Intergalactic Transmission and its Impact on the Ly{\alpha} Line

We study the intergalactic transmission of radiation in the vicinity of the Ly{\alpha} wavelength. Simulating sightlines through the intergalactic medium (IGM) in detailed cosmological hydrosimulations, the impact of the IGM on the shape of the line profile from Ly{\alpha} emitting galaxies at redshifts 2.5 to 6.5 is investigated. In particular we show that taking into account the correlation of the density and velocity fields of the IGM with the galaxies, the blue part of the spectrum may be appreciably reduced, even at relatively low redshifts. This may in some cases provide an alternative to the often-invoked outflow scenario, although it is concluded that this model is still a plausible explanation of the many asymmetric Ly{\alpha} profiles observed. Applying the calculated wavelength dependent transmission to simulated spectra from Ly{\alpha} emitting galaxies, we derive the fraction of photons that are lost in the IGM, in addition to what is absorbed internally in the galaxies due to dust. Moreover, by comparing the calculated transmission of radiation blueward of the Ly{\alpha} line, the total optical depth to Thomson scattering of cosmic microwave background, with corresponding observations, we are able to constrain the epoch when the Universe was reionized to z <~ 8.5.Comment: Substantially extended, ~30 references added, 1.5 page extra (article style) in particular on the impact of the IGM at z~5.8 and z~6.5, 2 extra figures, unnecessary fluff cut out, accepted for publication in Ap

Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations

We present results obtained with a 3D, Ly alpha radiative transfer code, applied to a fully cosmological galaxy formation simulation. The developed Monte Carlo code is capable of treating an arbitrary distribution of source Ly alpha emission, neutral hydrogen density, temperature, and peculiar velocity of the interstellar medium. We investigate the influence of resonant scattering on the appearance and properties of young galaxies by applying the code to a simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate 22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that resonant scattering of Ly alpha radiation can explain that young galaxies frequently are observed to be more extended on the sky in Ly alpha than in the optical. Moreover, it is shown that, for the system investigated, due to the anisotropic escape of the photons, the appearent maximum surface brightness can differ by a factor of ~15, and the total derived luminosity by a factor of ~4, depending on the orientation of the system relative to the observer.Comment: Letter updated to match version published in Ap

The Cosmological Constant in the Quantum Multiverse

Recently, a new framework for describing the multiverse has been proposed which is based on the principles of quantum mechanics. The framework allows for well-defined predictions, both regarding global properties of the universe and outcomes of particular experiments, according to a single probability formula. This provides complete unification of the eternally inflating multiverse and many worlds in quantum mechanics. In this paper we elucidate how cosmological parameters can be calculated in this framework, and study the probability distribution for the value of the cosmological constant. We consider both positive and negative values, and find that the observed value is consistent with the calculated distribution at an order of magnitude level. In particular, in contrast to the case of earlier measure proposals, our framework prefers a positive cosmological constant over a negative one. These results depend only moderately on how we model galaxy formation and life evolution therein.Comment: 18 pages, 4 figures; matches the version published in Phys. Rev.

Smooth geometries with four charges in four dimensions

A class of axially symmetric, rotating four-dimensional geometries carrying D1, D5, KK monopole and momentum charges is constructed. The geometries are found to be free of horizons and singulaties, and are candidates to be the gravity duals of microstates of the (0,4) CFT. These geometries are constructed by performing singularity analysis on a suitably chosen class of solutions of six-dimensional minimal supergravity written over a Gibbons-Hawking base metric. The properties of the solutions raise some interesting questions regarding the CFT.Comment: 1+32 pages, LaTeX, v2: references added, typographical errors correcte

Vibratory behavior of the sound generating structures of the bird syrinx

Journal ArticleRecent endoscopic studies of the bird syrinx during phonation suggest that sound is generated by vibrating membrane folds or labia (Goller and Larsen, J. exp. Biol., 200, 2165-2176, 1997; Goller and Larsen, Proc. Natl. Acad. Sci., 4,14787-14791, 1997) and not by a whistle mechanism (Nottebohm, J. comp. Physiol., 108, 157-170, 1976; Gaunt et a!., Auk, 99, 474-494, 1982)

Null Strings in Schwarzschild Spacetime

The null string equations of motion and constraints in the Schwarzschild spacetime are given. The solutions are those of the null geodesics of General Relativity appended by a null string constraint in which the "constants of motion" depend on the world-sheet spatial coordinate. Because of the extended nature of a string, the physical interpretation of the solutions is completely different from the point particle case. In particular, a null string is generally not propagating in a plane through the origin, although each of its individual points is. Some special solutions are obtained and their physical interpretation is given. Especially, the solution for a null string with a constant radial coordinate $r$ moving vertically from the south pole to the north pole around the photon sphere, is presented. A general discussion of classical null/tensile strings as compared to massless/massive particles is given. For instance, tensile circular solutions with a constant radial coordinate $r$ do not exist at all. The results are discussed in relation to the previous literature on the subject.Comment: 16 pages, REVTEX, no figure

Stable and Unstable Circular Strings in Inflationary Universes

It was shown by Garriga and Vilenkin that the circular shape of nucleated cosmic strings, of zero loop-energy in de Sitter space, is stable in the sense that the ratio of the mean fluctuation amplitude to the loop radius is constant. This result can be generalized to all expanding strings (of non-zero loop-energy) in de Sitter space. In other curved spacetimes the situation, however, may be different. In this paper we develop a general formalism treating fluctuations around circular strings embedded in arbitrary spatially flat FRW spacetimes. As examples we consider Minkowski space, de Sitter space and power law expanding universes. In the special case of power law inflation we find that in certain cases the fluctuations grow much slower that the radius of the underlying unperturbed circular string. The inflation of the universe thus tends to wash out the fluctuations and to stabilize these strings.Comment: 15 pages Latex, NORDITA 94/14-