters (the spectral index n s and the tensor-to-scalar ratio r) and compare them to the Planck data.Comment: 13 pages, 10 figure

Food Aid Impact on Poverty Reduction: Empirical Evidence from Rural Households in Ethiopia

Ethiopia is one of the highest food aid recipient countries in the Sub-Saharan Africa region. Despite the magnitude of the aid, its impact as development resource is inconclusive in both theoretical and empirical evidences. This paper evaluates the impact of food aid on poverty reduction making use of an Ethiopian rural longitudinal household survey data (ERHS) primarily collected in 1999 and 2004, with the purpose to add empirical evidences on the existing debate on food aid. Besides, it deals with the correlation of poverty assets which has fundamental importance for policy implication and the choice of appropriate development strategies. We used separate regressions on determinants of welfare growth for food-for-work (FFW) and free-food-distribution (FFD) programs. The results show that access to information, initial endowment, household characteristics, and shocks were the main determinants of escaping from poverty and food aid dependence. The results from the poverty profile, difference-indifferences matching and switching regression support the fact that participation in FFW or FFD programs reduces poverty. However, some indicators from the analysis showed that both FFW and FFD programs have targeting efficiency problems and differ across regions, which indicate that there is room for improvement in the distribution of food aid through targeting the poorest and geographically based intervention linked with productive investment.Ethiopia; food-for-work (FFW); free-food-distribution (FFD); welfare growth; difference-in-differences propensity score matching; endogenous switching regression