On extremal surfaces and de Sitter entropy

We study extremal surfaces in the static patch coordinatization of de Sitter space, focussing on the future and past universes. We find connected timelike codim-2 surfaces on a boundary Euclidean time slice stretching from the future boundary $I^+$ to the past boundary $I^-$. In a limit, these surfaces pass through the bifurcation region and have minimal area with a divergent piece alone, whose coefficient is de Sitter entropy in 4-dimensions. These are reminiscent of rotated versions of certain surfaces in the $AdS$ black hole. We close with some speculations on a possible $dS/CFT$ interpretation of 4-dim de Sitter space as dual to two copies of ghost-CFTs in an entangled state. For a simple toy model of two copies of ghost-spin chains, we argue that similar entangled states always have positive norm and positive entanglement.Comment: Latex, 20pgs, 3 figs, v3: clarifications added, some reorganizing of text, review of ghost-spin chains added, matches version to be published, v4: further minor clarifications adde

Black Holes in Astrophysics

This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience. Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star. These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei. A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons. Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analog of the Penrose process.Comment: To appear in a forthcoming Special Focus Issue on "Spacetime 100 Years Later" published by the New Journal of Physics (http://www.iop.org/EJ/njp) The article, finalized in October, 2004, consists of 21 pages of text, 3 figures and 6 movies (found at http://www.cfa.harvard.edu/~narayan/NJP

The inflation-output nexus:empirical evidence from India, Brazil and South Africa

In this paper we study the relationship between output and inflation for India, Brazil, and South Africa using the EGARCH model. For India and South Africa, we find evidence for: (1) the Cukierman and Meltzer hypothesis that inflation volatility raises inflation; (2) the Friedman hypothesis that inflation raises inflation volatility; and (3) the Black hypothesis that output volatility raises output growth, and that output volatility reduces inflation. For Brazil, we do not find any evidence of a systematic relationship between inflation and output growth.Output, inflation, EGARCH model, volatility

Did the US macroeconomic conditions affect Asian stock markets?

The aim of this paper is to examine the impact of US macroeconomic conditions—namely, exchange rate and short-term interest rate—on the stocks of seven Asian countries (China,India, the Philippines, Malaysia, Singapore, Thailand, and South Korea). Using daily data for the period 2000 to 2010, we divide the sample into pre-crisis period (pre-August 2007) and crisis period (post-August 2007) we find that in the short-run interest rate has a statistically insignificant effect on returns for all countries except the Philippines in the crisis period,while except for China, regardless of the crisis, depreciation had a statistically significant negative effect on returns. When the long-run relationship among the variables is considered,for four of the seven countries (India, Malaysia, Philippines, Singapore, and Thailand) while there was cointegration in the pre-crisis period, in the crisis period there was no such relationship, implying that the financial crisis has actually weakened the link between stock prices and economic fundamentals.Interest Rate; Exchange Rate; Financial Crisis; Depreciation