1997NewA....2..309P

Recent simulations, as well as weak lensing and X-ray data, indicate that gas is a good tracer of the total mass in clusters. The constancy of the gas fraction f_g in clusters of galaxies can thus be used as a novel distance indicator f_g prop h^3/2 Omega_b/Omega0. Assuming spherical symmetry and hydrostatic equilibrium allows us to infer f_g using only the beta-model parameters and an emission weighted temperature. Even though none of the factors h, Omega_b, Omega_0 is individually known to high precision, the relative distances between clusters at high redshift can be used to infer the deceleration parameter q_0. Using this new model, the local population of clusters has a scatter of only 15% in the Hubble relation. The 3 clusters at redshift z ∼ 0.5 suggest q_0 approx 0.85 ± 0.29 with 1 - sigma statistical error. This result 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_0rc^2 are well determined. However, this result may still be subject to unknown systematics such as cooling flows, which will require more detailed temperature maps, weak lensing data and numerical simulations to model accurately. Future lensing and X-ray data will dramatically improve the statistics.