Dark energy and the angular size - Redshift diagram for milliarcsecond radio sources

We investigate observational constraints on the cosmic equation of state from measurements of angular size for a large sample of milliarcsecond compact radio sources. The results are based on a flat Friedmann-Robertson-Walker (FRW) model driven by nonrelativistic matter, plus a smooth, dark energy component parameterized by its equation of state p(x) = omegarho(x) (-1 less than or equal to omega < 0). The allowed intervals for omega and Omega(m) are heavily dependent on the value of the mean projected linear size l. For l similar or equal to 20-30 h(-1) pc, we find Omega(m) <= 0.62, omega <= -0.2, and Omega(m) <= 0.17, omega <= -0.65 (68% c.l.), respectively. As a general result, this analysis shows that if one minimizes chi(2) for the parameters l, Omega m, and omega, the conventional Lambda CDM model (omega = -1) with Omega(m) = 0.2, and l = 22.6h(-1) pc is the best fit for these angular size data.