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) type models driven by non-relativistic matter plus a smooth dark energy component parametrized by its equation of state pₓ = \ω \ρₓ (-1 \≤ \ω < 0). The allowed intervals for \ω and \Ω_ are heavily dependent on the value of the mean projected linear size l. For l \ 20h⁻¹ - 30h⁻¹ pc, we find \Ω_ \≤ 0.62, \ω \≤ -0.2 and \Ω_ \≤ 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, \Ω_ and \ω, the conventional flat \ΛCDM model (\ω = -1) with \Ω_ = 0.22 and l = 22.6 h⁻¹pc is the best fit for these angular size data.