arXiv subjects

J C Jackson

Publications and source records attributed to J C Jackson.

3 recordsLinked to original sources

arXiv2006

Legacy data and cosmological constraints from the angular-size/redshift relation for ultra-compact radio sources

We have re-examined an ancient VLBI survey of ultra-comact radio sources at 2.29 GHz, which gave fringe amplitudes for 917 such objects with total flux density >0.5 Jy approximately. A number of cosmological investigations based upon this survey have been published in recent years. We have updated the sample with respect to both redshift and radio information, and now have full data for 613 objects, significantly larger than the number (337) used in earlier investigations. The corresponding angular-size/redshift diagram gives Omega_m=0.25+0.04/-0.03, Omega_Λ=0.97+0.09/-0.13 and K=0.22+0.07/-0.10. In combination with supernova data, and a simple-minded approach to CMB data based upon the angular size of the acoustic horizon, our best figures are Omega_m=0.298+0.025/-0.024, Omega_Λ=0.702+0.035/-0.036 and K= 0.000+0.021/-0.019. We have examined simple models of dynamical vacuum energy; the first, based upon a scalar potential V(phi)=omega_C^2 phi^2/2, gives w(0)=-1.00+0.06/-0.00, (dw/dz)_0=+0.00/-0.08; in this case conditions at z=0 require particular attention, to preclude behaviour in which phi becomes singular as z -->infinity. For fixed w limits are w=-1.20+0.15/-0.14. The above error bars are 68% confidence limits.

astro-ph↗

arXiv2006

Deceleration without dark matter

In homogeneous isotropic cosmological models the angular size theta of a standard measuring rod changes with redshift z in a manner that depends upon the parameters of the model. It has been argued that as a population ultracompact (milliarcsecond) radio sources measured by very long-baseline interferometry (VLBI) do not evolve with cosmic epoch, and thus comprise a set of standard objects, at least in a statistical sense. Here we examine the angular-size/redshift relation for 256 ultracompact sources with z in the range 0.5 to 3.8 for cosmological models with two degrees of freedom (Omega_0 and Lambda_0). The canonical inflationary cold dark matter model(Omega_0=1, Lambda_0=0) appears to be ruled out by the observed relationship, whereas low-density models with a cosmological constant of either sign are favoured.

astro-ph↗

arXiv2005

Mammographic image restoration using maximum entropy deconvolution

An image restoration approach based on a Bayesian maximum entropy method (MEM) has been applied to a radiological image deconvolution problem, that of reduction of geometric blurring in magnification mammography. The aim of the work is to demonstrate an improvement in image spatial resolution in realistic noisy radiological images with no associated penalty in terms of reduction in the signal-to-noise ratio perceived by the observer. Images of the TORMAM mammographic image quality phantom were recorded using the standard magnification settings of 1.8 magnification/fine focus and also at 1.8 magnification/broad focus and 3.0 magnification/fine focus; the latter two arrangements would normally give rise to unacceptable geometric blurring. Measured point-spread functions were used in conjunction with the MEM image processing to de-blur these images. The results are presented as comparative images of phantom test features and as observer scores for the raw and processed images. Visualization of high resolution features and the total image scores for the test phantom were improved by the application of the MEM processing. It is argued that this successful demonstration of image de-blurring in noisy radiological images offers the possibility of weakening the link between focal spot size and geometric blurring in radiology, thus opening up new approaches to system optimization.

physics.med-ph↗