On the origin of dark matter axions

We discuss the possible sources of dark matter axions in the early universe. In the standard thermal scenario, an axion string network forms at the Peccei-Quinn phase transition T\sim \fa and then radiatively decays into a cosmological background of axions; to be the dark matter, these axions must have a mass \ma \sim 100 \mu eV with specified large uncertainties. An inflationary phase with a reheat temperature below the PQ-scale T_{reh} \lapp \fa can also produce axion strings through quantum fluctuations, provided that the Hubble parameter during inflation is large H_1 \gapp \fa; this case again implies a dark matter axion mass \ma \sim 100 \mu eV. For a smaller Hubble parameter during inflation H_1 \lapp \fa, `anthropic tuning' allows dark matter axions to have any mass in a huge range below \ma\lapp 1 meV.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion

Towards Observing the Intercommutation of Flux Tubes in Superconductors

We propose a simple experiment to investigate the intercommutation of flux tubes in type II superconductors. Using this method the intercommutation of strings can be observed directly and the dependence of intercommutation on the angle of crossing of strings can also be analyzed.Comment: 8 pages (LATEX), three figures included at the end of the paper, NSF-ITP-94-2

Cosmic string induced CMB maps

We compute maps of CMB temperature fluctuations seeded by cosmic strings using high resolution simulations of cosmic strings in a Friedmann-Robertson-Walker universe. We create full-sky, 18-degree and 3-degree CMB maps, including the relevant string contribution at each resolution from before recombination to today. We extract the angular power spectrum from these maps, demonstrating the importance of recombination effects. We briefly discuss the probability density function of the pixel temperatures, their skewness and kurtosis.Comment: 5 pages, 4 figures, submitted to PRD; v2: 6 pages, 5 figures, matches published versio

The shape of primordial non-Gaussianity and the CMB bispectrum

We present a set of formalisms for comparing, evolving and constraining primordial non-Gaussian models through the CMB bispectrum. We describe improved methods for efficient computation of the full CMB bispectrum for any general (non-separable) primordial bispectrum, incorporating a flat sky approximation and a new cubic interpolation. We review all the primordial non-Gaussian models in the present literature and calculate the CMB bispectrum up to l <2000 for each different model. This allows us to determine the observational independence of these models by calculating the cross-correlation of their CMB bispectra. We are able to identify several distinct classes of primordial shapes - including equilateral, local, warm, flat and feature (non-scale invariant) - which should be distinguishable given a significant detection of CMB non-Gaussianity. We demonstrate that a simple shape correlator provides a fast and reliable method for determining whether or not CMB shapes are well correlated. We use an eigenmode decomposition of the primordial shape to characterise and understand model independence. Finally, we advocate a standardised normalisation method for $f_{NL}$ based on the shape autocorrelator, so that observational limits and errors can be consistently compared for different models.Comment: 32 pages, 20 figure