ar X iv : a st ro - p h / 01 09 04 7 v 3 2 2 M ay 2 00 2 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 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 ≃ 20h −1 − 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. Subject headings: cosmology: theory – dark matter – distance scale A large number of recent observational evidences strongly suggest that we live in a flat, accelerating Universe composed by ∼ 1/3 of matter (barionic + dark) and ∼ 2/3 of an exotic component with large negative pressure, usually named dark energy or " quintessence ". The basic set of experiments includes: observations from SNe Ia (Perlmutter et al. It is now believed that such results provide the remaining piece of information connecting the inflationary flatness prediction (Ω T = 1) with astronomical – 2 – observations, and, perhaps more important from a theoretical viewpoint, they have stimulated the current interest for more general models containing an extra component describing this unknown dark energy, and simultaneously accounting for the present accelerated stage of the Universe. The absence of a convincing evidence concerning the nature of this dark component gave origin to an intense debate and mainly to many theoretical speculations in the last few years. Some possible candidates for " quintessence " are: a vacuum decaying energy density, or a time varying Λ-term or still an extra component, the so-called " X-matter " , which is simply characterized by an equation of state p x = ωρ x , where ω ≥ −1 (Turner & White 1997; Chiba et al 1997) and includes, as a particular case, models with a cosmological constant (ΛCDM) (Peebles 1984). For " X-matter " models, several results …