Ionizing radiation from hydrogen recombination strongly suppresses the lithium scattering signature in the CMB

It has been suggested that secondary CMB anisotropies generated by neutral lithium could open a new observational window into the universe around the redshift z~400, and permit a determination of the primordial lithium abundance. The effect is due to resonant scattering in the allowed Li i doublet (2s2S1/2-2p2P1/2,3/2), so its observability depends on the formation history of neutral lithium. Here we show that the ultraviolet photons produced during hydrogen recombination are sufficient to keep lithium in the Li ii ionization stage in the relevant redshift range and suppress the neutral fraction by ~3 orders of magnitude from previous calculations, making the lithium signature unobservable

Hierarchical clustering and the baryon distribution in galaxy clusters

The baryon fraction of galaxy clusters in numerical simulations is found to be dependant on the cluster formation method. In all cases, the gas is anti-biased compared with the dark matter. However, clusters formed hierarchically are found to be more depleted in baryons than clusters formed non-hierarchically. There is a depletion of 10 to 15% for hierarchically formed clusters while the depletion is less than 10% for those clusters formed non-hierarchically. This difference is dependent on the mass of the clusters. The mean baryon enrichment profile for the hierarchically formed clusters shows an appreciable baryon enhancement around the virial radius not seen in the clusters formed without substructure. If this phenomenon applies to real clusters, it implies that determinations of the baryon fractions in clusters of galaxies require data extending beyond the virial radius of the clusters in order to achieve an unbiased value.Comment: 13 pages including 2 tables and 2 figures. Submitted to MNRA

Radar backscattering by the icy Galilean satellites and the radar glory effect

Radar echoes from the icy Galilean satellites are unique in character and they may be explained by a phenomenon that was called the radar glory effect. The radar glory backscattering from buried craters was analyzed as a possible model for this effect. These craters have a smaller refractive index below the crater than above. The possibility exists that the rays will be totally internally reflected at the crater walls. The rays which contribute to the backscattering will come from a circular annulus, when viewed before the refraction occurs at the ice-vacuum surface, in a plane orthogonal to the rays reflected from the crater

Federal Government Debt and Interest Rates

Does government debt affect interest rates? Despite a substantial body of empirical analysis, the answer based on the past two decades of research is mixed. While many studies suggest, at most, a single-digit rise in the interest rate when government debt increases by one percent of GDP, others estimate either much larger effects or find no effect. Comparing results across studies is complicated by differences in economic models, definitions of econometric approaches, and sources of data. Using a standard set of data and a simple analytical framework, we reconsider and add to empirical evidence on the effect of federal government debt and interest rates. We begin by deriving analytically the effect of government debt on the real interest rate and find that an increase in government debt equivalent to one percent of GDP would be predicted to increase the real interest rate by about two to three basis points. While some existing studies estimate effects in this range, others find larger effects. In almost all cases, these larger estimates come from specifications relating federal deficits (as opposed to debt) and the level of interest rates or from specifications not controlling adequately for macroeconomic influences on interest rates that might be correlated with deficits. We present our own empirical analysis in two parts. First, we examine a variety of conventional reduced-form specifications linking interest rates and government debt and other variables. In particular, we provide estimates for three types of specifications to permit comparisons among different approaches taken in previous research; we estimate the effect of: an expected, or projected, measure of federal government debt on a forward-looking measure of the real interest rate; an expected, or projected, measure of federal government debt on a current measure of the real interest rate; and a current measure of federal government debt on a current measure of the real interest rate. Most of the statistically significant estimated effects are consistent with the prediction of the simple analytical calculation. Second, we provide evidence using vector autoregression analysis. In general, these results are similar to those found in our reduced-form econometric analysis and consistent with the analytical calculations. Taken together, the bulk of our empirical results suggest that an increase in federal government debt equivalent to one percent of GDP, all else equal, would be expected to increase the long-term real rate of interest by about three basis points, though one specification suggests a larger impact, while some estimates are not statistically significantly different from zero. By presenting a range of results with the same data, we illustrate the dependence of estimation on specification and definition differences.

Vortices in self-gravitating disks

Vortices are believed to greatly help the formation of km sized planetesimals by collecting dust particles in their centers. However, vortex dynamics is commonly studied in non-self-gravitating disks. The main goal here is to examine the effects of disk self-gravity on the vortex dynamics via numerical simulations. In the self-gravitating case, when quasi-steady gravitoturbulent state is reached, vortices appear as transient structures undergoing recurring phases of formation, growth to sizes comparable to a local Jeans scale, and eventual shearing and destruction due to gravitational instability. Each phase lasts over 2-3 orbital periods. Vortices and density waves appear to be coupled implying that, in general, one should consider both vortex and density wave modes for a proper understanding of self-gravitating disk dynamics. Our results imply that given such an irregular and rapidly changing, transient character of vortex evolution in self-gravitating disks it may be difficult for such vortices to effectively trap dust particles in their centers that is a necessary process towards planet formation.Comment: to appear in the proceedings of Cool Stars, Stellar Systems and The Sun, 15th Cambridge Workshop, St. Andrews, Scotland, July 21-25, 200