Why Not Consider Closed Universes?

We consider structure formation and CMB anisotropies in a closed universe, both with and without a cosmological constant. The CMB angular power spectrum and the matter transfer function are presented, along with a discussion of their relative normalization. This represents the first full numerical evolution of density perturbations and anisotropies in a spherical geometry. We extend the likelihood function vs. Omega from the COBE 2-year data to Omega>=1. For large Omega the presence of a very steep rise in the spectrum towards low ell allows us to put an upper limit of Omega<=1.5 (95%CL) for primordial spectra with n<=1. This compares favorably with existing limits on Omega. We show that there are a range of closed models which are consistent with observational constraints while being even older than the currently popular flat models with a cosmological constant. Future constraints from degree scale CMB data may soon probe this region of parameter space. A derivation of the perturbed Einstein, fluid and Boltzmann equations for open and closed geometries is presented in an appendix.Comment: 24 pages, including 13 figures in a uuencoded self-unpacking shell script. Submitted to Ap

A Characteristic Scale on the Cosmic Microwave Sky

The current suite of results from Cosmic Microwave Background anisotropy experiments is fulfilling the promise of providing extraordinary levels of discrimination between cosmological models. We calculate a binned anisotropy power spectrum, which we tabulate, along with error bars and bin-to-bin correlations, so that it can be easily used for constraining models. The resulting power spectrum is flat at large angles, with a gradual rise to a prominent peak at around 0.5 degrees and a decrease thereafter. This is precisely the shape predicted by inflationary-inspired adiabatic models. Within that class of cosmologies, this characteristic scale imprinted on the CMB sky can be used to infer that the geometry of the Universe is very close to flat. The next wave of CMB results should add fuel to the debate about whether or not the Universe once inflated, as well as beginning in earnest the task of measuring cosmological parameters.Comment: 6 pages, 1 figure. A less technical article based on the same work has appeared in Science Perspectives under the title "How Flat is the Universe?" (Science, Mar 24, 2000, 2171-2172

"Sample Variance" in Small-Scale CMB Anisotropy Experiments

We discuss the effects of finite sky coverage and the uncertainty in extracting information about the power spectrum from experiments on small angular scales. In general the cosmic variance is enhanced by a factor of $4\pi/A$, where $A$ is the solid angle sampled by the experiment. As a rough guide, an experiment with sensitivity peaking at the $\ell$th multipole has to cover \simgt\ell independent patches to have a lower ``sample variance'' than for a whole-sky measurement of the quadrupole. Our approach gives a relatively simple way of attaching an error bar to the theoretical prediction for a particular experiment, and thereby comparing theories with experimental results, without the need for computationally-intensive Maximum Likelihood or Monte Carlo calculations.Comment: 6 pages in Plain TeX with 1 postscript figure appende

A generative model for feedback networks

We investigate a simple generative model for network formation. The model is designed to describe the growth of networks of kinship, trading, corporate alliances, or autocatalytic chemical reactions, where feedback is an essential element of network growth. The underlying graphs in these situations grow via a competition between cycle formation and node addition. After choosing a given node, a search is made for another node at a suitable distance. If such a node is found, a link is added connecting this to the original node, and increasing the number of cycles in the graph; if such a node cannot be found, a new node is added, which is linked to the original node. We simulate this algorithm and find that we cannot reject the hypothesis that the empirical degree distribution is a q-exponential function, which has been used to model long-range processes in nonequilibrium statistical mechanics.Comment: 11 pages, 6 figure

Does it work? evaluating a new pay system

This report focuses on the evaluation of the impact of new pay systems in large, unionised multi-site organisations by the organisations themselves. Evaluation of the effectiveness of a pay system, however, does not take place in a vacuum and relates to the aims and objectives of the pay system concerned. Moreover, evaluation is not an end in itself. It is, therefore, relevant to consider if any further steps were taken as a result of evaluation. Accordingly our research questions were: • What were the aims and objectives of organisations when introducing new pay arrangements? • What data did organisations collect and review to inform their evaluation? • What steps have organisations taken as a result of their evaluation? We re-appraised our data from 10 NHS trusts in England which had introduced some innovations in pay and grading in the 1990s. Additionally, we looked at seven multi-site unionised organisations outside the NHS in both the public and private sectors, which had recently made changes to their reward systems, carrying out interviews and inspecting documents. The main output is a template for the evaluation of Agenda for Change by NHS organisations

A space transportation system operations model

Presented is a description of a computer program which permits assessment of the operational support requirements of space transportation systems functioning in both a ground- and space-based environment. The scenario depicted provides for the delivery of payloads from Earth to a space station and beyond using upper stages based at the station. Model results are scenario dependent and rely on the input definitions of delivery requirements, task times, and available resources. Output is in terms of flight rate capabilities, resource requirements, and facility utilization. A general program description, program listing, input requirements, and sample output are included