122 research outputs found
CMB polarization power spectra contributions from a network of cosmic strings
We present the first calculation of the possible (local) cosmic string contribution to the cosmic microwave background polarization spectra from simulations of a string network (rather than a stochastic collection of unconnected string segments). We use field-theory simulations of the Abelian Higgs model to represent local U(1) strings, including their radiative decay and microphysics. Relative to previous estimates, our calculations show a shift in power to larger angular scales, making the chance of a future cosmic string detection from the B-mode polarization slightly greater. We explore a future ground-based polarization detector, taking the CLOVER project as our example. In the null hypothesis (that cosmic strings make a zero contribution) we find that CLOVER should limit the string tension mu to G mu < 0.12x10(-6) (where G is the gravitational constant), above which it is likely that a detection would be possible
Video in sociomaterial investigations : a solution to the problem of relevance for organizational research
This paper considers the application of video-based research to address methodological challenges for organizational scholars concerned with the sociomaterial foundations to work practice. In particular the claim that ‘all practices are always sociomaterial’ raises a ‘problem of relevance’ – that is on what grounds can we select material to include in the analytic account when there is a vast array of material in each setting? Furthermore, how can we grasp the sociality of material objects that are often taken for granted and that drift in and out of view? We address these methodological questions drawing on ethnomethodology and conversation analysis, and by making use of video recordings of everyday work and organizing. We demonstrate the approach with data from two service settings and explore the analysis both of single cases and collections. To conclude, the paper considers the distinctive contributions that these video-based studies have for our understanding of sociomateriality and organizational practice more generally
Detecting and distinguishing topological defects in future data from the CMBPol satellite
The proposed CMBPol mission will be able to detect the imprint of topological defects on the CMB provided the contribution is sufficiently strong. We quantify the detection threshold for cosmic strings and for textures, and analyze the satellite's ability to distinguish between these different types of defects. We also assess the level of danger of misidentification of a defect signature as from the wrong defect type or as an effect of primordial gravitational waves. A 0.002 fractional contribution of cosmic strings to the CMB temperature spectrum at multipole ten, and similarly a 0.001 fractional contribution of textures, can be detected and correctly identified at the 3 level. We also confirm that a tensor contribution of r=0.0018 can be detected at over 3, in agreement with the CMBPol mission concept study. These results are supported by a model selection analysis
Exhibiting interaction: conduct and collaboration in museums and galleries
This article explores how individuals, both alone and together, examine exhibits in museums and galleries. Drawing on ethnomethodology and conversation analysis, it focuses on the ways in which visitors encounter and experience exhibits and how their activities are organized, at least in
part, with intimate regard to the actions of others in the domain, both companions and "strangers." This study contributes to the long-standing concerns of symbolic interactionism with (mutual) attention and involvement, materiality and social relations, and interpersonal communication. The data consist of video recordings of naturally occurring action and interaction in various museums and galleries
CMB power spectra from cosmic strings: predictions for the Planck satellite and beyond
We present a significant improvement over our previous calculations of the
cosmic string contribution to cosmic microwave background (CMB) power spectra,
with particular focus on sub-WMAP angular scales. These smaller scales are
relevant for the now-operational Planck satellite and additional sub-orbital
CMB projects that have even finer resolutions. We employ larger Abelian Higgs
string simulations than before and we additionally model and extrapolate the
statistical measures from our simulations to smaller length scales. We then use
an efficient means of including the extrapolations into our Einstein-Boltzmann
calculations in order to yield accurate results over the multipole range 2 < l
3000 in
the case of the temperature power spectrum, which then allows cautious
extrapolation to even smaller scales. We find that a string contribution to the
temperature power spectrum making up 10% of power at l=10 would be larger than
the Silk-damped primary adiabatic contribution for l > 3500. Astrophysical
contributions such as the Sunyaev-Zeldovich effect also become important at
these scales and will reduce the sensitivity to strings, but these are
potentially distinguishable by their frequency-dependence.Comment: 18 pages, 16 figure
Approach to scaling in axion string networks
We study the approach to scaling in axion string networks in the radiation
era, through measuring the root-mean-square velocity as well as the scaled
mean string separation . We find good evidence for a fixed point in the
phase-space analysis in the variables , providing a strong indication
that standard scaling is taking place. We show that the approach to scaling can
be well described by a two parameter velocity-one-scale (VOS) model, and show
that the values of the parameters are insensitive to the initial state of the
network. The string length has also been commonly expressed in terms of a
dimensionless string length density , proportional to the number of
Hubble lengths of string per Hubble volume. In simulations with initial
conditions far from the fixed point is still evolving after half a
light-crossing time, which has been interpreted in the literature as a
long-term logarithmic growth. We show that all our simulations, even those
starting far from the fixed point, are accounted for by a VOS model with an
asymptote of (calculated from the string length in the
cosmic rest frame) and .Comment: 14 pages, 9 figures. v2: Minor changes, matches published versio
CMB power spectrum contribution from cosmic strings using field-evolution simulations of the Abelian Higgs model
We present the first field-theoretic calculations of the contribution made by
cosmic strings to the temperature power spectrum of the cosmic microwave
background (CMB). Unlike previous work, in which strings were modeled as
idealized one-dimensional objects, we evolve the simplest example of an
underlying field theory containing local U(1) strings, the Abelian Higgs model.
Limitations imposed by finite computational volumes are overcome using the
scaling property of string networks and a further extrapolation related to the
lessening of the string width in comoving coordinates. The strings and their
decay products, which are automatically included in the field theory approach,
source metric perturbations via their energy-momentum tensor, the unequal-time
correlation functions of which are used as input into the CMB calculation
phase. These calculations involve the use of a modified version of CMBEASY,
with results provided over the full range of relevant scales. We find that the
string tension required to normalize to the WMAP 3-year data at multipole
is , where we have quoted statistical and systematic errors
separately, and is Newton's constant. This is a factor 2-3 higher than
values in current circulation.Comment: 23 pages, 14 figures; further optimized figures for 1Mb size limit,
appendix added before submission to journal, matches accepted versio
Scaling Density of Axion Strings
In the QCD axion dark matter scenario with postinflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time t, by convention written zeta/t(2). The expectation has been that the dimensionless parameter zeta tends to a constant zeta(0), a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations zeta shows a logarithmic increase with time, while theoretical modeling suggests an inverse logarithmic correction. Either case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings, we compare the standard scaling (constant-zeta) model to the logarithmic growth and inverse-logarithmic correction models. In the standard scaling model, by fitting to linear growth in the mean string separation xi = t/root zeta, we find zeta(0) = 1.19 +/- 0.20. We conclude that the apparent corrections to zeta are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-zeta (linear xi) fit also show no evidence for logarithmic growth, restoring confidence that numerical simulations can be simply extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale. Reanalysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the postinflationary symmetry-breaking scenario we study should be increased by about 50%.Peer reviewe
Irreducible background of gravitational waves from a cosmic defect network : Update and comparison of numerical techniques
Cosmological phase transitions in the early Universe may produce relics in the form of a network of cosmic defects. Independently of the order of a phase transition, topology of the defects, and their global or gauge nature, the defects are expected to emit gravitational waves (GWs) as the network energy-momentum tensor adapts itself to maintaining scaling. We show that the evolution of any defect network (and for that matter any scaling source) emits a GW background with spectrum Omega(GW) proportional to f(3) for f > f(eq), where f(0) and f(eq) denote respectively the frequencies corresponding to the present and matter-radiation equality horizons. This background represents an irreducible emission of GWs from any scaling network of cosmic defects, with its amplitude characterized only by the symmetry-breaking scale and the nature of the defects. Using classical lattice simulations we calculate the GW signal emitted by defects created after the breaking of a global symmetry O(N) -> O(N - 1). We obtain the GW spectrum for N between 2 and 20 with two different techniques: integrating over unequal-time correlators of the energy-momentum tensor, updating our previous work on smaller lattices, and for the first time, comparing the result with the real-time evolution of the tensor perturbations sourced by the same defects. Our results validate the equivalence of the two techniques. Using cosmic microwave background upper bounds on the defects' energy scale, we discuss the difficulty of detecting this GW background in the case of global defects.Peer reviewe
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