Cosmological tests using the angular size of galaxy clusters

We use measurements of the galaxy-cluster angular size versus redshift to test and compare the standard model (Lambda CDM) and the R-h = ct Universe. We show that the latter fits the data with a reduced chi(2)(dof) = 0.786 for a Hubble constant H-0 = 72.6(-3.4)(+3.8) km s(-1) Mpc(-1), and H-0 is the sole parameter in this model. By comparison, the optimal flat Lambda cold dark matter (Lambda CDM) model, with two free parameters (including Omega(m) = 0.50 and H-0 = 73.9(-9.5)(+10.6) km s(-1) Mpc(-1)), fits the angular-size data with a reduced chi(2)(dof) = 0.806. On the basis of their chi(2)(dof) values alone, both models appear to account for the data very well in spite of the fact that the R-h = ct Universe expands at a constant rate, while Lambda CDM does not. However, because of the different number of free parameters in these models, selection tools, such as the Bayes Information Criterion, favour R-h = ct over Lambda CDM with a likelihood of similar to 86 per cent versus similar to 14 per cent. These results impact the question of galaxy growth at large redshifts. Previous work suggested an inconsistency with the underlying cosmological model unless elliptical and disc galaxies grew in size by a surprisingly large factor similar to 6 from z similar to 3 to 0. The fact that both Lambda CDM and R-h = ct fit the cluster-size measurements quite well casts some doubt on the suggestion that the unexpected result with individual galaxies may be due to the use of an incorrect expansion scenario, rather than astrophysical causes, such as mergers and/or selection effects.