Study of the Magnitude-Redshift Relation for type Ia Supernovae in a Model resulting from a Ricci-Symmetry

Models 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 require Λto 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 Λand a negative deceleration parameter. The best-fitting flat model is obtained as Ω_0 \approx 0.5 with q_0 \approx -0.2.

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