A linear evolution of the cosmological scale factor is a feature in several models designed to solve the cosmological constant problem via a coupling between scalar or tensor classical fields to the space-time curvature as well as in some alternative gravity theories .
In this paper , by assuming a general time dependence of the scale factor , R \sim t ^ { \alpha } , we investigate observational constraints on the dimensionless parameter \alpha from measurements of the angular size for a large sample of milliarcsecond compact radio sources .
In particular , we find that a strictly linear evolution , i.e. , \alpha \simeq 1 is favoured by these data , which is also in agreement with limits obtained from other independent cosmological tests .
The dependence of the critical redshift z _ { m } ( at which a given angular size takes its minimal value ) with the index \alpha is briefly discussed .