153 research outputs found
Lenses in the forest: cross-correlation of the Lyman-alpha flux with CMB lensing
We present a theoretical estimate for a new observable: the cross-correlation
between the Lyman-alpha flux fluctuations in quasar (QSO) spectra and the
convergence of the cosmic microwave background (CMB) as measured along the same
line-of-sight. As a first step toward the assessment of its detectability, we
estimate the signal-to-noise ratio using linear theory. Although the
signal-to-noise is small for a single line-of-sight and peaks at somewhat
smaller redshifts than those probed by the Lyman-alpha forest, we estimate a
total signal-to-noise of 9 for cross-correlating QSO spectra of SDSS-III with
Planck and of 20 for cross-correlating with a future polarization based CMB
experiment. The detection of this effect would be a direct measure of the
neutral hydrogen-matter cross-correlation and could provide important
information on the growth of structures at large scales in a redshift range
which is still poorly probed by observations.Comment: 5 pages, 4 figures, matches published versio
Improving the resolution in soft X-ray emission spectrometers through photon-counting using an Electron Multiplying CCD
In 2007, a study of back-illuminated Charge-Coupled Devices (CCDs) for soft X-ray photon detection demonstrated the improvements that could be brought over more traditional micro-channel plate detectors for X-ray spectrometers based on diffraction gratings and position sensitive detectors. Whilst the spatial resolution was reported to be improved dramatically, an intrinsic limit of approximately 25 micrometers was found due to the spreading of the charge cloud generated in the CCD across several pixels. To overcome this resolution limit, it is necessary to move away from the current integrated imaging methods and consider a photon-counting approach, recording the photon interaction locations to the sub-pixel level. To make use of photon-counting techniques it is important that the individual events are separable. To maintain the throughput of the spectrometer for high intensity lines, higher frame rates and therefore higher readout speeds are required. With CCD based systems, the increased noise at high readout speeds can limit the photon-counting performance. The Electron-Multiplying CCD shares a similar architecture with the standard CCD but incorporates a "gain register". This novel addition allows controllable gain to be applied to the signal before the read noise is introduced, therefore allowing individual events to be resolved above the noise even at much higher readout rates. In the past, the EM-CCD has only been available with imaging areas too small to be practical in soft X-ray emission spectrometers. The current drive for large area Electron-Multiplying CCDs is opening this technology to new photon-counting applications, requiring in-depth analysis of the processes and techniques involved. Early results indicate that through the introduction of photon-counting techniques the resolution in such systems can be dramatically improved
The Standard Cosmological Model
The Standard Model of Particle Physics (SMPP) is an enormously successful
description of high energy physics, driving ever more precise measurements to
find "physics beyond the standard model", as well as providing motivation for
developing more fundamental ideas that might explain the values of its
parameters. Simultaneously, a description of the entire 3-dimensional structure
of the present-day Universe is being built up painstakingly. Most of the
structure is stochastic in nature, being merely the result of the particular
realisation of the "initial conditions" within our observable Universe patch.
However, governing this structure is the Standard Model of Cosmology (SMC),
which appears to require only about a dozen parameters. Cosmologists are now
determining the values of these quantities with increasing precision in order
to search for "physics beyond the standard model", as well as trying to develop
an understanding of the more fundamental ideas which might explain the values
of its parameters. Although it is natural to see analogies between the two
Standard Models, some intrinsic differences also exist, which are discussed
here. Nevertheless, a truly fundamental theory will have to explain both the
SMPP and SMC, and this must include an appreciation of which elements are
deterministic and which are accidental. Considering different levels of
stochasticity within cosmology may make it easier to accept that physical
parameters in general might have a non-deterministic aspect.Comment: 16 pages, 2 figures, invited talk at "Theory Canada 1", June 2005,
Vancouve
Binary and Millisecond Pulsars at the New Millennium
We review the properties and applications of binary and millisecond pulsars.
Our knowledge of these exciting objects has greatly increased in recent years,
mainly due to successful surveys which have brought the known pulsar population
to over 1300. There are now 56 binary and millisecond pulsars in the Galactic
disk and a further 47 in globular clusters. This review is concerned primarily
with the results and spin-offs from these surveys which are of particular
interest to the relativity community.Comment: 59 pages, 26 figures, 5 tables. Accepted for publication in Living
Reviews in Relativity (http://www.livingreviews.org
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