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 = ωρ_x (- 1 ≤ ω < 0). The allowed intervals for ω and Ω_m are heavily dependent on the value of the mean projected linear size l. For l approx= 20-30h~(-1) pc, we find Ω_m ≤ 0.62, ω ≤ -0.2, and Ω_m ≤ 0.17, ω ≤ -0.65 (68% c.l.), respectively. As a general result, this analysis shows that if one minimizes χ~2 for the parameters l, Ω_m, and ω, the conventional flat ΛCDM model (ω = - 1) with Ω_m = 0.2 and l = 22.6h~(-1) pc is the best fit for these angular size data.
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