Vishwakarma, R. G.2012-03-142012-03-142001-07-05http://hdl.handle.net/11007/1470Models with a dynamic cosmological term Λ (t) are becoming popular as they solve the cosmological constant problem in a natural way. Instead of considering any ad-hoc assumption for the variation of Λ, we consider a particular symmetry, the contracted Ricci-collineation along the fluid flow, in Einstein’s theory. We show that apart from having interesting properties, this symmetry does demand Λ to be a function of the scale factor of the Robertson-Walker metric. In order to test the consistency of the resulting model with observations, we study the magnitude-redshift relation for the type Ia supernovae data from Perlmutter et al. The data fit the model very well and require a positive non-zero Λ and a negative deceleration parameter. The best-fitting flat model is obtained as Ω0 ≈ 0.5 with q0 ≈ −0.2.enRicci symmetriesVariable cosmological termR-Wmodels.Article