The Universal Deceleration and Angular Diameter Distances to Clusters of   Galaxies
  We show how the virial theorem can be applied to the hot gas in clusters of galaxies to obtain a yardstick, which could then be used to determine cosmological parameters. This yardstick relies on the assumptions of hydrostatic equilibrium and that the gas fraction is approximately constant. The constancy is checked empirically from a local population of clusters. By using the observed parameters consisting of temperature, surface brightness and radial profile $\beta$, one can calculate the expected core radius. Comparing it to the observed angular size, one can in principle calibrate the cosmological deceleration parameter $q_0$. We test this method on a small sample of 6 clusters, and show its promise and accuracy. The preliminary implications would be to suggest $q_0 \approx 0.85\pm 0.29$ with $1-\sigma$ statistical error bars, with several systematic uncertainties remaining. Taken at face value, this would argue against a cosmological constant. The method is robust to errors in the measurement of the core radius as long as the product of the central density and the core radius squared $\rho_0 r_c^2$ are well determined. New lensing and X-ray data can dramatically improve on the statistics. 
Ue-Li Pen