17,054 research outputs found
The Contribution of EUV from Clusters of Galaxies to the Cosmic Ionizing Background
Recent observations with the Extreme Ultraviolet Explorer (EUVE) suggest that
at least some clusters of galaxies are luminous sources of extreme ultraviolet
(EUV) radiation. It is not clear yet whether EUV emission is a general feature
of clusters; for the purposes of limiting the contribution to the background
radiation, we assume that it is true of most clusters. Assuming that the source
of the EUV emission is inverse Compton (IC) scattering of the Cosmic Microwave
Background photons by relativistic electrons, we construct a simple model for
the expected average emission from clusters as a function of their mass and the
redshift of interest. Press-Schechter theory is used to determine the abundance
of clusters of various masses as a function of redshift. We determine the
amount of background radiation produced by clusters. The total mean intensity,
spectrum, and the ionization rates for HI and HeII are determined at present
and at a variety of redshifts. Because clusters form by the merger of smaller
subclusters, the amount of EUV background radiation should be larger at present
than in the past. We compare our results to the ionizing background expected
from quasars. We find that while clusters do contribute a significant EUV
background, it is less than a percent of that expected from quasars.Comment: 13 pages in emulateapj5 style with 7 figures, accepted for
publication in Astrophysical Journa
Synthetic aperture radar operator tactical target acquisition research
A radar target acquisition research study was conducted to access the effects of two levels of 13 radar sensor, display, and mission parameters on operator tactical target acquisition. A saturated fractional-factorial screening design was employed to examine these parameters. Data analysis computed ETA squared values for main and second-order effects for the variables tested. Ranking of the research parameters in terms of importance to system design revealed four variables (radar coverage, radar resolution/multiple looks, display resolution, and display size) accounted for 50 percent of the target acquisition probability variance
X ray opacity in cluster cooling flows
We have calculated the emergent x-ray properties for a set of spherically symmetric, steady-state cluster cooling flow models including the effects of radiative transfer. Opacity due to resonant x-ray lines, photoelectric absorption, and electron scattering have been included in these calculations, and homogeneous and inhomogeneous gas distributions were considered. The effects of photoionization opacity are small for both types of models. In contrast, resonant line optical depths can be quite high in both homogeneous and inhomogeneous models. The presence of turbulence in the gas can significantly lower the line opacity. We find that integrated x-ray spectra for the flow cooling now are only slightly affected by radiative transfer effects. However x-ray line surface brightness profiles can be dramatically affected by radiative transfer. Line profiles are also strongly affected by transfer effects. The combined effects of opacity and inflow cause many of the lines in optically thick models to be asymmetrical
Precision of Hubble constant derived using black hole binary absolute distances and statistical redshift information
Measured gravitational waveforms from black hole binary inspiral events
directly determine absolute luminosity distances. To use these data for
cosmology, it is necessary to independently obtain redshifts for the events,
which may be difficult for those without electromagnetic counterparts. Here it
is demonstrated that certainly in principle, and possibly in practice,
clustering of galaxies allows extraction of the redshift information from a
sample statistically for the purpose of estimating mean cosmological
parameters, without identification of host galaxies for individual events. We
extract mock galaxy samples from the 6th Data Release of the Sloan Digital Sky
Survey resembling those that would be associated with inspiral events of
stellar mass black holes falling into massive black holes at redshift z ~ 0.1
to 0.5. A simple statistical procedure is described to estimate a likelihood
function for the Hubble constant H_0: each galaxy in a LISA error volume
contributes linearly to the log likelihood for the source redshift, and the log
likelihood for each source contributes linearly to that of H_0. This procedure
is shown to provide an accurate and unbiased estimator of H_0. It is estimated
that a precision better than one percent in H_0 may be possible if the rate of
such events is sufficiently high, on the order of 20 to z = 0.5.Comment: 9 pages, 4 figures, submitted to Phys. Rev. D; new references adde
New remarks on the Cosmological Argument
We present a formal analysis of the Cosmological Argument in its two main
forms: that due to Aquinas, and the revised version of the Kalam Cosmological
Argument more recently advocated by William Lane Craig. We formulate these two
arguments in such a way that each conclusion follows in first-order logic from
the corresponding assumptions. Our analysis shows that the conclusion which
follows for Aquinas is considerably weaker than what his aims demand. With
formalizations that are logically valid in hand, we reinterpret the natural
language versions of the premises and conclusions in terms of concepts of
causality consistent with (and used in) recent work in cosmology done by
physicists. In brief: the Kalam argument commits the fallacy of equivocation in
a way that seems beyond repair; two of the premises adopted by Aquinas seem
dubious when the terms `cause' and `causality' are interpreted in the context
of contemporary empirical science. Thus, while there are no problems with
whether the conclusions follow logically from their assumptions, the Kalam
argument is not viable, and the Aquinas argument does not imply a caused
origination of the universe. The assumptions of the latter are at best less
than obvious relative to recent work in the sciences. We conclude with mention
of a new argument that makes some positive modifications to an alternative
variation on Aquinas by Le Poidevin, which nonetheless seems rather weak.Comment: 12 pages, accepted for publication in International Journal for
Philosophy of Religio
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Digital inclusion - the vision, the challenges and the way forward
This paper considers the vision and aspiration of digital inclusion, and then examines the current reality. It looks beyond the rhetoric to provide an analysis of the status quo, a consideration of some facilitators and challenges to progress and some suggestions for moving forward with renewed energy and commitment. The far-reaching benefits of digital inclusion and the crucial role it plays in enabling full participation in our digital society are considered. At the heart of the vision of universal digital inclusion is the deceptively simple goal to ensure that everyone is able to access and experience the wide-ranging benefits and transformational opportunities and impacts it offers. The reality is a long way from the vision: inequality of access still exists despite many national campaigns and initiatives to reduce it. The benefits and beneficiaries of a digital society are not just the individual but all stakeholders in the wider society. Research evidence has shown that the critical success factors for successful digital participation are (i) appropriate design and (ii) readily available and on-going ICT (Information and Communication Technology) support in the community. Challenges and proven solutions are presented. The proposition of community hubs in local venues to provide user-centred ICT support and learning for older and disabled people is presented. While the challenges to achieve digital inclusion are very considerable, the knowledge of how to achieve it and the technologies which enable it already exist. Harnessing of political will is necessary to make digital inclusion a reality rather than a vision. With the cooperation and commitment of all stakeholders actualisation of the vision of a digitally inclusive society, while challenging, can be achieved and will yield opportunities and rewards that eclipse the cost of implementation
Current research in oxidation-resistant carbon-carbon composites at NASA. Langley Research Center
The significant potential of carbon-carbon composites for high-temperature structural applications is well established. For hypersonic vehicle applications, desirable properties include low density, high specific strength and stiffness, low coefficients of thermal expansion, and retention of mechanical properties above 3000 F. A significant problem associated with carbon materials, however, is that they oxidize rapidly in air at temperatures above about 800 F, and therefore must be protected from oxidation. Successful development of effective methods of oxidation protection is key to the eventual utilization of carbon-carbon composites on hypersonic vehicles such as NASP. In this presentation, the basic elements of an oxidation-protection system are described. Results from oxidation-performance evaluations of state-of-the-art ACC-4 type material in simulated airframe vehicle environments (temperature, pressure, and time) conducted at NASA Langley are also presented. NASA Langley has an active research effort to improve the oxidation resistance of carbon-carbon materials for airframe structural and vehicle thermal protection applications. Conversion coating and sealant development research is highlighted
NASA Langley Research Center National Aero-Space Plane Mission simulation profile sets
To provide information on the potential for long life service of oxidation resistant carbon-carbon (ORCC) materials in the National Aero-Space Plane (NASP) airframe environment, NASP ascent, entry, and cruise trajectories were analytically flown. Temperature and pressure profiles were generated for 20 vehicle locations. Orbital (ascent and entry) and cruise profile sets from four locations are presented along with the humidity exposure and testing sequences that are being used to evaluate ORCC materials. The four profiles show peak temperatures during the ascent leg of an orbital mission of 2800, 2500, 2000, and 1700 F. These profiles bracket conditions where carbon-carbon might be used on the NASP vehicle
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