Dark Energy and the Angular Size–Redshift Diagram for Milliarcsecond Radio Sources

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Abstract
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 px = ωρx (-1 ≤ ω < 0). The allowed intervals for ω and Ωm are heavily dependent on the value of the mean projected linear size l. For l 20-30h-1 pc, we find Ωm ≤ 0.62, ω ≤ -0.2, and Ωm ≤ 0.17, ω ≤ -0.65 (68% c.l.), respectively. As a general result, this analysis shows that if one minimizes χ2 for the parameters l, Ωm, and ω, the conventional flat ΛCDM model (ω = -1) with Ωm = 0.2 and l = 22.6h-1 pc is the best fit for these angular size data.