35451533
Runaway dilatonic domain walls
We explore the stability of domain wall and bubble solutions in theories with compact extra dimensions. The energy density stored inside of the wall can destabilize the volume modulus of a compactification, leading to solutions containing either a timelike singularity or a region where space decompactifies, depending on the metric ansatz. We determine the structure of such solutions both analytically and using numerical simulations, and analyze how they arise in compactifications of Einstein-Maxwell theory and type IIB string theory. The existence of instabilities has important implications for the formation of networks of topological defects and the population of vacua during eternal inflation.
18383227
Intergalactic Dust and Observations of Type Ia Supernovae
Estimates of the cosmic star formation rate and of cluster metallicities independently imply that at z 0.5 the gas in the universe has substantial average metallicity: 1/10 Z/Z☉ 1/3 for Ωgas = 0.05. This metal density probably cannot be contained in known solar-metallicity galaxies of density parameter Ω* ≈ 0.004, implying significant enrichment of the intergalactic medium (IGM) by ejection of metals and dust from galaxies via winds, in mergers or in dust efflux driven by radiation pressure. Galaxies have a dust-to-metal ratio of ~0.5 in their interstellar media, but some fraction (1 - f) > 0 of this must be destroyed in the IGM or during the ejection process. Assuming the Draine & Lee dust model and preferential destruction of small grains (as destruction by sputtering would provide), I calculate the reddening and extinction of a uniform cosmological dust component in terms of f and the minimum grain size amin. Very small grains provide most of the reddening but less than half of the opacity for optical extinction. For f 0.3 and amin 0.1 μm, the intergalactic dust would be too gray to have been detected by its reddening, yet dense enough to be cosmologically important: it could account for the recently observed Type Ia supernova dimming at z ~ 0.5 without cosmic acceleration. It would also have implications for galaxy counts and evolutionary studies and would contribute significantly to the cosmic infrared background (CIB). The importance of gray intergalactic dust of the described type can be tested by observations of z = 0.5 supernovae in (rest) R-band or longer wavelengths and by the fluxes of a large sample of supernovae at z > 1.
118974302
Steady state eternal inflation
Since the advent of inflation, several theorems have been proven suggesting that although inflation can (and generically does) continue eternally into the future, it cannot be extended eternally into the past to create a ``steady-state'' model with no initial time. Here we provide a construction that circumvents these theorems and allows a self-consistent, geodesically complete, and physically sensible steady-state eternally inflating universe, based on the flat slicing of de Sitter space. This construction could be used as the background space-time for creation events that form big-bang-like regions, and hence could form the basis for a cosmology that is compatible with observations and yet which avoids an initial singularity or beginning of time.
118896902
Gamma-Ray Burst Afterglows as Probes of Galactic and Intergalactic Dust
The amount and properties of high-redshift galactic and intergalactic (IG) dust are largely unknown but could be investigated using multiwavelength photometry of high-z objects that have a known intrinsic spectrum. Observations of γ-ray burst (GRB) afterglows appear to support the theoretical model of an adiabatic blast wave expanding into an external medium. In this model the synchrotron peak flux is independent of frequency, providing a flat spectrum when observed over time, and therefore allowing straightforward measurement of the relative attenuation of afterglow flux in widely separated bands. Applying this method to dust extinction, we show that for a sample of afterglows that have been corrected by galactic extinction comparison between the number counts of peak fluxes in X-ray versus optical can provide constraints on an intergalactic component of dust. A similar technique can probe the redshift dependence of extinction in GRB-forming regions without requiring an assumed relation between extinction and reddening by the dust. Probing systematic changes in extinction with redshift, particularly in IG and/or nonreddening dust, is crucial to a proper interpretation of the Type Ia supernova Hubble diagram and similar observations and useful for understanding GRB progenitor environments.
14712841
On the Superradiance of Spin-1 Waves in an Equatorial Wedge around a Kerr Hole.
Recently Van Putten has suggested that superradiance of magnetosonic waves in a toroidal magnetosphere around a Kerr black hole may play a role in the central engine of gamma-ray bursts. In this context, he computed (in the WKB approximation) the superradiant amplification of scalar waves confined to a thin equatorial wedge around a Kerr hole and found that the superradiance is higher than for radiation incident over all angles. This Letter presents calculations of both spin-0 (scalar) superradiance (integrating the radial equation rather than using the WKB method) and spin-1 (electromagnetic/magnetosonic) superradiance in Van Putten's wedge geometry. In contrast to the scalar case, spin-1 superradiance decreases in the wedge geometry, decreasing the likelihood of its astrophysical importance.
16799264
Eternal Inflation, past and future
Cosmological inflation, if it occurred, radically alters the picture of the `big bang', which would merely point to reheating at the end of inflation. Moreover, this reheating may be only local, so that inflation continues elsewhere and forever, continually spawning big-bang-like regions. This chapter reviews this idea of `eternal inflation', then focuses on what this may mean for the ultimate beginning of the universe. In particular, I will argue that given eternal inflation, the universe may be free of a cosmological initial singularity, might be eternal (and eternally inflating) to the past, and might obey an interesting sort of cosmological time-symmetry.
119173827
Inverse scattering approach for massive Thirring models with integrable type-II defects
We discuss the integrability of the bosonic and Grassmannian massive Thirring models in the presence of defects through the inverse scattering approach. We present a general method to compute the generating functions of modified conserved quantities for any integrable field equation associated with the m × m spectral linear problem. We apply the method to derive in particular the defect contributions for the number of occupation, energy and momentum of the massive Thirring models.
119172404
N=1 super sinh-Gordon model with defects revisited
A bstractThe Lax pair formalism is considered to discuss the integrability of the N = 1 supersymmetric sinh-Gordon model with a defect. We derive associated defect matrix for the model and construct the generating functions of the modified conserved quantities. The corresponding defect contributions for the modified energy and momentum of the model are explicitly computed.