44,204 research outputs found

    The old and new universe in the era of precision cosmology

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    These are privileged times to be a cosmologist. Recent years have witnessed unprecedented progress in observational and computational techniques and we now are able to quantify cosmological properties with unprecedented accuracy. My work builds upon this observational accuracy by establishing a connection with viable theoretical models. I focus on two specifics eras of the universe’s evolution, namely inflation and today’s cosmic acceleration. In the context of single field inflationary models I illustrate the relation between the spectra of curvature and gravitational wave perturbations. I conclude that their mutual interdependence extends beyond the usual amplitude consistency relation and can be traced all the way to infinite order of accuracy. This yields an infinite hierarchy of consistency relations between these spectra and their derivatives. On a observational perspective, using WMAP’s data, I explore the dependence of CMB constraints on inflation with the cosmological scale at which these are chosen to be presented. I develop a technique that allows for an appropriate choice of this scale and show that this way constraints may be improved by as much as 5 times. In the context of the particle physics motivated quintessence models I have looked at the ability of early universe probes - namely Big Bang Nucleosynthesis - for distinguishing between different dark energy proposals when combined with standard distance modulus or the Hubble rate techniques. I conclude that more yet more accurate measurements are required if observations are to successfully confirm or rule out these models as potential candidates against a cosmological constant. I also analyze possible effects that may mimic or underlie cosmic acceleration effects. I focus on a potential lack of knowledge of the precise values of particular cosmological parameters such as the curvature and matter content of the universe. I find that even a small uncertainty in any of this two quantities leads to significant bias on the reconstruction of dark energy properties, when typical probes like the distance luminosity and the Hubble rate are considered. I conclude that in order to disentangle between these effects a combination of distance and expansion history measurements is required

    Reconstructing the Inflaton Potential---in Principle and in Practice

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    Generalizing the original work by Hodges and Blumenthal, we outline a formalism which allows one, in principle, to reconstruct the potential of the inflaton field from knowledge of the tensor gravitational wave spectrum or the scalar density fluctuation spectrum, with special emphasis on the importance of the tensor spectrum. We provide some illustrative examples of such reconstruction. We then discuss in some detail the question of whether one can use real observations to carry out this procedure. We conclude that in practice, a full reconstruction of the functional form of the potential will not be possible within the foreseeable future. However, with a knowledge of the dark matter components, it should soon be possible to combine intermediate-scale data with measurements of large-scale cosmic microwave background anisotropies to yield useful information regarding the potential.Comment: 39 pages plus 2 figures (upon request:[email protected]), LaTeX, FNAL--PUB--93/029-A; SUSSEX-AST 93/3-

    Coupled dark matter-dark energy in light of near Universe observations

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    Cosmological analysis based on currently available observations are unable to rule out a sizeable coupling among the dark energy and dark matter fluids. We explore a variety of coupled dark matter-dark energy models, which satisfy cosmic microwave background constraints, in light of low redshift and near universe observations. We illustrate the phenomenology of different classes of dark coupling models, paying particular attention in distinguishing between effects that appear only on the expansion history and those that appear in the growth of structure. We find that while a broad class of dark coupling models are effectively models where general relativity (GR) is modified --and thus can be probed by a combination of tests for the expansion history and the growth of structure--, there is a class of dark coupling models where gravity is still GR, but the growth of perturbations is, in principle modified. While this effect is small in the specific models we have considered, one should bear in mind that an inconsistency between reconstructed expansion history and growth may not uniquely indicate deviations from GR. Our low redshift constraints arise from cosmic velocities, redshift space distortions and dark matter abundance in galaxy voids. We find that current data constrain the dimensionless coupling to be |xi|<0.2, but prospects from forthcoming data are for a significant improvement. Future, precise measurements of the Hubble constant, combined with high-precision constraints on the growth of structure, could provide the key to rule out dark coupling models which survive other tests. We shall exploit as well weak equivalence principle violation arguments, which have the potential to highly disfavour a broad family of coupled models.Comment: 34 pages, 6 figures; changes to match published versio

    Investigating dark energy experiments with principal components

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    We use a principal component approach to contrast different kinds of probes of dark energy, and to emphasize how an array of probes can work together to constrain an arbitrary equation of state history w(z). We pay particular attention to the role of the priors in assessing the information content of experiments and propose using an explicit prior on the degree of smoothness of w(z) that is independent of the binning scheme. We also show how a figure of merit based on the mean squared error probes the number of new modes constrained by a data set, and use it to examine how informative various experiments will be in constraining the evolution of dark energy.Comment: A significantly expanded version with an added PCA for weak lensing, a new detailed discussion of the correlation prior proposed in this work, and a new discussion outlining the differences between the Bayesian and the frequentist approaches to reconstructing w(z). Matches the version accepted to JCAP. 8 pages, 2 figure

    Reconstructing the shape of the correlation function

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    We develop an estimator for the correlation function which, in the ensemble average, returns the shape of the correlation function, even for signals that have significant correlations on the scale of the survey region. Our estimator is general and works in any number of dimensions. We develop versions of the estimator for both diffuse and discrete signals. As an application, we examine Monte Carlo simulations of X-ray background measurements. These include a realistic, spatially-inhomogeneous population of spurious detector events. We discuss applying the estimator to the averaging of correlation functions evaluated on several small fields, and to other cosmological applications.Comment: 10 pages, 5 figures, submitted to ApJS. Methods and results unchanged but text is expanded and significantly reordered in response to refere

    "A wealth of knowledge": A survey of the employment experiences of older nurses and midwives in the NHS

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    Background: The United Kingdom's National Health Service workforce is ageing, and the specific needs of this sector of its workforce need to be addressed. Nursing, and midwifery shortage is a worldwide issue, and with increasing demands for care the retention of older nurses and midwives is crucial. Objectives: To report on the employment experiences of nurses and midwives with it particular focus on issues relating to age, ethnicity, ill-health and disability. Design: The postal survey was developed following a literature review and analysis of National Health Service and Government policy documents. Settings: This was a UK-wide Survey of nurses and midwives working in National Health Service Trusts and Primary Care Trusts. Participants/methods: A postal Survey of nurses and midwives was undertaken between May and December 2005. National Health Service Trusts and Primary Care Trusts (n = 44) identified as having policies relevant to the Study were contacted regarding the procedure for seeking research governance approval. Thirteen National Health Service Trusts and Primary Care Trusts participated, with 2610 surveys distributed; 510 Surveys were returned (20% response rate). Results: Nurses and midwives aged 50 years and over had undertaken fewer Continuing Professional Development activities than nurses and midwives Under 50. Whilst not related to age, the study also found that 20% of the survey sample reported experiencing some form of discrimination. Nurses and midwives did not differ on either quality of life or psychological health using standard instruments. Having a disability did not lead to greater psychological morbidity but did have a negative effect on quality of life. Having a work-related illness had a negative impact on both quality of life and psychological morbidity. hi relation to ethnicity, black nurses and midwives reported lower psychological morbidity than other ethnic groups; that is, they enjoyed a higher level of mental well-being. Conclusion: The nursing and midwifery workforce is ageing worldwide with a significant proportion now approaching, or having already reached, potential retirement age. With the recent introduction of the age legislation the working lives of older nurses and midwives in the National Health Service have never been more relevant. Whilst access to Continuing Professional Development is pertinent to the retention of nurses and midwives of all ages, in this study, older nurses reported less access that younger nurses. (C) 2008 Elsevier Ltd. All rights reserved
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